Lessons on Chemistry SS3 –Edudelight.com

WEEKS                          TOPICS

1.              ALKANOIC ACIDS
2.              ALKANOATES
3.              FATS AND OILS
4.              AMINO ACIDS
5.              NATURAL AND SYNTHETIC POLYMERS
6.              CARBOHYDRATES
7.              METALS AND THEIR COMPOUNDS &(SODIUM)
8.              CALCIUM
9.              TIN &TRANSITION METALS
10.              IRON&ALLOYS

WEEK  1                     

SOURCES, PREPARATION AND PROPERTIES OF ALKANOIC ACIDS

Carboxylic acid s are organic compound s containing carboxy1 group  These compounds possess sufficient acidic character and are called carboxylic acids. Many common chemicals such as citric acid (lemon juice), ethanoic acid (vinegar) are carboxylic acids. The carboxyl group is made up carbonyl,  and hydroxyl, -OH group, hence, its name is carboxyl group (carb from carbonyl and oxyl from hydroxyl). Carboxylic acids may be aliphatic (R-COOH) or aromatic (Ar-COOH) depending upon whether -COOH group is attached to aliphatic alkyl chain or aryl groups respectively.

Aliphatic monocarboxylic acids (containing <?ne carboxyl group) are known as fatty acids because .some of their higher members (C₁₂-C₁₈) like palmitic acid (C₁₅H₃₁COOH) and stearic acid (C₁₇H₃₅COOH) exist in natural fats as esters and are obtained by their hydrolysis.

Sources of Alkanoic Acids

GENERAL METHODS OF PREPARATION

1. By Oxidation Reactions

(i) By Oxidation of Primary Alcohols. Carboxylic acids can be easily prepared by oxidation of primary alcohols with acidified potassium permanganate or acidified potassium dichromate.

(ii) By Oxidation of Aldehydes. Aldehydes on oxidation with usual oxidizing agents give carboxylic acids with same number of carbon atoms as in the aldehyde.

R – CHO + [O]           à        R – COOH

CH₃ CHO + [O]          à        CH₃ COOH + H₂ O

Ethy1 alcohol                          Acetic acid

(iii) By Oxidation of Methyl Ketones. Methyl ketones can also be easily oxidized to carboxylate ion by hypohalite solutions. The salts of carboxylic acid so formed, can be hydrolysed to corresponding carboxylic acid. In this method, the acid formed has one carbon atom less than the parent ketone.

2. From Alkyl Cyanides

Alkyl cyanides on hydrolysis with dilute acids or alkalies give carboxylic acids.

This method serves as a very good synthetic method for preparation of carboxylic acids.

3. By Carbonation of Grignard Reagents

Carboxylic acids can be obtained By carbonation of Grignard reagents. The reaction is carried out by bubbling CO₂ (dry ice is employed as source of carbon dioxide) through ethereal solution of Grignard reagent and subsequent hydrolysis with dil. acids.

This method gives carboxylic acid with one carbon more than the starting compound.

4. From Sodium Alkoxide and Carbon Monoxide 

Heating of sodium alkoxide with CO under pressure yields sodium salt of fatty acids which on subsequent acidification gives acid.

5. From Olefins

Higher fatty acids can be obtained on large scale by heating an olefin with CO and steam under pressure at 570 K-675 K in the presence of phosphoric acid as catalyst.

Benzoic acid can be prepared by oxidation of alkyl benzenes with alkaline potassium permanganate, chromic anhydride or cone. nitric acid. The alkyl side chain gets oxidised to -COOH group irrespective of the size of the chain. Many aromatic acids are obtained industrially by this method.

PHYSICAL PROPERTIES OF ALKANOIC ACIDS

1. Physical State. The first three aliphatic acids are colourless liquids with pungent smell. The next six are oily liquids with an odour of rancid butter while the higher members are colourless, odourless waxy solids. Benzoic acid is a crystalline solid.

2. Solubility. The first four aliphatic members are soluble in water due to intermolecular hydrogen bonding with water molecules.

With increasing size of the alkyl group, the non-polar part of the molecule predominates thereby reducing the solubility in water. The higher members are practically insoluble in water.

3. Boiling Points. Carboxylic acids have quite high boiling points due to the presence of intermolecular hydrogen bonding which results in the formation of dimeric structures.

Due to dimeric structure, the effective molecular mass of the acid becomes double the actual mass. Hence, carboxylic acids have higher boiling points than alcohols and alkanes of comparable molecular masses. Moreover, O – H bond in carboxylic acids is more polar than O-H bond in alcohols. This is due to electron withdrawing effect of carbon group on O-H. Hence, H-bonds in carboxylic acids are relatively stronger than those in alcohols.

4. Melting Points. In first ten members of the homologous series, the alternation effect is observed. The alternation effect implies that the melting point of an acid with even number of carbon atoms is higher than the acid with odd number of carbon atoms above and below it. However, no such effect is observed in homologues with more than ten carbons. The alternation effect can be explained on the basis of the fact that in the carboxylic acids with even number of carbon atoms, the terminal methyl group and carboxylic group are on the opposite sides of zig-zag carbon chain. Hence, they fit better in the crystal lattice and it results in stronger intermolecular forces. On the other hand, acids with odd number of C atoms have carboxyl and terminal methyl group & on the same side of zig-zag carbon chain. Therefore, such molecules being relatively unsymmetrical, fit poorly in the crystal lattice. This causes weaker intermolecular forces and accounts for the relatively lower melting points.

The melting and boiling points of aromatic acids are usually higher than those of aliphatic acids of comparable molecular masses. This is presumably due to the fact that planar benzene ring in these acids can pack closely in the crystal lattice than zig-zag aliphatic acids.

The physical properties of some homologous of alkanoic acids are tabulated below:

CHEMICAL CHARACTERISTICS OF ALKANOICACIDS

In carboxylic acids, the functional group is carboxyl group Carboxylic acids are resonance  hybrid of the following structures

From these structures, it is clear that the carbonyl part of the carboxyl group does not have a double bond character but a reduced double bond character. Thus, it does not give the reactions of the carbonyl group. Also it is evident that the two contributing structures of carboxylic acid are not equivalent, therefore, resonance stabilization in them is not much. Moreover, structure II involves charge separation, so this structure has higher energy and hence makes less contribution. It may be noted here that oxygen atom of –OH group has positive charge in structure n, this indicates its electron deficient nature. Hence, the shared pair of electrons of O-H bond will be strongly pulled towards oxygen and this makes the O -H bond quite polar. Thus, the reactions of carpoxylic acids are characteristic of the carboxyl group and alkyl group. Now, let us study their chemical characteristics.

1. ACIDIC NATURE

Carboxylic acids are quite strong acids because of the presence of polar O-H group. They ionize to give hydrogen ions and hence, behave as acids.

Carboxylic acids behave as fairly strong acids. This can be explained as follows:

Carboxylic acids as well as carboxylate ion both are stabilized by resonance. However, carboxylate ion is more stabilized by resonance because its contributing structures are exactly identical. On the other hand, the contributing structures of carboxylic acid involve charge separation. Since carboxylate ion is more stabilized by resonance than carboxy lie acid, therefore, equilibrium in Eqn. ( 46.1) lies very much in forward direction, i.e., in favour of ionized form. Hence, carboxylic acids behave as fairly strong acids. They tum blue litmus red. Some chemical reactions showing the acidic nature of carboxylic acids are:

           (a) Reaction with Metals. Carboxylic acids react with metals such as Na, K, Zn, etc., and liberate hydrogen gas.

2R – COOH + ZN  à (RCOO)₂ ZN + H₂

2CH₃ COOH + ZN à (CH₃COO)₂ ZN + H₂

(b) Reaction with Alkalies. Carboxylic acids react with alkalies (NaOH, KOH) to form salt and water.

RCOOH + NaOH               à        RCOONa + H₂O

CH₃COOH + NaOH              à        CH₃COONa + H₂O

Acetic acid                                          Sodjum acetate

(c) Reaction with Bicarbonates and Carbonates. Carboxylic acids react with bicarbonates and carbonates and produce brisk effervescence due to liberation of CO₂.

RCOOH + NaHCO₃                     RCOONa + C0₂ + H20

CH₃COOH + NaHCO₃               CH₃COONa + CO₂ + H₂O

Acetic acid      Sodium                        Sodium acetate

bicarbonate

 (d) Reaction with Ammonia. Carboxylic acid react with ammonia to form ammonium salts. Ammonium salts on heating give amides.

Comparison of acidic strengths of carboxylic  acids and alcohols.

In order to understand, the relative acidic strengths of carboxylic acids and alcohols, Jet us consider their dissociation to give H+ Ions.

In Eqn. (46.2) though both carboxylic acid and carboxylate anion are resonance stabilised but resonance stabilization is larger in carboxylate anion than in carboxylic acid. It is because the contributing structures of carboxylate anion are identical but in carboxylic acid, the contributing structures are nonequivalent. Therefore, the equilibrium lies more towards right. On the other hand, in case of alcohols (Eqn. 46.3) neither alcohol nor alkoxide ion are stabilised by resonance and hence alcohols are weakly dissociated and behave as weaker acids than carboxylic acids.

Effect of substituents on acidic strength of carboxylic acids. The factors which increase the stability of carboxylate ion more than the carboxylic acids, increase the acidic strength of acid and the factors that decrease the stability of carboxylate ion decrease the acid strength.

The electron withdrawing groups stabilize the carboxylate anion by dispersal of the negative charge and increase the strength of the acid. On the other hand, the electron releasing groups cause concentration of negative charge, destabilize the carboxylate anion and hence, decrease the strength of the acid.

Electron withdrawing groups such as halo group,-NO2, -CN, etc., increase the acidity of carboxylic acids whereas electron donating groups like alkyl groups decrease the acidity of carboxylic acids.

The effect of various substituents on the strength of acids has been further illustrated with the help of following examples.

1. The effect of number of the substituents is illustrated by the chloro substituted acetic acids. The acid strength increases from chloroacetic acid to trichloroacetic acid.

CICH₂ COOH < Cl₂CHCOOH < Cl₃CCOOH

It is due to the reason that the increase ‘n the number of chloro substituents on a-carbon atom of acetic acid makes the electron withdrawing effect more pronounced and increases the stability of corresponding conjugate base,. i.e., carboxylate ion.

This causes the increase in the strength of their corresponding acids.

2. The effect of nature of the substituent is illustrated by the various halo acetic acids. Their strength increases as

The above order is due to the fact that the electron withdrawing effect of halo-groups increase from

I< Br< Cl<F.

Consequently, the strength of acids also increases.

3. The effect of the position of the substituent is illustrated by the acidity of a-chloro and  β chloro propionic acids.

The effect of the substituent decreases as its distance from the – COOH group increases. Thus, electron withdrawing effect in β-chloropropionic acid is less pronounced  because -Cl group is relatively away from -COOH group. Thus, α-chloropropionic acid is stronger acid than β-chloropropionic acid.

Measurement of acidity of carboxylic acids. The acid strength of carboxylic acids is measured from their dissociation constant. For example,

where K a is the dissociation constant of acid. Higher the value of K a stronger the acid will be. As the numerical values of Ka vary by large magnitude, therefore, it is more convenient to express the acidic strength in terms of its p K a value(= -log K). Now, smaller is the value of p KO , stronger is the acid and vice versa. The K a and p K a values of some acids are given in Table 

Table

 K a and Pk a Values of Some Acids

SYSTEMATIC NAMES AND STRUCTURE OF ALKANOIC ACIDS

In carboxylic acids, The carbon of the -COOH group is Sp² hybridised and hence the carbon along with two oxygen atoms lie in one plane and are separated by about 120°. The orbital structure of -COOH is shown below:

group and QH group. The 1t pair of electrons of the C group are in conjugation with the lone pair of electrons on the -OH group. As a result of this, the -COOH group can be represented by different electronic structures, called canonical forms or mesomers.

This phenomenon in which a compound/group can be written in two or more than two structures is known as resonance or mesomerism.

Due to resonance, the electron density around oxygen of O- H group decreases due to displacement of electrons towards –C- group. The displacement of electrons is due to resonance is known as mesomeric effect or resonance effect. The phenomenon of resonance effect or mesomeric effects in carboxylic acid group helps to explain the acidic character of these compounds (see acidic nature).

NOMENCLATURE

The common or trivial names are derived from the source of individual acids. Formic acid, for example, was so named because it was first obtained by the distillation of ants (Latin fomica meaning ·ants). Similarly, acetic acid has derived its name from vinegar (Latin : acetum meaning vinegar) Butyric acid is present in butter fat (Latin : butyrum meaning butter). The positions of substituents present is specifted by Greek letters a, β, y, etc. The carbon atom next to the carboxyl carbon is considered as a-carbon.

1 In IUPAC system, the monocarboxylic acids are named as alkimoic acids. The name of the acid is derived by replacing the terminal ‘e-’ of the corresponding alkane with ‘-oic acid’.

1 Carboxyl carbon is always given number one in chain of carbon atoms.

The trivial and IUPAC names of some common alkanoic acids are given in Table below.

Table

The Trivial and IUPAC Names of Some Acids

NOMENCLATURE  OF  POLYCARBOXYLIC ACIDS

In order to write the name IUPAC names of polycarboxylic acids i.e;, when two or more carboxylic acids groups are present the following rules are followed:

(a) When Two COOH Groups an! Present. In such co1npound< the Carbon atom.’ of both the carboxylic acid group  are to be counted in the chain. For example.

(b) When Three or More COOH Groups are Present. According to 1993 recommendations when three or more COOH groups are present group like ram rule is followed According to this rule the carbon at of the carboxylic group are not counted in the chain. Some examples are discussed as follow:

ISOMERISM OF ALKANOIC ACIDS

(i) Chain (nuclear isomerism):

(ii) Functional isomerism. CnH₂NO₂ represents saturated monocarboxylic acids. Este  hydroxy carbonyl compounds and hydroxy  oxiranes  For example.

(iii) Optical isomerism. Carboxylic acids with one chiral atom can show optical isomerism.

 ETHANOIC ACID (ACETIC ACID)

From ethyl alcohol. In the method. ethyl alcohol is first dehydrogenated with hot reduced copper to acetaldehyde obtained as oxidized with air in the presence of manganous acetate as catalyst to acetic acid

It can also is produced from carboxylation of methanol in the pr6cnce of rhodium catalyst

Acetic add is also obtained in dilute form (called vinegar by the fermentation of ethyl alcohol with bacteria. Acetobacter  in the presence of air. This a very old method for obtaining vinegar.

Uses of Ethanoic Acid

1. Solvent for many or11anic chemicals especially in the textile. paint and colour making industry.

2. Starting material for the synthesis of many organic compounds. manufacture of rayon fibre.

3. Preservative, flavoring and tenderizer for foods, pick led foods.

4. Used in cooking (vinegar)

Uses of Phenylmethanoic Acid.

1. Disinfectant

2. Preservative for foods. such as canned fruits and m juices; pharmaceuticals and cosmetics.

3. Treatment of ringworm as an ointment (Whitfield) . In all the above ~ the main work the acid does is to prevent and n:duce growth of microbes or germs

4. Primary standard in titrimetry to Standardize alkalis

EVALUATION

1.      Fatty acids are

a)      Unsaturated dicarboxylic acids

b)      Long-chain alkanoic acids

c)      Aromatic carboxylic acids

d)     Aromatic dicarboxylic acids

     Write down the structure of

i.            propane-1, 2, 3-triol

ii.            hexadecanoic acid.

3.What is the functional group of Alkanoic acids?

4.Name the first three members of the group.

5.Using chemical equations only, describe the preparation of ethanoic acids.

6.With chemical equations only,show the reactions of Alkanoic acids with the following.

i.PCl₃

ii. Zn

iii.CaCO₃

iv.ROH

Lessons on Chemistry SS3 –Edudelight.com

WEEK  2

SOURCES, PREPARATION AND PROPERTIES OF ALKANOATES

ESTERS (R’COR”)

Esters are the derivatives of the carboxylic acids in which the -OH part of the carboxylic group has been replaced by -OR group where, R may be alkyl or aryl group.

 SOURCES, PREPARATION AND PROPERTIES OFALKANOATES

Alkyl alkanoates are found widely in nature. Short carbon chain simple alkyl alkanoates exist as liquids and have a characteristic pleasant odour. They occur in essential oils, many fruits and flowers and are sometimes called fruit essences because of their pleasant odours.

More complex alkyl alkanoates are found in fats and oils and some waxes.

Fats and oils contain one, two or three alkanoate groups which generally do not have pleasant odours. It is worth noting that there is a mixture of different alkanoate derivates of  propane-1, 2, 3-triol and long-chain fatty acids in fats and oils. Table 48.1 tabulates common and direct sources of some alkyl alkanoates which give pleasant or fruity smells. It includes some waxes which do not have a fruity smell and are formed from long carbon chain fatty acids. Esters and their mixtures are prepared and used as artificial scents in many foods, drinks and perfumes.

Examples of Sources and Uses of Alkyl Alkanoates

PREPARATION OF ALKANOATES

1. Acylation. Esters are prepared by the acylation of alcohols or phenols. The acylating agents can be any of the following:

Carboxylic acid / H 2SO 4 or Acyl chloride or Acid anhydride

(i) Condensation of alcohols with carboxylic acid This reaction involves esterification of alkanols by alkanoic acid

(ii) Esterification through acid derivatives

2. By Reaction of Acids with Diazomethane. Acids on being treated with ethereal solution of diazomethane yield methyl esters.

RCOOH + CH₂N₂ à  RCOOCH₃ + N₂

Acid                Diazo               Ester

methane

3. By Tischenko Reaction. When aldehydes containing a-hydrogen atoms are treated with aluminium ethoxide. They undergo condensation to produce esters.

PHYSICAL PROPERTIES OF ALKANOATES

1. Physical state. Esters are colourless, oily liquids with a characteristic fruity odour. The odour of the most of the flowers and fruits is due to esters present in them. The characteristic tastes and odours of different esters find applications in the manufacture of artificial flavouring and perfuming agents. The flavours of some of the esters are given below:

Ester                                        Flavour            Ester                            Flavour

n-Pentyl ethanoate                  Banana            Amyl but)’rate             Apricot

Octyl ethanoate                       Orange                        Isobutyl                      Raspberry

methanoate

Ethyl butanoate                       Pineapple         Benzyl ethanoate        Jasmine

2. Solubility. Esters are sparingly soluble in water but are quite miscible in organic solvents like alcohols and ethers. In fact, most of the esters are themselves very good solvents for plastics and nitrocellulose.

3. Boiling points. The boiling points of esters are always less than the corresponding carboxylic acids because esters do not form hydrogen bonds.

CHEMICAL PROPERTIES

1. Hydrolysis. Esters are hydrolysed slowly by water at boiling temperature. The reaction is catalysed by small amount of acid or base. The basic hydrolysis is also known as saponification. It is because of the fact that the esters with high molecular mass acids (C₁₂-C ₁₇) give soap on hydrolysis with a base. Soaps are sodium or potassium salts of carboxylic acids with high molecular mass (C₁₂-C₁₇).

2. Reduction. Esters are reduced to alcohols by the reducing agents like (sodium/ethanol) or (lithium aluminium hydride).

3. Reaction with Ammonia. Esters on treatment with alcoholic ammonia yield acid amides. This reaction is known as ammonolysis of esters.

4. Reaction with Phosphorus Pentachloride. Esters are converted into acid chlorides and alkyl halides by heating with phosphorus pentachloride.

RCOOR’ + PC1₅                    →        RCOCl + R’Cl + POC1₃

C₆H₅COOC₂H₅ + PCl             →       C₆H₅ COCl + C₂H₂OH

Ethyl benzoate                                    Benzoyl chloride

5. Alcoholysis. An ester on refluxing with a large excess of an alcohol in the presence of a little acid or alkali, undergoes exchange of alcohol residues, i.e., alkoxy parts as shown below:

This reaction is known as alcoholysis or trans-esterification.

6. Reaction with Grignard’s Reagent. All esters except the esters of formic acid react with Grignard’s reagent to give tertiary alcohols. However, the esters of formic acid, i: e., alkyl formates form secondary alcohols.

Similarly, if we start with ethyl formate (HCOOC2H5) and methyl magnesium iodide, the product will be isopropyl alcohol.

USES OF ALKANOATES

1. Soap making. The alkyl alkanoates of higher fatty acids like palmitic, stearic or oleic acids are used in the process of soap making. The reaction is known as saponification.

The saponification process can be represented generally for fats and oils which contain one or more ester groups.

2. Flavoring and perfuming agents. Esters or alkanoates ‘have characteristic odours and are therefore used in the manufacture of artificial flavouring and perfuming agents (see table 48.1 for details).

3. As plasticizers. Many esters are used as plasticizers i.e., substances which are added to thermoplastics to soften them so as to be reused. Some common examples are, di-n butyl pthalate, triceryl phosphate, etc.

4. As solvents. Esters are used as solvents due to their ability to dissolve various greases

Methyl acetate is used as solvents for many oils and resins.

Ethyl acetate is used as extraction solvent in food processing and pharmaceutical industry.

Butyl acetate is used as solvent for inks and as a cleaning liquid for surfaces.

Some commercially important carboxylic acids are ethanoic acid Phenyl methanoic and benzoic acid. Let us study their industrial preparation :md uses.

EVALUATION

 1.    Write down the structure of

1. sodium octadecanoate.

2. What do you understand by decarboxylation?

3. What is Esterification?

4. Give the structure of the following compounds

i. ethyl ethanoate

ii. ethyl propanoate

WEEK  3

FATS AND OILS

Fats and oils are important constituents of balanced diet for human beings. Fats are solids while oils are liquids at room temperature. A substance which behaves as an oil in a tropical country such as Ghana may behave as a fat in a country with colder climate. However, fats and oils have the same basic structure. Fats and oils are generally insoluble in water but are soluble in organic solvents.

GENERAL STRUCTURE OF FATS AND OILS

Fats and oils are triesters of glycerol with long chain fatty acids. Each molecule of fat is
composed of one molecule of glycerol and three molecules of fatty acids. A molecule of fat may be represented by the general formula,

These triesters of glycerol and fatty acids are known as triglycerides. If the triglyceride contains all the three same acid groups, it is known as simple glyceride and if the acid groups are different, it is called a mixed glyceride. Glycerides consisting predominantly of saturated fatty acids are solid at room temperature while those with a high proportion of unsaturated acids are usually liquid at room temperature and are called oils.

The fats containing saturated fatty acids are also called saturated fats and the fats containing unsaturated fatty acids are called unsaturated fats, Saturated fats have higher melting point than unsaturated fats. Saturated fatty acids occur in animal fats, margarine and butter. Unsaturated fatty acids occur in vegetable oils like ground-nut oil, palm kernel oil, olive oil, com oil, cotton seed oil, mustard oil, etc. Because of the presence of double bonds in them, unsaturated fats are
more reactive than the saturated fats. Saturated fats being more stable tend to accumulate and get stored under the skin. Therefore, persons consuming more amounts of saturated fat gain weight and become obese. Excessive use of saturated fats leads to many diseases such as hypertension.

Vegetable and marine oils can be hardened and turned into solid fats by hydrogenation in the presence of some appropriate catalyst such as nickel. This-process converts most of the unsaturated fatty acids into saturated fatty acids.

Among saturated fatty acids palmitic acid (C₁₅H₃₁COOH) is widely present in fats. Oleic acid (C₁₇H₃₃COOH), an unsaturated fatty acid, is the most widely distributed in nature. In most fats of animal as well as plant origin it forms 30 per cent or more of the total fatty acids.

ESSENTIAL FATTY ACIDS

Cells, in our body, can synthesize most of the fatty acids that it needs, from the carbohydrates. However, a few polyunsaturated fatty acids cannot be synthesized in the body. The fatty acids which cannot be synthesized in the body are known as essential fatty acids (EFA). Linoleic acid and linolenic acids are examples of essential fatty acids.

CH₃(CH₂)₄-CH=CH-CH₂-CH=CH-(CH₂)₇,COOH
Linoleic Acid

CH₃CH₂CH=CHCH₂CH=CHCH₂-CH=CH₃CH₂COOH
Linolenic Acid

Essential fatty acids are present in large amounts in many vegetable oils. A diet rich in essential fatty acids (linoleic acid and linolenic acid) can be obtained by eating plenty of vegetable seed oils. Essential fatty acids are drastically reduced during hydrogenation of oils. For example, hydrogenation of groundnut oil reduces essential fatty acids from 28 to 2 per cent.

Some common food items which are rich in fats and the type of fatty acids they contain are given in Table 58.1.

Oils and fats are widely distributed both in plants and, animals.

In plants oils and fats are stored in their seeds, roots and fruits. Ground nuts, cotton seeds, palm kernels, castor beans, olives are rich in fats and oils. These are rich in unsaturated fats.

Major Fatty Acids in Some Fats

Types of Fat	Major Fatty Acid	Formula of the Fatty Acid
1. Butter, Cream or milk	Butyric acid	CH3-CH2-CH2-COOH
2. Coconut oil	Octanoic acid	CH3-(CH2)6-COOH
3. Animal fat	Oleic acid and	CH3-(CH2)7-CH=CH-(CH2)7-COOH
	Palmitic acid	CH3-(CH2)14-COOH
4. Plant fats (Seed Oils)	Oleic acid,	CH3-(CH2)7-CH=CH-(CH2)7-COOH
	Palmitic acid and	CH3-(CH2)14-COOH
	Essential fatty acids

Animals are also an important source of fats. Milk and milk products such as butter, cream, cheese are rich in fats. Meat and eggs also contain fat. Creamish white fat found in pig meat (pork) is called lard. Tallow, the fat from sheep and cattle is used largely for making soap.

ats and oils are extracted from the natural sources by the following processes:

1. Rendering. The animal tissues containing the fat are heated dry or with water until the fat melts and can be removed.

 2. Pressing. Oils are obtained from seeds by crushing between steel rollers and then pressing in a hydraulic press.

3. Solvent Extraction. It is often applied to the residue after pressing or rendering for complete removal of oil or fat. The solvents used include petroleum ether and benzene.

PHYSICAL PROPERTIES

1. Oils and fats are liquids or solids having a greasy feel. When pure, they are colourless, odourless and tasteless.
2. They are insoluble in water but soluble in organic solvents such as ether, chloroform and benzene.
3. They have a lower density than water and consequently float on the surface when mixed with water.

 CHEMICAL PROPERTIES

Fats and oils are triesters of glycerol with saturated arid unsaturated fatty acids. Their reactions are those of ester groups in triplicate and carbon-carbon double bonds.

1.      Hydrolysis. They are readily hydrolysed by heating with acids or alkalies or superheated steam. When boiled with sodium or potassium hydroxide solution, the hydrolysis products are glycerol and sodium or potassium salts of long-chain fatty acids. The latter are called soaps and alkaline hydrolysis is known as saponification.

2.      Hydrogenation. On catalytic hydrogenation, hydrogen adds across the carbon-carbon double bonds of the acid components of the triglycerides. This results in the formation of saturated glycerides which are solid fats at room temperature. This hydrogenation process is called hardening of oils. For example,

Partial hydrogenation of vegetable oils is used for the manufacture of margarine and other hardened oils.

 Add To Your Knowledge

For the manufacture of margarine, oils such as groundnut oil and palm oil are treated to remove undesirable material, bleached and heated. Hydrogen gas at a pressure of about 4 atmospheres is passed through the hot oil in the presence of nickel as catalyst. By controlling the extent of hydrogenation the oil is hardened to the desired level. The hardened oil is deodorized and then blended with milk. The product is then mixed with salt, vitamins, flavouring agents and
preservatives to obtain margarine. Margarine is used as a substitute for butter.

TEST FOR FATS AND OILS

SOLUBILITY TEST

This test is based on the fact that oils and fats are soluble in organic solvents such as chloroform, ethanol, etc., but are insoluble in water. Shake a small amount of the given sample with 5 cm³ each of water, ethanol and chloroform in three test tubes. Observe the solubility and draw inferences.

TRANSLUSCENT SPOT TEST

Press a little of the substance in the folds of the filter paper. On unfolding the filter paper, the appearance of transluscent or greasy spot on filter paper indicates the presence of fat or oil. The spot grows larger on heating and drying the filter paper.

 ACROLEIN TEST

Heat a little of the sample with some crystals of KHSO₄ in a test-tube. A pungent irritating odour of acrolein confirms the presence of fat or oil.

he word ‘detergent’ means ‘cleansing agent’ and so the detergents are substances which remove dirt and have cleansing action in water. According to this definition of detergents, soap is also a detergent and has been used for more than two thousand years. There are two types of detergents:

1. Soapy detergents or soaps
2. Non-soapy detergents or soapless soaps.

 Soap

A soap is a sodium or potassium salt of some long chain carboxylic acids (fatty acid).

Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acids are known as soft soaps. Hard soaps are prepared from cheap oils and fats and sodium hydroxide. They contain free alkali and are used for washing purposes. Soft soaps are prepared from good oils and potassium hydroxide. They do not contain free alkali, produce more lather and are used as toilet soaps, shaving creams and shampoos.

A soap has a large non-ionic hydrocarbon group and an ionic COO^–Na⁺ group. So for simplicity the structure of soap can be represented as

Some examples of soaps are: sodium stearate, C₁₇H₃₅COO^–Na⁺, sodium palmitate, C₁₅H₃₁COO^–Na⁺ and sodium oleate, C₁₇H₃₃COO^–Na⁺.

Soaps of metals other than sodium and potassium are usually water-insoluble and do not find application as a cleansing agent. Therefore, hard water, which contains salts of magnesium and calcium, reacts with soap to form magnesium salt of fatty acid and calcium salt of fatty acid.

These calcium and magnesium salts of fatty acids are insoluble in water and separate as curdy white precipitate. Thus, a lot of soap is wasted if water is hard.

Study of Action of Soap in Hard Water

• Take two test tubes and label them as A and B.
• In test tube A, put 10 cm³ of distilled water (or rain water) and in test tube B take 10 cm” of hard water.
• Add 5 drops of soap solution to both the tubes.
• Shake the test tubes vigorously for an equal period of time.

What do you observe?

It is observed that more foam is formed in the test tube containing distilled water. In the test tube containing hard water less foam is formed and at the same time a curdy white precipitate is formed.

Note. If hard water is not available, it can be prepared by adding small amount of calcium chloride or magnesium sulphate to the ordinary water.

PREPARATION OF SOAP

Soap is prepared by heating oil or fat of vegetable or animal origin with calculated quantity of concentrated sodium hydroxide solution (caustic soda solution). Hydrolysis of fat takes place and a mixture of sodium salts of fatty acids and glycerol is formed.

Soap which is formed as a result of alkaline hydrolysis of oil or fat is separated from the solution by the addition of sodium chloride (common salt). Salt decreases the solubility of soap which is, therefore, released soap from the solution. The process is known as salting out. The crude soap that separates out is called grained soap. Now, soap being lighter than water floats on its surface from where it is removed. The lower aqueous layer is called lye.

After the removal of soap the solution which is left behind contains glycerol and sodium chloride. Glycerol is then recovered from this mixture as it is an important chemical
which finds use in drugs, paints, cosmetics and explosives.

TRADITIONAL SOAP MAKING

In Ghana and other parts of Western Africa, soap was prepared by traditional methods long before the modern methods were available. In the traditional methods, ashes from the burning of plantain peels, cocoa husk and wood are used in place of alkali. The ashes provide potassium hydroxide. Oils or fats are obtained from vegetable sources such as palm, coconut, cocoa, etc.

The oil is heated with dry ash or its aqueous solution, with constant stirring. In the traditional soap, glycerol is not recovered. The product thus obtained is usually coloured due to impurities and is not as good as the soap manufactured by modern methods.

Laboratory Preparation of Soap

• Take about 25 cm³ of castor oil or vegetable oil in a beaker. To this add about 50 cm³ of 20 per cent sodium hydroxide solution slowly with constant shaking.
• Heat the mixture slowly to boil and let it continue boiling for about ten minutes.

• Remove the beaker from the burner and add about 5 g of common salt to the beaker.
• Allow the mixture to cool.

What do you observe?

After sometime a solid crust of soap will be seen floating in the beaker.

DETERGENTS

These are also called synthetic detergents or syndets or soapless soaps or just detergents. So in our routine language when we say detergent, it means the synthetic detergent. These synthetic detergents are designed in such a way that while using them problems do not arise with the
hardness of water just as they arise with the use of soap. They do not form insoluble calcium and magnesium salts with hard water and can be used for washing even with hard water.

A synthetic detergent is the sodium salt of a long chain benzene sulphonic acid or the sodium salt of a long chain alkyl hydrogen sulphate.

Like soaps they contain an ionic group such as sulphonate group, SO₃^–Na⁺ or sulphate group, OSO₃^–Na⁺ and long chain hydrocarbon which is a non-ionic group.

PREPARATION OF SYNTHETIC DETERGENTS

Synthetic detergents are prepared by reacting hydrocarbons from petroleum with conc. tetraoxosulphate (VI) acid and converting the product into its sodium salt. Examples of synthetic detergents are sodium p-dodecyl benzenesulphonate and sodium lauryl sulphate.

Add To Your Knowledge

Washing powders available in the market contain about 15 to 30 per cent detergents by weight and the remaining part is other chemicals which are added to it. These are:

• Tetraoxophosphate(V) compounds which prevent formation of insoluble compounds with Ca²⁺ ions and fatty acids
• Bleaching agents to remove stains and to produce whiteness in clothes.
• Perfumes
• Fluorescents
• Additives to prevent corrosion of metals, and to maintain the powdery texture of the detergent.

ADVANTAGES OF SYNTHETIC DETERGENTS OVER SOAPS

Synthetic detergents are widely used these days as cleansing agents, Synthetic detergents have the following advantages over soaps:

1. Synthetic detergents can be used even in hard water whereas some of the soap gets wasted if water is hard.
2. Synthetic detergents can be used even in acidic medium as they are the salts of strong acids and are not decomposed in acidic medium.
3. Synthetic detergents have a stronger cleansing action than soaps.
4. Synthetic detergents are more soluble in water than soaps.
5. Synthetic detergents are prepared from the hydrocarbons obtained from petroleum. This saves vegetable oils which are otherwise used in preparation of soap.

The uses of fats and oils are as follows:

1. Oils and fats are important constituent of food. They are used as food mainly to provide energy.
2. Oils are used for the preparation of margarine and soaps.
3. Some oils such as cod liver oil find medicinal application.
4. Fats and oils are also used in the manufacture of cosmetics such as cold creams, lipsticks, lotions; etc.
5. Certain drying oils such as linseed oil are used in oil-based paints.
6. Some oils are used for lubrication of maChine parts.

FTop of Form

UNCTIONS OF FATS IN THE BODY

1. The main function of fats in the body is to provide energy. By supplying energy, fats save proteins from being used for energy and allow them to perform their more important role of building and repairing tissues. Fats on oxidation provide almost twice as much energy as that given by carbohydrates. The fats provide on oxidation about 37 kJ of energy per gram as compared to about 17 kJ of energy per gram of carbohydrates. Fats yield more energy than carbohydrates because fats contain less percentage of oxygen and higher percentage of carbon and hydrogen as compared with carbohydrates. Fats can also be stored in body for subsequent use. When we consume food which has more energy than is required by the body for performing various functions, the excess food is deposited under our skin in the form of subcutaneous fat.
2. In addition to supplying energy, fats also help in forming structural material of cells and tissues such as the cell membrane.
3. Fats also carry the fat soluble vitamins A, D, E and K into the body and help in the absorption of these vitamins in the intestines.
4. Fat stored under skin protects animals from cold because it is poor conductor of heat.

SUMMARY

• Fats and oils are triesters of glycerol with long chain fatty acids.
• Animal fats are rich in saturated fats whereas vegetable oils are rich in unsaturated fats.
• Fats and oils are insoluble in water but are soluble in organic solvents such as benzene, hexane, ether, petroleum ether, etc.
• Saponification. The alkaline hydrolysis of fats and oils is called saponification. The products of saponification are glycerol and sodium or potassium salts of long chain fatty acids (soaps).
• Hydrogenation of oils transforms them into fats.
• Oil can be extracted from plant seeds by pressing and solvent extraction.
• Soaps. These are sodium or potassium salts of long chain fatty acids.
• Synthetic Detergents. These are sodium salts of long chain benzene sulphonic acids or long chain alkyl hydrogen sulphates.
• Synthetic detergents can be used in acidic medium or in hard water.
• The cleansing action of soaps and detergents is based on the presence of both hydrophobic and hydrophilic groups in their molecules which helps to emulsify the oily dirt which can then be rinsed out.

EVALUATION

1.      Fats and oils are

a)      monoesters of glycerol

b)      diesters of glycerol

c)      triesters of glycerol

d)     diesters of glycol

3.      Liquid oils can be converted to solid fats by

a)      Hydrogenation

b)      Saponification

c)      Hydrolysis

d)     Oxidation of double bonds.

4.      Alkaline hydrolysis of oils and fats is known as

a)      Saponification

b)      Rancidification

c)      Diazotization

d)     Hydrogenation

5.      Sodium or potassium salts of fatty acids are known as

a)      Carbohydrates

b)      Soaps

c)      Non-soapy detergents

d)     Proteins.

II. Fill in the Blanks

6.      Complete the following sentences by supplying appropriate words:

i.            Triglycerides containing unsaturated fatty acids are        ………. at room temperature and are called ……….

ii.            The products of saponification of fats are soaps and       ……….

iii.            Hard soaps are      ………. salts of long chain fatty acids.

7.      What are fats and oils? How do they differ from each other?

Is it possible to convert oil into fat?

8.      Give structure of a saturated fat and an unsaturated fat.

9.      Write down the structure of

i.            propane-1, 2, 3-triol

ii.            hexadecanoic acid

iii.            sodium octadecanoate.

10.  What are the various steps involved in the extraction of oil from groundnuts?

11.  What are the uses of fats and oils?

12.  What are the major functions of fats and oils in the human body?

13.  What is a detergent? Name two types of detergents.

14.  What are soaps? Why does it not produce lather easily in hard water?

15.  What are hard and soft soaps?

16.  What is the basic structure of a soap molecule?

17.  Why is sodium chloride added during the manufacture of soap from oils?

18.  Why is soap not suitable for washing clothes when water is hard?

19.  How do synthetic detergent differ from soaps? What is the basic structure of a synthetic detergent molecule?

20.  Why are synthetic detergents called soapless soaps?

21.  What are synthetic detergents? Give one example of a synthetic detergent.

22.  What are the advantages of synthetic detergents over soaps?

23.  How is soap prepared in laboratory?

24.  Name the raw materials required for the manufacture of soap.

25.  What are the advantages of synthetic detergents?

26.  Explain the cleansing action of soap.

27.  Explain why detergents are preferred over soaps?

28.  Explain the water pollution by the use of synthetic detergents.

29.  Name the two ways by which soaps or detergents help in cleansing.

30.  Would you be able to check if water is hard by using a detergent?

Lessons on Chemistry SS3 –Edudelight.com

 WEEK 4

PROTEINS

Proteins are the complex organic substances which are the basis of protoplasm and are found in all living organisms. The name protein (Greek, proteios means first) was introduced by Mudlar (1839) because of prime importance of such substances to animal life. In human beings, proteins constitute about 18% by mass of the body.

In general, proteins are polymers of a-amino acids. The amino acid units in proteins are held by peptide (-CONH-) linkage. Polymeric products of α-amino acids with molecular mass up to 10,000 are called polypeptides while those having molecular mass more that 10,000 are considered as proteins. However, there is no sharp demarcation between polypeptides and proteins.

SOURCES OF PROTEINS

Proteins are found in the animal as well as plant foods. Animal foods such as meat, poultry fish, eggs” milk and cheese contain <proteins with many essential amino acids. Plant products such as beans, groundnuts, cashew nut, cereals (maize and wheat) and pulses are also good protein food, though they contain fewer essential amino acids.

Top of Form

These are amino substituted carboxylic acids. Of various amino acids, the α-amino acids or 2-acids are quite important because they are the building blocks of peptides and proteins. In α-amino acids, the amino group is present on the α-carbon atom (i.e., C atom next to COOH group): Thus, they can be represented by the general formula,

R – CH – COOH          |        NH2

Group R – is different for different α-amino acids.

About 20 of the a-amino acids have been identified as the constituents of most of the animal and plant proteins. The formulae of such amino acids along with their common and abbreviated names have been given in Table 56.1.

CLASSIFICATION OF AMINO ACIDS

The amino acids containing equal number of amino and carboxyl groups are known as neutral amino acids. The amino acids which contain more number of carboxyl groups than amino groups are known as acidic amino acids while those containing more number of amino groups are called basic amino acids. For example, glutamic acid and aspartic acid are acidic amino acids while lysine is basic amino acid. Alanine is a neutral amino acid.

Add To Your Knowledge

ESSENTIAL AND NON-ESSENTIAL AMINO ACIDS

Out of 20 amino acids which are required for protein synthesis, the human body can synthesize 10. The amino acids which the body can synthesize are called non-essential amino acids. The remaining 10 amino acids which the body is not able to synthesize are called essential amino acids. The essential amino acids must be supplied to our bodies through our diet because they are required for proper health and growth. The deficiency of essential amino acids may cause diseases like Kwashiorkor in which water balance of the body is disturbed.

STRUCTURE OF AMINO ACIDS

It has been found by spectroscopic means that -COOH and –NH₂ groups of amino acids do not exist as such but they react with each other to form internal salt structure which is also called zwitter ion structure. In the formation of zwitter ion, a proton from -COOH part of the molecule is released and attaches itself to –NH₂ part to constitute a dipolar ion as shown below:

Zwitter ion is a neutral species but carries both positive and negative charges.

 PROPERTIES OF AMINO ACIDS

The amino acids are usually colourless, crystalline and high melting solids. They are moderately soluble in water. They are insoluble in organic solvents such as ether, benzene
and petroleum ether. In acidic solution amino acids exist as cations and migrate towards cathode in an electric field whereas ill basic solutions they exist as anions and migrate towards anode. At the intermediate pH, however, they exist as zwitter ion (a dipolar ion) and do not migrate towards either electrode. This pH is known as the isoelectric point of the α-amino acid.

Different amino acids have different isoelectric points.

The isoelectric point of an amino acids depends upon the functional groups present in the amino acid. Neutral amino acids have isoelectric point in the pH range of 5.6-6.3.

At isoelectric point the amino acids have least solubility in water. This property of amino acids is exploited in the separation of different α-amino acids obtained by hydrolysis of proteins.

α-Amino acids exhibit chemical reactions characteristic of primary -NH₂ group and -COOH group.

NAMING THE AMINO ACIDS

Amino acids are known by their common names. However, they can also be named by following the IUPAC rules. In protein or peptides, the amino acid units are represented by a three letter symbol or a one letter symbol. For example, the abbreviation for glycine and alanine are gly, ala respectively. The names, structures, and abbreviations of various α-amino acids are given in the Table below

 Structure and Names of Some a-Amino Acid

H₂N – CH – COOH

             |

             R

6	Serine	Ser	S	-CH2-OH
7	TyrosineAcidic amino acids	Tyr	Y
8	Aspartic acid	Asp	D	-CH2-COOH
9	Glutamic acidBasic amino acids	Glu	E	-CH2-CH2-COOH
10	*Lysine	Lys	K	-CH2-(CH2)3-NH2

Study of α-Amino Acids

Draw structures of the following a-amino acids

      i.            Alanine

      ii.            Phenylalanine

iii.            Tyrosine

iv.            Aspartic acid

v.            Lysine

vi.            Glutamic acid

Give IUPAC names of these α-amino acids. Use the information from Table 56.1. .

 SOLVED EXAMPLE

Give reasons for the following:

On electrolysis in acidic solution amino acids migrate towards in alkaline solution these migrate towards anode.

R – CH – COO–          |       NH2

R – CH – COOH          |      +NH3

Solution. In acidic medium, amino acids exist as cationsand in basic medium they exist as anions

Therefore, on electrolysis in acidic medium they migrate towards cathode while in alkaline medium they migrate towards anode.

Peptides are the products formed by the condensation of two or more amino acids through their amino and carboxylic groups involving elimination of water molecules. They may be classified as dipeptides, tripeptides, tetrapeptides, etc., depending upon whether the number of amino acid molecules taking part in condensation is two, three or four respectively. When the number of such amino acids is more than ten, the product is called polypeptide. A polypeptide having molecular mass more than 10000 u is called protein. Thus, polypeptides and proteins are condensation polymers of α-amino acids. The linkage (-CO-NH-) which unites various amino acid units in a peptide molecule is called peptide linkage or peptide bond.

A simple convention is used to write the structure and name of the peptide. The amino acid unit having free -NH2 group is called N-terminal end whereas the amino acid unit with free -COOH group is called C-terminal end. The structure is written with N-terminal end to the left and C-termina0l end to the right. The base name of the peptide is taken from the C-terminal amino acid unit. Other amino acid units are taken to be substituents of this acid and the suffix ine of their name is replaced with yl. In case of polypeptides and proteins, the abbreviated names of amino acid units are used. Let us write the structure of a tripeptide formed from glycine, alanine and serine .

A section of polypeptide with different amino acid units is given below:

HYDROLYSIS OF PROTEINS

Polypeptides and proteins can be hydrolysed by dilute acids or enzymes. The ultimate product of hydrolysis of proteins is a mixture of α-amino acids.

When food is digested, the proteins present in it are broken into constituent amino acid molecules. During digestion the peptide linkage that joins the amino acids in proteins gets hydrolysed. Hydrolysis of proteins takes place in the stomach and small intestine and the amino acids produced in the process are absorbed from the intestine by the blood. These amino acids are then regrouped to form specific proteins in the cells of our body.

Hydrolysis of tripeptide glycylalanylglycine (gly.ala.gly) is shown below:

CLASSIFICATION BASED UPON MOLECULAR STRUCTURE

Proteins can be classified into two broad classes on the basis of their molecular structure:

Fibrous proteins and Globular proteins.

1. Fibrous Proteins

Such proteins have thread like molecules which tend to lie side by side to form fibres. The molecules are held together at many points by hydrogen and disulphide bonds: They are
generally insoluble in water.

2. Globular Proteins

Such proteins have molecules which are folded into compact units that often approach spheroidal shapes. The areas of contact between the molecules are small. Therefore, the intermolecular forces, here, are comparatively weak. These proteins are soluble in water or aqueous solutions of acids, bases or salts.

 CLASSIFICATION BASED UPON THE

CORRESPONDING HYDROLYSIS PRODUCTS

On the basis of chemical composition or the nature of products of hydrolysis, proteins are classified into three categories.

1. Simple Proteins

These proteins on hydrolysis give mixture of α-amino acids only. Some examples are albumins in white of egg, oxyzenin in rice, keratin in hair, nails, horns and glutenin in wheat.

2. Conjugated Proteins

These proteins on hydrolysis give a non-protein part in addition to the a-amino acids. This non-protein portion is called the prosthetic group. The, main function of the prosthetic group is to control the biological functions of the protein. Some examples of conjugate proteins along with their corresponding prosthetic groups are given below in tabular form:

Name of the Protein	Prosthetic Group
Nucleoproteins	Nucleic acids
Glycoproteins	Sugars
Lipoproteins	Lipids such as lecithin
Phosphoproteins	Phosphoric acid residues
Chromoproteins	Some colouring matter
Metalloproteins	Some metal.

3. Derived Proteins
These are the degradation products obtained by partial hydrolysis of simple or conjugated proteins with acids, alkalies or enzymes. Some examples are, proteoses, peptones and polypeptides.
Peptones → Polypeptides → Peptones → Polypeptides

The structure of proteins is generally studied at four different levels, i.e., primary, secondary, tertiary and quaternary structures as discussed below:

1.      PRIMARY STRUCTURE OF PROTEINS

It refers to the sequence in which amino acids are arranged in proteins. Each protein has a specific sequence of amino acid units which is critical of its biological activity. The change of just one amino acid in the sequence can alter the biological activity. For example, haemoglobin a protein responsible for carrying oxygen in the blood consists of 574 amino acid units. The change of just one amino acid in the sequence produces ineffective haemoglobin (which is found in the patients suffering from sickle cell anaemia).

– Val – His – Leu – Thr – Pro – Glu – Glu – Lys –

                       (Normal haemoglobin)

– Val – His – Leu – Thr – Pro – Val – Glu – Lys

                    (Sickle cell haemoglobin)

The procedure for the determination of primary structure of protein is quite complex. Such procedure was first of all developed by Frederic Sanger in 1953, who determined the amino acid sequence in insulin.

2.      SECONDARY STRUCTURE OF PROTEINS

It refers the arrangement of polypeptide chains giving rise to a particular shape, which arises as a result of hydrogen bonding.

The two common secondary structures are α-helix and β-pleated sheet structure.

3.      TERTIARY STRUCTURE OF PROTEINS

It refers to the definite geometric pattern in which the entire protein molecule folds up to produce a specific shape. It arises due to folding and superposition of various secondary structural elements. The two important tertiary structures of proteins are fibrous structure and globular structure. At normal pH and temperature, each protein will take a shape that is energetically most stable. This shape is called native state of the protein.

4.      QUARTERNARY STRUCTURE

Many proteins exist as aggregates of two or more polypeptide chains. The overall structure of the protein which arises due to specific spatial arrangement of multiple sub-units is called quarternary structure. For example, the quarternary structure of haemoglobin involves four polypeptide chains.

SOLVED EXAMPLE

Example 56.2. What type of linkages are responsible for the formation of,

(i) Primary structure of proteins
(ii) α-Helix formation

Solution.

(i) Primary structure of proteins arises due to peptide bonds (-CO-NH- bonds) between various constituent of amino acids.

(ii) Hydrogen bonds between -C- and -N- groups of

||            |

O           H

the same polypeptide chain.

All proteins in a living system possess definite configuration and biological activity. A protein found in a living system with definite configuration and biological activity is called a native protein. When a protein in its native form is subjected to physical change such as change in
temperature, addition of solvents miscible with water or chemical change such as addition of acids, alkalies or salts, the hydrogen bonds are disturbed. As a result, globules unfold
and helix get uncoiled and protein loses its biological activity and gets coagulated. The proteins in this state are said to be denatured.

The process that brings about changes in physical as well, as biological properties of the proteins is called denaturation of proteins.

Some important features of denaturation are

        i.            Chemically, denaturation does not change the primary structure but changes the secondary and tertiary structures of proteins.

      ii.            Denaturation of proteins can be caused by changes in pH, temperature or by adding other denaturating agents.

    iii.            Denatured protein loses its biological activity.

     iv.            During denaturation, the protein molecule uncoils from an ordered and specific conformation into a more random        conformation leading to precipitation. Thus, denaturation leads to increase in entropy.

       v.            Denaturation may be reversible or irreversible.

The most common example of irreversible denaturation of proteins is coagulation of albumin present in white part of an egg. When the egg is boiled, hard the soluble globular proteins present in it denature resulting in the formation of insoluble fibrous proteins.

Another example of irreversible denaturation is curdling of milk which is caused due to formation of lactic acid by bacteria present in milk.

In case of reversible denaturation process, the proteins recovers its original properties and biological activity when the disruptive agent is removed. The reverse of denaturation is called renaturation.

Enzymes are the important group of globular proteins which act as biological catalysts in living systems: They may be simple or conjugated proteins, Some enzymes can be non-proteins also. Enzymes increase the rate of biochemical reactions by providing alternative path of lower activation energy. About 3000 different kinds of enzymes have been identified so far. Some common enzymes along with their functions have been given in Table  below.

Some Common Enzymes

Enzymes	Reaction Catalysed
Amylase	Starch → Glucose,
Maltase	Maltose → Glucose
Lactase	Lactose → Glucose + Galactose
Invertase	Sucrose → Glucose + Fructose
Pepsin	Proteins → Amino acids
Trypsin	Proteins → Amino acids

USES OF PROTEINS FOR FOOD

Proteins are an important component of our food. Proteins build up, maintain and replace the tissues in our body. Our muscles, our organs, and our immune systems are made up mostly of protein. Our body uses the protein we eat to make large number of specialized protein molecules that have specific functions.

The best sources of proteins are poultry, fish, eggs, nuts, dairy products, (such as milk, cheese, curd) seeds and legumes.

Proteins, in our food, supply the essential amino acids which our body is unable to synthesize. When we eat food that contains proteins, the digestive juices in the stomach and intestines break down the proteins present in food into amino acids. Theamino acids then can be used to give energy that our body needs to perform various functions.

FUNCTIONS OF PROTEINS

Some of the different forms of proteins and their important functions in human body are given below:

1. Proteins are needed for growth and repair of the body growing children need proteins in large quantities for making new tissues. Proteins as muscle, skin, hair and other tissues constitute the bulk of body’s non-skeletal structure.
2. Some proteins as hormones regulate many body functions. For example, the hormone insulin is a protein. It regulates sugar level in the blood.
3. Some proteins as enzymes catalyse or help in biochemical reactions. For example, pepsin and trypsin.
4. Some proteins act as antibodies, and protect the body from the effect of invading species or substances.
5. Transport proteins carry different substances in the blood to different tissues. For example, haemoglobin is a transport protein. It carries oxygen from lungs to the tissues and carbon dioxide from tissues to the lungs.
6. Contractile proteins help in contraction of muscle and other cells of our body. Myosin is an example of contractile protein.
7. Proteins also serve as a source of energy when the carbohydrates are not available in the food, especially during long fasting or starvation.
8. Proteins act as buffers and maintain appropriate pH required for many biochemical reactions.

1. BIURET TEST

To the dispersion of the substance to be tested (say 5% solution of egg albumin) add about 2 ml of NaOH solution. Now add 4-5 drops of 1 % CuSO₄ solution.

Bluish violet colouration indicates the presence of protein.

2. MILLON’S TEST

This test is given by proteins containing phenolic amino acids. Gelatin does not give this test.

To 1-2 ml of egg albumin dispersion add 2 drops of Millon’s reagent.

White ppt. which changes to brick red on boiling, confirms the presence of proteins.

Millon’s Reagent is prepared by dissolving 5 g each of HgNO₃ and Hg(NO₃)₂ in 100 ml of dil.
HNO₃.

Caution: Millon’s reagent is poisonous. Avoid contact with skin and do not inhale fumes.

3. NINHYDRIN TEST

Take about 2 ml of egg albumin dispersion in a test-tube and add 1-2 ml of Ninhydrin solution. Boil the contents.

Intense blue or purple colouration confirms the presence of proteins.

Ninhydrin Solution is prepared by dissolving 0.1 g of ninhydrin in about 100 m1 of distilled water. This solution is unstable and can be kept only for two days.

SUMMARY

• α-Amino Acids: The organic compounds containing -COOH group and an amino group at the α-carbon atom. These are the building blocks of proteins and peptides.
• Zwitter ion or dipolar ion structure of α-amino acids

+

H₃N –  CH – COO^–

|

R

• Proteins: The complex nitrogenous organic molecules which are essential for the growth and maintenance of life.
• Proteins are condensation polymers of α-amino acids.

In proteins various amino acids are joined by peptide linkage.

• Hydrolysis product of peptides and proteins is α-amino acids
• Denaturation. It is the process that brings about changes in the physical as well as biological properties of the protein.
• Enzymes: The biological catalysts produced by living cells which catalyse the biochemical reactions in living organism. Most enzymes are globular proteins.
• Presence of proteins in a food item can be detected by

– Biuret test

– Millon’s test

– Ninhydrin test.

 EVALUATION

1.      Which of the following is not a source of proteins?

a)      fish

b)      potato

c)      egg

d)     milk.

2.      The number of naturally occurring α-amino acids is about

a)      10

b)      50

c)      40

d)     20

3.      The structure of phenylalanine is 

Its IUPAC name is

a)      2-amino-l-phenylpropanoic acid

b)      1-phenyl-2-aminopropanoic acid

c)      3-phenyl-2-aminopropanoic acid

d)     2-amino-3-phenylpropanoic acid

4.      The correct statement in respect of protein haemoglobin is that it

a)      maintains blood sugar level

b)      acts as an oxygen carrier in the blood

c)      forms antibodies and offers resistance to diseases

d)     functions as a catalyst for biological reactions.

5.      Which one of the following structures represents the peptide chain?

6.      The helical structure of protein is stabilized by

a)      hydrogen bonds

b)      ether bonds

c)      peptide bonds

d)     dipeptide bonds.

7.      The presence of an amino acid or protein will change the colour of ninhydrin solution from red to

a)      green

b)      yellow

c)      blue

d)     orange.

8.      Give examples of two natural polymers.

9.      What is the final product obtained by hydrolysis of proteins?

10.      Draw structures of glycine and alanine, and give their IUPAC names.

11.      List any five sources of proteins.

12.      Discuss the following:

i.            zwitter ion structure of amino acids

ii.            peptide linkage

iii.            isoelectric point.

13.      List four important physical properties of a-amino acids.

14.      Explain briefly:

i.            primary structure of proteins

ii.            denaturation of proteins

iii.            enzymes

15.  List any four functions of proteins in human body.

16.  Describe two tests to detect the presence of proteins in a food item.

WEEK 5       

CLASSIFICATION OF POLYMERS

Polymers are classified in a number of ways depending upon one criterion or the other as described below.

On the basis of source or origin, the polymers are classified into two types:

Natural polymers and Synthetic polymers.

1.      Natural Polymers. Polymers found in nature, mostly in plants and animal sources, are called natural polymers. A few examples are:

a. Polysaccharides. Starch and cellulose are very common examples of polysaccharides. They are the polymers of glucose. Starch is a chief food reserve of plants while cellulose is chief structural material of plants.

b. Proteins. These are the polymers of α-amino acids. They are building blocks of animal cells. They constitute indispensable part of our food. Wool, natural silk, leather, etc., are proteins.

c. Nucleic Acids. These are the polymers of various nucleotides. RNA and DNA are common examples.

d. Natural Rubber. Substance obtained from latex is a polymer of 2-methyl buta-I, 3-diene (isoprene).

It may be noted that polymers like polysaccharides, nucleic acids, proteins, etc., which control various life processes in plants and animals are also called biopolymers.

2.      Semi-Synthetic Polymers. These are mostly derived from naturally occurring polymers by carrying out chemical modifications. For example,

H₂SO₄

Cellulose + (CH₃CO)₂O                →                   Cellulose diacetate

Acetic anhydride

Cellulose diacetate is used in making threads, films, glasses, etc.

3.      Synthetic Polymers. The polymers which are prepared in the laboratory are referred to as synthetic polymers or man-made polymers. Some examples of the synthetic polymers are polyethylene, polystyrene, PTFE synthetic rubber, nylon, PVC, bakelite, teflon, orion, etc.

The synthetic polymers can be further classified as these made up of monomers and comonomer units.

The polymers are also classified based on:

a)      Structure

b)      molecular forces

c)      modes of polymerisation.

PROPERTIES OF POLYMERS ON THE BASIS OF THEIR STRUCTURAL FEATURES

There are several factors which determine the physical as well as chemical properties of a polymer, for example, nature of chain packing; type of molecular force etc. These factors influence the crystallinity of polymers, which in turn, along with other factors influence properties as melting point, tensile strength, viscosity, toughness, etc. Based on their structure, i.e., how the monomers are linked to each other we have the following types of polymers:

Linear polymers,

Branched chain polymers, and

Cross-linked polymers.

1.      Linear Polymers. These are the polymers in which monomeric units are linked together to form long straight chains. The polymeric chains are stacked over one another to give a well packed structure. As a result of close packing, such polymers have high densities, high tensile strength and high melting points. Common examples of such type of polymers are polyethylene, nylons and polyesters. 

2.      Branched Chain Polymers. In this type of polymers, the monomeric units are linked to constitute, long chains (called the main-chain). There are side chains of different lengths which constitute branches. Branched chain polymers are irregularly packed and thus, they have low density, lower tensile strength and lower melting points as compared to linear polymers. Amylopectin and glycogen are common examples of this type.

3.    Cross-linked or Network Polymers. In this type of polymers, the monomeric units are linked together to constitute a three-dimensional network. The links involved are called cross-links. Cross-linked polymers are hard, rigid and brittle because of their network structure. Common examples of this type of polymers are bakelite, melamine formaldehyde resin, etc.

Low Density (LDPE) and High density (HDPE) polyethylene

Polyethylene is a branched chain polymer. This means that the polymer chains do not be in parallel but instead form a tangled mass (Fig. 59.4). Now since the chains cannot be close together, the density of polyethylene is low. It is therefore called low-density polyethylene (or LDPE). LDPE has a lower tensile strength, low melting point (= 130° C)

Linear chains of polyethylene i.e., chains with no branching can be close together and hence results in increase in the density. Such a polymer is called high density polyethylene (HDPE). It was first produced in 1953 in Germany by Karl Ziegler.

As the HDPE chains form ordered structures, therefore it has higher density than LDPE. Its melting point and tensile strength is also higher than LDPE.

High Density Polythene (HDPE)	Low Density Polythene (LDPE)·
333-343 K nCH2= CH2          →            -(CH2-CH2)-n 6-7 atm. TiCl4 + Al(C2H5)3 It consists of linear chains polymers molecules Density = 0.97 g cm-3 Melting point 403 K It is translucent polymer It is chemically inert, having relativelygreater toughness and high tensilestrength than LDP Used in the manufacture of containers,pipes, bottles, toys, bags,etc.	350-570 K nCH2= CH2          →            -(CH2-CH2)-n 1000-1500 atm. Traces of O2 It consist of branched chain structure of polymer molecules Density= 0.92 g cm-3 Melting point 383 K It is transparent polymer It is also chemically inert with moderate tensile strength and high toughness Used as packing material in the form of thin film or sheet, also as insulation for electrical wires and cables.

The molecular forces like van der Waals’ forces and hydrogen bonds existing in the monomeric units effect many mechanical properties of polymers such as tensile strength, toughness, elasticity, etc. depend upon. Although these intermolecular forces are present in simple molecules also, but their effect is less significant in them as compared to that in macromolecules. It is because of the fact, that in polymers there is a combined effect of these forces all along the long chains. Obviously, longer the chain, more intense is the effect of intermolecular forces.
On the basis of the magnitude of inter-molecular forces present in the polymers, we have,
Elastomers, Fibers, Thermoplastics, and Thermosetting polymers.

1. Elastomers. These are the polymers in which the polymer chains are held up by weakest attractive forces. They are amorphous polymers having high degree of elasticity. The weak forces permit the polymer to be stretched out .about 10 times their normal length but they return to their original position when the stretching force is withdrawn. In fact,
these polymers consist of randomly coiled molecular chains having few cross-links. When the stress is applied, these randomly coiled chains straighten out and the polymer gets stretched. As soon as the stretching force is released, the polymer regain the original shape because weak forces do not allow the polymer to remain in the’ stretched form.

2. Fibers. These are the polymers which have quite strong interparticle forces such as H-bonds. They have high tensile strength and high modulus. They are thread-like polymers and can be woven into fabrics. Nylon, dacron silk are some common examples of this types of polymers. The H’bonds in nylon-66 have been shown in this Fig.

3.      Thermoplastics. These are the polymers in which interparticle forces of atttaction are in between those of elastomers and fibers. The polymers can be easily moulded into desired shapes by heating and subsequent cooling to room-temperature. There is no cross-linking between the polymer chains. In fact, thermoplastic polymers soften on heating and become fluids but on cooling they become hard. They are capable of undergoing such reversible changes on heating and cooling repeatedly. Common example of thermoplastics are polyethene, polystyrene, polyvinyl chloride, etc.

4.      Thermosetting Polymers. These are the polymers which become hard and infusible on heating. They are normally made from semi-fluid substances with low molecular masses, by heating in a mould. Heating results in excessive cross-linking between the chains forming three dimensional network of bonds as a consequence of which a non-fusible and insoluble hard material is produced. Bakelite is a common example of thermosetting polymer. In short, a thermoplastic material can be remelted time and again without change, while a thermosetting material undergoes a permanent change upon melting and thereafter sets to a solid which cannot be remelted.

MODIFICATION OF PROPERTIES OF POLYMERS

Plasticizers, fillers and dyes are often added in various quantities to modify the physical properties of plastics. For example, it is a common feature among many plastics that they do not soften to workable extent on heating.

a)      Synthetic resins and cellulose derivatives are horny tough materials. Such plastics can be easily softened by the addition of some organic compounds which are known as plasticizers. For example, polyvinyl chloride (PVC) is extremely- hard and brittle. It was
found that it often decomposed before it could be moulded. However, it was discovered in 1926 that the addition of certain substances like, di-n-butylphthalate, makes it soft and workable. The plasticizing effect is due to the solubilization action and an accompanying reduction in intermolecular forces which permits free movement of molecules relative to each other. In simple word plasticizers are molecules which get in between the chains, allowing them to slide over one another more easily.

Some important plasticizers are:

(i)                 Triceryl phosphate

(ii)               Dimethyl pthalate

(iii)             Triphenyl phosphate

(iv)              Camphor.

 b)      The moulding of a polymer can also be achieved by using the thermoplastic or thermosetting polymers. The method of moulding is to use an expansion method. In this method either a chemical reaction is done or a gas is bubbled into the polymer. For example,

                    i.            Polyurethane foam is made by mixing isocyanates and diols. The isocyanate reacts to give carbon dioxide gas, which produces foaming. The polyurethane foams are used as filling for furniture.

                  ii.            Similarly expanded polystyrene is made by bubbling gas into the polymer. Pentane has been used for this purpose. However, CFCs (Chlorofluoro hydrocarbons) have been increasily used for making polymer foams.

                iii.            Vulcanisation of rubber. The natural as well as synthetic rubber are very soft and sticky. These are hardened by a process known as vulcanisation. Charles Goodyear found that when rubber is heated with sulphur, it becomes hard and more flexible. He called the heating of rubber with sulphur as vulcanisation, (The name is coined after vulcan, the Roman god of fire.)

Heating rubber with sulphur causes cross-linking of polymer chains through disulphide bonds. Thus, the individual chains which were entangled together in the rubber now get together into a giant molecule through vulcanisation. As the polymer has double bonds, the chains have bends and kinks that prevent them from forming a tightly packed crystalline polymer. When rubber is stretched, the chains straighten out along the direction of the pull. Cross-linking prevents the polymer from being torn when it is stretched. When the stress is removed it springs back to its original shape and size. This is shown in Fig. 59.6.

 RESINS AND PLASTICS

I.  Resins. Resins are amorphous organic solids or sticky organic liquids or semi-solids which usually have a typical metallic lustre and are often transparent or translucent Natural resins are secreted or flow out of plants (and some insects) when they are cut or
wounded. They can harden when exposed to the air to form brittle, non-crystalline solids. Resins are insoluble in water but soluble in ether, ethanol and other organic solvents. Examples of natural resins are amber (fossil resin) and shellac.

II. Plastics. A plastic is a substance which is capable of being moulded whereas a resin lacks this property. Typical plastics on heating become soft but do not give mobile melts. In contrast, resins on heating give mobile melts. Plastics usually have much higher molecular mass than resins. Nevertheless, this differentiation is not very clear and the two terms are often used interchangeably .

CHEMICAL TESTS ON PLASTICS

Identification of an unknown sample of a plastic, rubber or a fiber may be required for a variety of reasons. Many of these polymers carry their trade name only and their detailed structure is not” disclosed. In this age of competition, a manufacturer may like to develop a product similar to that of another producer. In such cases, it may be necessary to analyse the polymer sample and identify it. Some common methods are discussed as follows:

1.      Visual Examination. A visual examination of the sample may reveal useful information. For example, the. presence of a hard, inflexible flash line would indicate a thermoset moulded material. Similarly, the presence of a gate scar would indicate an injection moulded material. The physical form of the sample, i.e., whether granules, film, sheet or fiber and its flexibility or rigidity would give some indication of its identity. The colour of the product can also be used for identification of a thermoset material. For example, a pastel shade would rule out a phenolic and probably indicate” that it is a urea or melamine-formaldehyde plastics material.

2.      Cutting. The cutting of a plastics material with a pen knife can also provide some information. For example, one can differentiate between crystal clear cellulose acetate and poly (styrene). Thermoset and thermoplastic material can also be differentiated in this way. The ivory like cut of casein and cast phenolics is noteworthy.

3.      Heating tests. A small amount of the material to be tested is taken in a spoon type of spatula and heated on a small bunsen flame. The ease of burning, whether the burning continues after removal from the flame, colour of flame and so on, all give an indication of the possible identity of the material. If the material explodes or burns away rapidly, it is possibly a cellulose nitrate composition.

A second heating test can be conducted with the help of a clean copper wire. The wire is first heated in a clear: bunsen flame and then touched with a small quantity of the material, It is heated again and the colour of the flame is noted. Blue and green colours indicate the presence of halogens in the composition, that is, chlorine, fluorine and rarely, bromine. Presence of poly (vinyl chloride) or its co-polymers, poly (vinylidine chloride), Polyttetrafluoroethylene), chlorinated rubber, rubber hydrochloride or cellulose acetate containing a plasticizer like tricresyl phosphate is indicated by this test.

A third heating test is carried out by heating a small sample in a hard glass tube. The gas evolved is condensed in another tube and very carefully smelled. This can then be compared with the gas evolved from a known polymer.

4.      Fusion Test. The metallic sodium fusion test, can show the presence or absence of nitrogen and halogens. Similarly, the potassium nitrate/potassium carbonate fusion test will indicate the presence or absence of phosphorus in the material under examination. Thus, one can easily distinguish between Nylon and Terylene.

 Identification of Polymers

From the preliminary tests, a great deal of information can be gathered regarding the possible identity of the unknown polymer. If the polymer does not show rubber like elasticity,
an elastomer is ruled out. Further, if on heating it does not melt or flow, a thermosetting polymer is indicated. If it does melt, a thermoplastic polymer is indicated.

 

PREPARATION AND USES OF POLYMERS

1. Polythene or Polyethylene. This is addition polymer of ethene. Two types of polythenes namely; high density polythene and lo\; density polythene, are being produced these days using different conditions for polymerisation.

nCH2= CH2     -(CH2-CH2)-n

ethane                        polythene

Characteristics and uses. It is used in film wraps and bags for packaging, water pipes, coating of telephones, electric wires and cables.

2. Polypropylene. The monomer units are propylene molecules. It is generally manufactured by passing propylene through n-hexane (inert solvent) containing Ziegler-Natta catalyst (a mixture of triethyl aluminium and titanium chloride).

   CH3                                           CH3

|                           Al(C2H5)3         |

nCH = CH2                  →         -(CH-CH2-)n

Propylene                                              Polypropylene

Characteristics and uses. It is harder, stronger and lighter than polyethene. It is used in: packing of textile material and food, liners of bags, gramophone records, ropes, carpet fibres etc.

3.  Polystyrene or Styron. The monomer units are styrene molecules. It is prepared by free radical polymerisation of styrene in the presence of benzoyl peroxide.

(C6H5CO)2O

nCH = CH2                  →         -(CH-CH2-)n

|                                                              |

C6H5                                                       C6H5

Styrene                                                Polystyrene

Characteristics and uses. It is a white thermoplastic material which is transparent and flats on water. It is used for making toys, combs, model construction kits, ceiling tiles, packing for delicate articles and lining material for refrigerators and TV cabinets.

Polystyrene is solid under the name Styrofoam or Styron.

4. Teflon or Poly Tetrafluoro Ethylene (PTFE). The monomer unit is Tetrafluro-ethylene molecules. The prepared by heating tetrafluroethylene under pressure in the presence of ammonium peroxo-sulphate [(NH4)2S2O8].

(NH4)2S2O6

nCF2 = CF2             →         -(CF-CF2-)n

Heat, pressure

Tetrafluroethylene                              Teflon

Characteristics and uses. It is a very tough material and is resistant towards heat, action of acids or bases. It is a bad conductor of electricity. It is used in:

Coating utensils to make them non-sticking, making seals and gaskets which can withstand high pressures, insulations for high frequency electrical installations.

5. Poly Monochloro Trifluoro Ethylene (PCTFE). The monomer unit are chloro-trifluoro ethylene molecules. It is prepared by the polymerization of monochlorotrifluoro ethylene

nCIFC = CF2             →         -(CIFC-CF2-)n

Chlorotrifluoro ethylene                                PCTFE

The properties of PCTFE are similar to Teflon. However, this is relatively less resistance to heat and chemicals.

6. Poly Vinyl Chloride (PVC). The monomer units are vinyl chloride molecules. It is prepared by heating vinyl chloride in an inert solvent in the presence of dibenzoyl peroxide.

Dibenzoyl peroxide

nCH2 = CH             →         -(CH2-CH-)n

|                                                    |

Cl                                                   Cl

Vinyl chloride                                      PVC

Characteristics and uses.PVC is a hard horny material. However, it can be made to aquire any degree of pliability by the addition of a plasticizer. It is resistant to chemicals as well as heat. It is used for making:

Rain coats, hand bags, toys, hosepipes, gramophone records, electrical insulations and floor covering.

7. Neoprene. This is synthetic rubber which resembles natural rubber in its properties. It is obtained by polymerization of chloroprene (2-chloro-1, 3-butadiene) in the presence of potassium persulphate.

nCH2 = C             →         CH = CH2

|

Cl

Chloroprene

↓ K2S2O8

-(CH2 – C = CH – CH2-)n

|

Cl

Neoprene

Characteristics and uses.Neoprene is superior to natural rubber in its stability to aerial oxidation and also in its resistance to oils and other solvents. It is generally used for making.

Hoses, shoeheels, stoppers and belts.

8. Buna-S. It is a copolymer of 1-3-butadiee and styrene, It is obtained by the polymerization of butadiene and styrene in the ratio of 3:1 in the presence of sodium. It is also known as styrene butadiene rubber (SBR)

nCH2 = CH – CH = CH2 + nC6H5CH5 =CH2

Butadiene                     Styrene

Na, ↓ Heat

-(CH2 – CH = CH – CH2 – CH – CH2 -)n

|

C6H5

Buna-S

In buna-S, Bu stands for butadiene; na for sodium which is polymerizing agent and S stands for styrene.

Characteristics and uses.SBR has slightly less tensile strength than natural rubber. It is used in the manufacture of:

Automobile tyres, rubber soles, belts, hoses, etc.

9. Terelene. It is a polymer obtained by the condensation reaction between ethylene glycol and terephthalic acid.

Characteristics and uses. Terelene is resistant to the action of chemical and biological substances and also to abrasion. It has a low moisture absorbing power. As such it is widely used in making wash and wear fabrics. The polyester textile fibers made from the polymer are marketed under the trade name terelene or dacron.

It is used as ablend with cotton and wool in clothing. It is also used in seat belts and sails. The polymer is also used as mylar in the preparation offilms, magnetic recording tapes and for packing frozen food. Dacron (and teflon) tubes are good substitutes for human blood vessels in heart by-pass operations.

10.      Glyptal or Alkyd Resin. Glyptal is a general name of all polymers obtained by condensation of di-basic acids, and polyhydric alcohols. The simplest glyptal is poly (ethylene glycol phthalate) which is obtained by the condensation of ethylene glycol and phthalic acid.

Characteristics and uses. These are three dimensional cross-linked polymers. Poly (ethylene glycol phthalate) dissolves in suitable solvents and the solution on evaporation leaves a tough and non-flexible film. Thus, it is used in:

adherant paints, lacquers and building materials like asbestos and cement.

      11. Nylon-6,6. It is a polymer of adipic acid , (1, 6-hexanedioic acid) and hexamethylene diamine (1, 6-diarninohexane)

O             O

||              ||

n H₂N-{CH₂)₆-NH₂ + nHOC-{CH₂)₄-COH

Hexamethylene diamine                          Adipic acid

– 525 K ↓ Heat

H             H                 O

|                    |                        ||

-( N-{CH₂)₆-N-C-{CH₂)₄-C)- + 2nH₂O

||

O

(Nylon-66)

Characteristics and uses. Nylon-66 (read as nylon-six-six) can be cast into a sheet or fibers by spinning devices. Nylon fibers have high tensile strength. They are tough. and resistant to abrasion. They are also somewhat elastic in nature. Nylon finds uses in:

making bristels and brushes, carpets and fabrics in textile industry, elastic hosiery in the form of crinkled nylon.

12.      Nylon 6,10. It is’ a polymer of hexamethylene diarnine (six carbon atoms) and sebacoyl chloride (ten carbon atoms)

O               O

||                     ||

nH₂N(CH₂)₆-NH₂ + nCI- C (CH₂)₈ – C – Cl

heat ↓

H O             O

|    ||                   ||

-(HN-(CH₂)₆-N-C-(CH₂)₈– C )_n +2nHCI

Nylon 610

13.      Nylon-6 (or Perlon), It is obtained from the monomer caprolactum. Caprolactum is obtained from cyclohexane , according to the reaction sequence given below:

Caprolactum on heating with traces of water hydrolyses to 6-amino caproic acid which on continued heating undergoes self-condensation and polymerises to give nylon-6. 

Nylon-6 is used for the manufacture of tyre cords, fabrics and ropes.

14.      Phenol-Formaldehyde Resin (Bakelite). These are made by the reaction of phenol and formaldehyde in basic medium. The reaction involves of formation of methylene bridges in ortho, para or ortho as well as para position as shown in the following reactions.

Characteristics and uses. Bakelite is a cross-linked thermosetting polymer. Soft bakelites with low degree of polymerisation are used as bonding glue for laminated wooden planks, in the preparation of-varnishes and lacquers .

High degree of polymerisation leads to formation of hard bakelite which is used for making combs, fountain pen barrels, gramophone records, electrical goods, formica table tops and
many other products. Sulphonated bakelites are used as ion-exchange resins for softening of hard water.

The reaction starts with the initial formation of o-and / or p-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through – CH₂ groups. The initial product could be a linear product – Novolac used in paints.

Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called bakelite.

15.      Melamine Formaldehyde Resin. Melamine and formaldehyde copolymerise to give another polymer.

Characteristics and uses. Melamine polymer is quite hard and is used in making plastic crockery under the trade name melmac. Cups, plates and other articles made from melamine polymer do not break on being dropped.

16.      Natural Rubber, Rubber is a natural polymer and possesses elastic properties. It is obtained from a rubber tree. When the bark of the tree is cut, a sticky white liquid, latex, oozes out. It is a suspension of rubber particles in water.

Natural rubber is a linear polymer of 2-methyl-l, 3- butadiene (isoprene). It is also called as cis-l,4-poly isoprere. On an average it contains 5000 isoprene units. All the double bonds in natural rubber are Cis. Rubber is a waterproof material.

The cis-poly isoprene molecule consists of various chains held by weak van der Waals interactions and has a coiled structure. Thus it can be stretched like a spring and exhibits elastic properties. Gutta-percha (getah means gum and percha means tree) is a naturally occurring isomer of rubber in which all the double bonds are trans. Like rubber, gutta percha is exuded by certain trees. It is harder and more brittle than rubber

SYNTHETIC RUBBER

To meet human needs, scientists have started preparing synthetic rubbers. Besides having similar properties as natural rubbers they are tougher, more flexible and more durable than natural rubber. They are capable of getting stretched to twice its length. However, it returns to its original shape and size as soon as the external stretching force is released. Synthetic rubbers have been made by the polymerisation of dienes other than isoprene. The polymerisation is carried
out in the presence of Zeigler-Natta catalyst. For example, Polymerisation of I, 3-butadiene

PREPARATION OF SYNTHETIC RUBBERS

 1. Neoprene or polychloroprene is formed by the free radical polymerisation of chloroprene.

It has superior resistance to vegetable and mineral oils. It is used for manufacturing of conveyer belts, gaskets and hoses.

 BIODEGRADABLE POLYMERS

These are polymers that can be broken into small segments by enzyme-catalysed reactions. The required enzymes are produced by microorganism. It is a known fact that the carbon-carbon bonds of chain growth polymers are inert to enzyme-catalysed reactions, and .hence they are non-biodegradable. To make such polymers biodegradable we have to insert certain bonds in the chains so that these can be easily broken by the enzymes. Now when. such polymers are buried as waste, microorganisms present in the ground can degrade the polymer.

One of the best methods of making a polymer biodegradable is by inserting hydrolysable ester group into the polymer.

For example if acetal is added to an alkene undergoing radical polymerisation, ester group will be inserted into the polymer.

The weak links in the polymer are susceptible to enzyme catalysed hydrolysis.

Aliphatic polyesters are one of the important class of biodegradable polymers. Some other examples of biodegradable polymers are described below:

i.            PHBV (Poly-hydroxybutrate-co-β-hydroxy valerate). It is a copolymer of 3-hydroxy butyric acid and 3~hydroxypen1anoic acid.

PHBV is used in orthopaedic devices and controlled drug release. The drug put in PHBV capsule is released after this polymer is degraded by enzymatic action. It can also be degraded by bacterial action.

i.            Poly glycolic acid and. poly lactic acid. These are also biodegradable polymers and are used for post operative stiches. These are bioabsorbable structures.

ii.            Nylon-2-Nylon-6. It is an alternating polyamide copolymer of glycine (H₂N-CH₂-COOH) and amino caproic acid (H₂N-(CH₂)₅COOH) and is biodegradable.

 DISADVANTAGES OF POLYMERS

Environmental Pollution by Plastics

Plastic materials are used to make a large number of domestic and industrial materials. Most of the plastics are strong and not easily destroyed by bacteria when disposed of after use. They are non-biodegradable. When disposed of on the surface of the ground, inside the ground or elsewhere, they do not decay and remain there for about one hundred years.

For example, they may collect together in drains and block the drainage, thus creating sewage problems. The drains may have to be dugout to remove the non-biodegradable plastic materials such as polythene.

Plastics can be. burnt at refuse dumps, but the products of burning can be hazardous. Burning of PVC plastics, which contain chlorine, produces pungent smelling hydrogen chloride gas and chlorine gas which are poisonous to humans and plants.

Pollution by plastics may be reduced by reduction in plastic use, more use of biodegradable plastics, incineration and recycling.

SUMMARY

• Polymer: A substance with giant molecules having high molecular mass.
• Polymerisation: The process in which large number of small molecules combine to form a giant molecule or a macromolecule. The small molecules are called monomers.
• Homopolymer: A polymer formed by combination of only one type of monomers.
• Copolymer: A polymer formed by combination of two or more different types of monomers.
• Addition Polymers: The polymers formed by combination of large number of monomers without the elimination of any smaller molecules.
• Cross-linked Polymers: Monomers cross-link to form a network. These are hard and rigid.
• Condensation Polymers: During formation of such polymers, elimination of smaller molecules such as H₂O, NH₃, etc., takes place.
• Polythene: It is a polymer of ethene.
• Poly vinylchloride: It is formed by the polymerisation of viny1chloride (CH₂ = CH -Cl).
• Phenol-methanal plastics: It is commercially known as Bakelite.
• Elastomers: In this type of polymers, the polymeric chains are held by weak intermolecular forces. For example, rubber.
• Fibres: These are thread like polymers having hydrogen bonds as intermolecular forces between different polymeric chains. For examples, cotton, nylon-66.
• Thermoplastics: These are the polymers that soften on heating and can be. moulded into any shape. Intermolecular forces in them are intermediate between fibres and elastomers.
• Thermosetting Polymers: These polymers are very hard, infusible and have extensive .cross links. For example, bakelite.
• Plastic: A polymer which is readily deformable and can be moulded into any shape.
• Vulcanisation: Heating of natural rubber with sulphur to make it strong.
• Resilency Property: The property of returning to the original shape after distortion within elastic limit.
• Isoprene: 2-Methyl-l, 3-butadierre. Monomer of natural rubber.
• Plasticizer: A substance that when added to a thermoplastic improves its workability.
• Caprolactum: Starting material for Nylon-6, a synthetic polyamide fibre.
• Wool: A natural polyamide fibre.
• Poly Urethane: It is formed by reaction of. di-isocyanate with a polyester having hydroxy groups on ends. These are used as leather substitute.

EVALUATION

1.      Polymers are

a)      micromolecules

b)      macromolecules

c)      sub-micromolecules

d)     none of these.

2.      Bakelite is

a)      addition polymer

b)      elastomer

c)      thermoplastic

d)     thermosetting

3.      The S in Buna-S refers to

a)      sodium

b)      sulphur

c)      styrene

d)     just a trade name

4.      The repeating units of PTFE am

a)      Cl₂CH-CH₃

b)      F₂C = CF₂

c)      F₃C-CF₃

d)     FCIC = CF₂

5.      The inter-particle forces between linear chains in Nylon-66 are

a)      H-bonds

b)      covalent bonds

c)      ionic bonds

d)     unpredictable.

6.      Nylon-66 is a polyamide of

a)      viny1chloride and formaldehyde

b)      adipic acid and methyl amine

c)      adipic acid and hexamethylene diamine

d)     formaldehyde and malamine.

7.      Which of the following is not a condensation polymer?

a)      Glyptal

b)      Nylon-66

c)      Dacron

d)     PTFE.

8.      Which of the following is a condensation polymer?

a)      Polystyrene

b)      Neoprene

c)      PAN

d)     Poly (ethylene glycol phthalate).

9.      The monomer of PVC is

a)      ethylene

b)      tetrafluoroethylene

c)      chloroethene

d)     none of the above.

10.  Which of the following polymers is, a copolymer?

a)      Polypropylene

b)      Nylon-66

c)      PVC

d)     Teflon

11.  Which of the following polymers is a homopolymer?

a)      Bakelite

b)      Nylon-66

c)      Terylene

d)     Neoprene.

12.  Which of the following types of polymers has the strongest       interparticle forces?

a)      Elastomers

b)      Thermoplastics

c)      Fibers

d)     Thermosetting polymers.

13.  Bakelite is obtained from phenol by reacting with

a)      ethanol

b)      methanal

c)      vinyl chloride

d)     ethylene glycol.

14.  Polymer used. in, bullet proof glass is

a)      PMMA

b)      lexan

c)      nomex

d)     kevlar

15.  Nylon-6 is made from

a)      1, 3-Butadiene

b)      chloroprene

c)      adipic acid

d)     caprolactam.

16.  F₂C = CF₂ is a monomer of

a)      Teflon

b)      glyptal

c)      nylon-6

d)     buna-5.

17.  Soft drinks and baby feeding bottles are generally made up of

a)      polyester

b)      polyurethane

c)      poly urea

d)     polyamide

e)      polystyrene.

18.  Nylon threads are made of

a)      polyvinyl polymer

b)      polyester polymer

c)      polyamide polymer

d)     polyethylene polymer.

19.  (-NH(CH₂)₆-NHCO(CH₂)₄CO-)_n is a

a)      homopolymer

b)      copolymer

c)      addition polymer

d)     thermosetting polymer.

20.  Which is not a polymer?

a)      Sucrose

b)      enzyme

c)      Starch

d)     teflon

II. Fill in the Blanks

21.  Complete the following sentences by supplying the appropriate words:

i.            On the basis of their origin polymers are classified as …… and …… polymers.

ii.            The three natural fibres are ……

iii.            Natural rubber is a polymer of ……

iv.            Phenol formaldehyde resin is commonly called ……

v.            Nylon-6 is a polymer of ……

vi.            The monomer units of PAN is ……

vii.            The monomer units of PMMA is ……

viii.            In Buna-S, S stands for …….

ix.            Lucite is a polymer of ……

x.            The starting material of PCTFE is ……

22.  Explain the following terms with examples:

a)      Polyester

b)      Polyamide

c)      Natural polymers

d)     Synthetic polymers.

23.  Give the classification of polymers:

a)      On the basis of their mode of synthesis; and

b)      On the basis of nature of forces between the macromolecules.

Give suitable example in each case.

24.  What is a polyamide? How is Nylon-6 synthesised?

25.  How does vulcanised rubber differ from natural rubber?

26.  Define the terms: Thermoplastic polymers, Thermosetting polymers, Fibers and Elastomers.

27.  Give the starting material of each of the following:

a)      PMMA

b)      PCTFE

c)      PAN

d)     PVC

28.  What is the difference between nylon-6 and nylon-66?

29.  Give the structures of monomers of bakelite. How is it formed? To which class, thermosetting or thermoplastic does it belong? Give reasons.

30.  Write the names and structures of the monomer of each of the following. Give one use of each of the polymers.

a)      Terelene

b)      Neoprene

c)      Natural rubber

d)     Lucite.

31.  What do you understand by linear, branched chain and cross-linked polymers?

32.  Given examples of polymers belonging to following categories. Give their structures:

a)      Polyamide

b)      Polyhaloolefins

c)      Polyolefins

d)     Polyesters

e)      Polyacrylates.

33.  Arrange the following polymers in increasing order of their intermolecular forces. Also classify them as addition and condensation polymers. Nylon-6, Neoprene, PVC.

34.  Mention which of the following are addition polymers?

a)      Terylene

b)      Nylon-66

c)      Neoprene

d)     Teflon.

35.  Write the information asked for the following polymers:

a)      Bakelite: materials required for preparation

b)      PVC: monomer unit

c)      Synthetic rubber: monomer unit.

36.  Give one method of synthesis of each of the following:

a)      Buna-S

b)      Teflon

c)      Nylon-66.

37.  What is the difference between homopolymer and co-polymer? Give one example.

38.  Define any two plasticizers.

39.  How are polymers classified into different categories on the basis of intermolecular forces? Give one example of a polymer of each of these categories.

40.  How do double bonds in rubber molecule influence their structure and reactivity?

41.  Differentiate between addition and condensation polymers based on mode of polymerisation. Give one example of each type.

Answers

I. Objective Type Questions

1.      (b)

2.      (d)

3.      (c)

4.      (b)

5.      (a)

6.      (c)

7.      (d)

8.      (d)

9.      (c)

10.  (b)

11.  (d)

12.  (d)

13.  (b)

14.  (b)

15.  (d)

16.  (a)

17.  (e)

18.  (c)

19.  (b)

20.  (a)

II. Fill in the blanks

21.  (i) natural, synthetic

(ii) wool, cotton and silk

(iii) isoprene

(iv) bakelite

(v) caprolactum

(vi) acrylonitrile

(vii) methylmethacrylate

(viii) sulphar
(ix) methyl methacrylate

(x) chlorotrifluroethylene

WEEK  6

CARBOHYDRATES

Carbohydrates constitute an important class of compounds like glucose, fructose, sucrose, starch, cellulose, etc., which play a vital role in our everyday life. They are the ultimate source of most of our food. We clothe ourselves with cellulose in the form of cotton; rayon and linen. We build furniture and houses from cellulose in the form of wood. Thus, carbohydrates provide us with basic necessities of life, food, clothing and shelter.

Carbohydrates are also known as saccharides. Originally, the name carbohydrate was given to the compounds pertaining to general formula C_x(H₂O)_y, and they were considered to be hydrates of carbon. However, this definition could not hold ground for long because many compounds like formaldehyde (CH₂O), acetic acid (C₂H₄O₂), etc., conform to formula C_x(H₂O)_y but they do not exhibit the characteristic properties of carbohydrates.

According to the modem definition carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or the compounds that yield such compounds on hydrolysis.

CLASSIFICATION OF CARBOHYDRATES

Carbohydrates are classified into three major categories depending upon their behaviour towards hydrolysis:

 1.      MONOSACCHARIDES

These are simple carbohydrates which cannot be hydrolysed to simpler carbohydrates.

About 20 monosaccharides are known to occur in nature. Glucose and fructose are common examples.

2.      OLIGOSACCHARIDES

These are the carbohydrates which on hydrolysis give two to ten units of monosaccharides. Accordingly, they may be further divided into di, tri or tetrasaccharides depending upon the actual number of monosaccharide units formed by the hydrolysis of a particular oligosaccharide.

Disaccharides give two units of monosaccharides on hydrolysis. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. Common examples are sucrose and maltose. Both have molecular formula C₁₂H₂₂O₁₁.

Sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose on hydrolysis gives two molecules of glucose only.

Trisaccharides give three units of monosaccharides on hydrolysis. Raffinose, C₁₈H₃₂O₁₆ is a common example.

Tetrasaccharides give four units of monosaccharides on hydrolysis. Stachyose, C₂₄H₄₂O₂₁ is a common example.

3.      POLYSACCHARIDES

These are the carbohydrates which are polymeric molecules and can be hydrolysed to give large number of monosaccharide units. The commonly occurring polysaccharides have the general formula (C₆H₁₀O₅)_n The common examples are starch, glycogen and cellulose.

It may be noted that the carbohydrates which are sweet in taste are collectively called sugars while those which are not sweet are called non-sugars. Monosaccharides and disaccharides are sugars but polysaccharides are non-sugars.

The relative degree of sweetness of various sugars is given below in tabular form:

Sugar	Lactose	Maltose	Galactose	Glucose	Sucrose	Fructose
Relative Sweetness	16	32	32	74	100	173

Do you know? 

Besides carbohydrates, some other compounds are far more sweet. For example,

• Saccharin is about 500 times as sweet as sucrose.
• Aspartame, a peptide, is about 160 times sweeter than sucrose.

REDUCING AND NON-REDUCING CARBOHYDRATES

The carbohydrates may also be classified as reducing and non-reducing sugars. The carbohydrates which can reduce Tollen’s reagent or Fehling’s or Benedict’s solution are classified as reducing sugars, while those which do not reduce these reagents are called non-reducing sugars:

Reducing sugars contain free aldehyde or ketonic group. All monosaccharides are reducing sugars. Disaccharides may be reducing or non-reducing. If the carbonyl groups of both the monosaccharides are involved in linkage, the disaccharide is non-reducing. On the other hand, if one of the carbonyl groups is free, the disaccharide is reducing: Sucrose is a non-reducing sugar while maltose is a reducing sugar.

SOURCES OF CARBOHYDRATES

Glucose occurs in sweet fruits such as grapes, mangoes, oranges, etc. Honey is also rich in glucose. In combined state it is present in maltose, sucrose, starch, cellulose, etc.

Fructose is found in ripe fruits and honey.

Sucrose. Major sources of sucrose are sugarcane and sugarbeet

Lactose is present in milk.

Starch. Major sources of starch are wheat, rice, cassava, root tubers such as potatoes, legumes and vegetables.

Cellulose. Cellulose is present in cotton, wood and jute. 

As pointed out earlier, these are the simplest carbohydrates and cannot be hydrolysed to give still simpler carbohydrates. Monosaccharides containing aldehyde group are called aldoses while those containing ketonic group are called ketoses. They can be further classified into different
categories depending upon the number of carbon atoms. For example,

Monosaccharides with three carbon atoms are called trioses.

Monosaccharides with four carbon atoms are called tetroses.

Monosaccharides with five, six and seven carbon atoms are called pentoses, hexoses and heptoses respectively. 

GLUCOSE

(DEXTROSE OR GRAPE SUGAR), C₆H₁₂O₆

Glucose occurs in nature in free state as well as in combined state. In free state, it occurs in sweet fruits (such as grapes, mangoes, oranges) and honey. Ripe grapes contain about 20% glucose and hence the name grape sugar. In combined state, glucose is present in di-and polysaccharides. Maltose and starch on hydrolysis yield only glucose.

Preparation of Glucose 

1.      By Hydrolysis of Cane-sugar. In laboratory, glucose can be prepared by hydrolysis of cane-sugar in the presence of alcohol using dilute hydrochloric acid. Glucose and fructose are formed in equal amounts. Glucose, being less soluble in ethyl alcohol than fructose, crystallizes out.

H⁺

C₁₂H₂₂O₁₁ + H₂O     →     C₆H₁₂O₆ + C₆H₁₂O₆

                                                                                 Glucose      Fructose

2.      By Hydrolysis of Starch. Glucose is obtained, on commercial scale, by hydrolysis of starch by boiling it with dilute H₂SO₄ acid at 393 K under a pressure of 2-3 atm.

H⁺

(C₆H₁₀O₅)_n + nH₂O                     →                nC₆H₁₂O₆

Starch                                 393K.2-3 atm.           Glucose

Physical Properties of Glucose

1.      Glucose is a white crystalline solid. Its melting point is 419 K.

2.      It is sweet in taste.

3.      It is very soluble in water due to its ability to form hydrogen bonds with water. Glucose is insoluble in organic solvents such as ether.

Structure of Glucose

Glucose is an aldohexose. It is monomer of many of the larger carbohydrates such as starch, cellulose. The reactions of glucose indicate that its molecule contains one  primary (-CH₂OH) and four secondary (-CHOH) by hydroxyl groups. Glucose was assigned the following structure:

Glucose also exists in two cyclic forms, α-D-glucose and β-D-glucose. The cyclic structures of glucose are shown below:

α-D-glucose

β-D-glucose

FRUCTOSE (LAEVULOSE), C₆H₁₂O₆

Fructose is a ketohexose. It is obtained along with glucose by the hydrolysis of sucrose. It is found in nature in ripe fruits and honey.

Physical Properties of Fructose

1.      Fructose is a white crystalline solid. Its melting point in 377 K

2.      It is highly soluble in water. It is insoluble in organic solvents such as ether.

3.      It is very sweet in taste and is used in making sweets,

Structure of Fructose

Fructose has the molecular formula C₆H₁₂O₆ and on the basis of its reactions it was found to contain a ketonic functional group at C-2 and six carbon atoms in straight chain as in case of glucose. Its open chain structure may be written as:

Fructose also exists in two cyclic forms, (α- D- fructose and β-D-fructose.

The two cyclic structures ‘of fructose are represented by Haworth structures as given:

These are the carbohydrates which give two units of monosaccharides on hydrolysis with dilute acids or enzymes. Some examples are: Sucrose, maltose and lactose.

H₂O/H⁺

C₁₂H₂₂O₁₁     →     C₆H₁₂O₆ + C₆H₁₂O₆

Sucrose                  Glucose       Fructose

H₂O/H⁺

C₁₂H₂₂O₁₁     →     C₆H₁₂O₆ + C₆H₁₂O₆

Lactose                  Galactose     Fructose

H₂O/H⁺

C₁₂H₂₂O₁₁     →     2C₆H₁₂O₆

Maltose                  Glucose

This implies that a disaccharide is formed by condensation of two monosaccharide units. The two monosaccharide units in a disaccharide are joined together by an oxide (or ether) linkage formed by loss of a water molecule. Such a linkage between two monosaccharide units
through oxygen atom is called glycosidic linkage.

Now, let us study about some disaccharides in somewhat details.

SUCROSE (CANE-SUGAR), C₁₂H₂₂O₁₁

Sucrose is the most widely occurring disaccharide. It is found in all photosynthetic plants. It is obtained commercially from sugarcane or sugarbeets. On hydrolysis with dilute acids or enzyme invertase, 1 mole of sucrose gives 1 mole of glucose and 1 mole of fructose.

H⁺

C₁₂H₂₂O₁₁ + H₂O     →     C₆H₁₂O₆ + C₆H₁₂O₆

                                                                                       Glucose             Fructose

Sucrose is a non-reducing sugar. Haworth (1927) suggested the following structure of sucrose.

Haworth’s Representation of Sucrose

MALTOSE,C₁₂H₂₂O₁₁

When starch is hydrolysed by the enzyme diastase, maltose is formed as one of the products.

Diastase

2(C₆H₁₀O₅)_n + nH₂O                     →                nC₁₂H₂₂O₁₁

Starch                                                                      Maltose

On hydrolysis with dilute acids, one mole of maltose yields 2 moles of glucose.

H⁺

C₁₂H₂₂O₁₁ + H₂O     →     2C₆H₁₂O₆

Maltose                               Glucose

Maltose is a reducing sugar. In maltose the two glucose units are linked through α-glycosidic linkage between C-1 of one glucose unit .and C-4 of the other. The free aldehyde group can be produced at C-1 of the second glucose in solution. Hence, maltose shows reducing properties.

The structure of maltose is given below:

LACTOSE, (MILK SUGAR),C₁₂H₂₂O₁₁

Lactose is present in the milk and is also known as milk sugar. On hydrolysis with dilute acids one mole of lactose yields 1 mole of .glucose and one mole of galactose

H⁺

C₁₂H₂₂O₁₁     →     C₆H₁₂O₆ + C₆H₁₂O₆

                                                                     +H₂O     Glucose             Fructose

Lactose is a reducing sugar. In lactose, glucose and galactose units are linked through β-glycosidic linkage between C-1 of galactose and C-4 of glucose unit. The structure of lactose is given as follows:

POLYMERS OF CARBOHYDRATES – POLYSACCHARIDES

These are the carbohydrates which are made of large number of monosaccharide units. They are naturally occurring condensation polymers in which large number of monosaccharide units are joined together through glycosidic linkages. They are colourless, tasteless amorphous powders. They play vital role in plant and animal life.

Let us study the structure of three important polysaccharides starch, cellulose and glycogen.

 1.      STARCH (AMYLUM), (C₆H₁₀O₅)_n

It is a most abundant source of carbohydrate in human diet. It is a major food reserve material of plants and occurs mainly in seeds, fruits, tubers and roots of the plants. The important sources of starch are wheat, rice, cassava, cocoyam maize, potatoes, legumes and other vegetables. repared foods such as fufu, akple, omotuo, tapioca and kokonte have a high starch content.

Starch is the polymer of D-glucose. It consists of two components:

a)      Amylose, the water soluble fraction, and a linear, polymer of D-glucose units. It gives blue colour with iodine.

b)      Amylopectin, the water insoluble fraction, and consists of linear as well as branched chain polymers of D-glucose.

Natural starch contains approximately 15-20% of amylose and 80-85% of amylopectin. Amylopectin does not give blue colour with iodine. A molecule of amylose may contain 200-1000 glucose units whereas a molecule of amylopectin may contain 2000-3000 glucose units. In amylose. as well as amylopectin the D-glucose units are linked through a-glycosidic linkages between C-l of one glucose unit and C-4 of the next glucose unit. Branching in amylopectin occurs through C₁ – C₆ glycosidic linkage.

The section of amylose as well amylopectin has been given as follows:

On hydrolysis with dilute acids or enzymes, starch breaks down into maltose and finally glucose.

Diastase

(C₆H₁₀O₅)_n        →         C₁₂H₂₂O₁₁        →         C₆H₁₂O₆

Starch                              Maltose                            Glucose

Starch is a white amorphous powder. It is tasteless and is insoluble in cold water. 

2. CELLULOSE, (C₆H₁₀O₅)_n

Cellulose occurs exclusively in plants. It is the most abundant organic substance in plant kingdom. It is a chief structural material of cell walls of all the plants. It is also the chief component of cotton, wood and jute. Wood contains 45-50% while cotton contains 90-95% cellulose. It may be noted that over 50% of the total organic matter of the living world is cellulose.

Structurally, cellulose is a linear polymer of glucose units joined by β-glycosidic linkage between C-1 of one glucose unit and C- 4 of the next glucose unit as shown below:

The chains are held together by hydrogen bonds between glucose units of adjacent strands. The chains are so arranged as to constitute the bundles. This lends rigidity to its structure

Cellulose does not reduce Tollen’s reagent or Fehling’s solution. It is not fermented by yeast. Cellulose cannot be hydrolysed easily. However, when it is heated with dilute sulphuric acid, under pressure, it undergoes hydrolysis and yields glucose.

 Add To Your Knowledge

• Cellulose forms many useful products when treated with suitable chemical reagents. Some of the important products are:
• Celluloid, which is used in films.
• Gun cotton, which is an explosive.
• Cellulose acetate is used in plastics, for wrapping films and nail polishes.
• Methyl cellulose is used in cosmetics, pastes and for fabric sizing.
• Ethyl cellulose is used in plastic coats and films.

On treatment with concentrated sodium hydroxide, cellulose forms a transluscent mass which imparts a silky lustre to cotton. The process is called mercerisation and cotton so produced is called mercerised cotton.

 3. GLYCOGEN

Glycogen is the principal reserve of carbohydrates in animals. The molecular structure of glycogen is similar to amylopectin. That is why it is also known as animal starch. However, one main difference between glycogen and amylopectin is that average chain length in glycogen is
10-14 while in amylopectin it is 25-30 glucose units.

Glycogen is present in liver, muscles and brain. When the body needs glucose, enzymes break glycogen into glucose.

In addition to starch and cellulose, a number of other polysaccharides are used as food components. These are the gums and pectins. Gums are polysaccharides made up of more than one type of monosaccharides. They are used for thickening and improvement of texture in food industry. Pectins are found in fruit skins and are extracted by boiling. Jelly preparations contain pectin dissolved in a fruit juice. The pectin causes jelly to set into a semi-solid.

USES OF CARBOHYDRATES

Some important uses of carbohydrates are discussed below:

1.      Glucose is widely used in food industry in production of fruit drinks and sweets. .

2.      Glucose is soluble in water and is absorbed rapidly into the blood stream. Therefore, it is useful for sick people and sportsmen who need instant energy.

3.      Fructose is the sweetest sugar. It is used as a sweetening agent.

4.      Sucrose is commonly used as table sugar. It is also used as sweetening agent.

5.      Sucrose is used to prepare glucose and fructose by hydrolysis. Sucrose is also used in the manufacture of ethanol by fermentation.

6.      Starch can be used for the production of ethanol by hydrolysis and fermentation. It is extensively used in food.

7.      Starch is used for stiffening cloth after laundering. Its suspension in water is used as an adhesive.

8.     Cellulose is used for the production of paper.

FUNCTIONS OF CARBOHYDRATES

The carbohydrates perform many important functions in living bodies.

They act as biofuels to provide energy for functioning of living organisms.

In human system, all the carbohydrates except cellulose can serve as source of energy. Starch and various sugars which are taken as food are the first hydrolysed to glucose by the enzymes present in the digestive system.

Amylase

2(C₆H₁₀O₅)_n + nH₂O                     →                nC₁₂H₂₂O₁₁

Starch                                                                      Maltose

Maltase

C₁₂H₂₂O₁₁ + H₂O                     →                2C₆H₁₂O₆

Maltose                                                           Glucose

Glucose on slow oxidation to carbon dioxide and water in the presence of enzymes liberates large amount of energy which ‘is used by the body for carrying out various functions.

C₆H₁₂O₆ + 6O₂     →     6CO₂ + 6H₂O + 2832 kJ

In order to fulfil the emergency requirements, our body also stores some of the carbohydrates as glycogen in the liver. Glycogen on hydrolysis gives glucose.

It may be noted that cellulose cannot be hydrolysed in our body because enzymes required for its hydrolysis are not present in our body. However, grazing animals are capable of hydrolysing cellulose to glucose. In these animals cellulolytic bacteria present in the rumen, break down cellulose with the help of enzyme cellulose and is subsequently digested and converted into glucose.

1.      They act as constituents of cell membrane of plants and bacteria.

2.     D-ribose and 2-deoxy-D-ribose are present in nucleic acids.

SOLVED EXAMPLES

Example

What are reducing and non-reducing sugars?

Solution. Carbohydrates which reduce Tollen’s reagent and Fehling solution are called reducing sugars, while those which do not reduce these are called non-reducing sugars. For example, glucose, fructose, maltose and lactose are reducing sugars while sucrose is a non-reducing sugar.

Example

Draw open chain structures of an aldopentose and an aldohexose:

Solution.

Example

Amylose and cellulose are both straight chain polysaccharides containing only D-glucose units. What is the structural difference between the two?

Solution. In amylose, Dvglucose units are joined together by α-glycosidic linkages involving C-l of one glucose molecule and C-4 of the next glucose molecule. In cellulose, D-glucose units are joined together by β-glycosidic linkages between C-l of one molecule and C-4 of the next glucose molecule.

SUMMARY

• Carbohydrates are the polyhydroxy aldehydes or polyhydroxy ketones or the compounds which can be hydrolysed to such compounds.
• Carbohydrates are classified as monosaccharides, oligosaccharides or polysaccharides.
• The carbohydrates which can reduce Tollen’s reagent, Fehling’s solution or Bededict’s solution are known as reducing carbohydrates.

Glucose, fructose and lactose are reducing sugars sucrose is a non reducing sugar.

• Carbohydrates which are sweet in taste are known as sugars. Fructose is the sweetest sugar.
• Sucrose on hydrolysis gives glucose and fructose.
• Lactose on hydrolysis gives glucose and galactose.
• Maltose, starch and cellulose on hydrolysis give only glucose on hydrolysis.
• All monosaccharides and disaccharides are soluble in water.
• Starch, cellulose and glycogen are examples of polysaccharides.
• Carbohydrates are the major source of energy for functioning of living beings. They are the constituents of cell membrane of plants.

EVALUATION

1.      Which of the following is an example of aldohexose?

a)      Ribose

b)      Fructose

c)      Sucrose

d)     Glucose.

2.      Maltose on hydrolysis gives

a)      α-D-glucose

b)      α and β-D-glucose

c)      glucose and fructose

d)     fructose only;

3.      Fructose has/is

a)      bitter taste

b)      sweet taste

c)      tasteless

d)     nothing certain.

4.      The constituent units of sucrose are

a)      lactose and glucose

b)      glucose and fructose

c)      galactose and glucose

d)     glucose and maltose.

5.      Which disaccharide is present in milk?

a)      Maltose

b)      Galactose

c)      Sucrose

d)     Lactose.

6.      Which of the following is a ketohexose ?

a)      Fructose

b)      Maltose

c)      Glucose

d)     Ribose.

7.                  The linkage that holds monosaccharide units together in a polysaccharide is called

a)      peptide linkage

b)      glycoside linkage

c)      ester linkage

d)     ionic linkage.

8.      Lactose on hydrolysis yields

a)      Glucose

b)      fructose

c)      glucose and fructose

d)     glucose and galactose.

9.      Which of the following carbohydrates is not a reducing sugar

a)      Sucrose

b)      Lactose

c)      Glucose

d)     Fructose.

II. Fill in the Blanks

10.  Complete the following sentences by supplying appropriate words:

                    i.            Sucrose on hydrolysis yields …………

                  ii.            ………… is the sweetest sugar.

                iii.            ………… (carbohydrate) is not digested in our body.

                 iv.            Two monosaccharide units in a disaccharide are held by ………… linkage

                  v.            Molecular formula of fructose is …………

11.  What are carbohydrates? How are they classified? Give examples. What are the important sources of glucose and fructose.

12.  What are reducing and non-reducing sugars?

13.  Give an example of aldohexose and an aldotriose.

14.  What is the name given to the linkage which holds together two monosaccharides units in a polysaccharide?

15.  Give the structure of:

                                i.            sucrose

                             ii.            α-D-fructose

                            iii.            maltose.

16.  Out of amylose and amylopectin which form of starch is soluble in water?

17.  Glucose or sucrose are soluble in water but cyclohexane or benzene (simple six membered ring compounds) are insoluble in water. Explain.

18.  What products are expected when lactose is hydrolysed?

19.  Draw structures of α-D-fructose and β-D-glucose.

20.  What are the hydrolysis products of?

                                i.            sucrose

                              ii.            lactose

21.  What is glycogen?

22.  Describe the functions of carbohydrates in living organism.

23.  List some important uses of carbohydrates.

24.  Draw the structure of cellulose, What is the product of hydrolysis of cellulose?

Answers

I.  Objective Type Questions

1.      (d)

2.      (a)

3.      (b)

4.      (b)

5.      (d)

6.      (a)

7.      (b)

8.      (d)

9.      (a)

II.                Fill in the Blanks

10.  (i) glucose and fructose

(ii) Fructose

(iii) cellulose

(iv) glycoside

(v) C₆H₁₂O₆

Lessons on Chemistry SS3 –Edudelight.com

 WEEK  7

METALS AND THEIR COMPOUNDS

It is a well-known fact that elements are the building blocks of various kinds of materials.

Although there are only about 114 elements known, yet they constitute the entire matter on this earth. The elements are classified into three categories, namely: metals, non-metals and metalloids.

Metals are generally lustrous, solids, malleable, ductile and good conductors of heat and electricity. They constitute about two-third of the known elements. Some of the metals which flnd common use in our daily life are : iron, copper, aluminium, lead, gold, silver, mercury, etc.

PROPERTIES OF METALS

The characteristic properties of metals are summarized in Table 50.1

Properties of Metals

REACTIVITY SERIES OR ACTIVITY SERIES

By observing the relative reactivities of different metals in their reactions with oxygen, dilute acids, water and chlorine, the metals have been arranged in the order of their chemical reactivity as given in Table 50.2. Reactivities of certain metals are also compared by studying displacement reactions. A more reactive metal displaces a less reactive metal from the solution of its salt. Such a list is called the activity series or reactivity series. Thus, the activity series is an arrangement of metals in the order of decreasing reactivity. In the reactivity series,  the most reactive metal is placed at the top whereas the least reactive metal is placed at the bottom. From this series it becomes possible to predict the general behaviour and relative chemical activity of the common metals.

Reactivity Series of Metals

Metals occupying higher position in the series have more tendency to lose electrons and are more reactive. The metals at the bottom of the series are least reactive.

Elements occur in nature in two states: native state and combined state.

1. 1.     NATIVE STATE

The metals are said to be in native state if they are found in their elementary form. Generally, less active metals are found in native state. The common examples of metals which occur in native state are gold, silver, copper, platinum, etc.

1. 2.     COMBINED STATE

The metals are said to occur in the combined state if they are found in nature in the form of their compounds. Generally, the reactive metals occur in the form of their compounds

In the combined state the metals are found in the crust of the earth as oxides, carbonates, sulphides, silicates, phosphates, etc. The naturally occurring chemical substances in the earth’s crust which can be obtained by mining are known as minerals. Minerals extracted from the crust of the earth are not pure but instead they are associated with large number of earthly, rocky and silicious impurities. The impurities associated with the minerals are collectively known as gangue or matrix

Every mineral of the metal cannot be used for its extraction. In some cases, economic factors while in others availability of the mineral may be hinderance. The mineral from which the metal can be economically and conveniently extracted is called ore. For example, earth’s crust contains aluminium in the form of two well-known minerals, bauxite (AI₂O₃.2H₂O) and china clay (AI₂0₃.2SiO₂.2H₂O), but the extraction of aluminium is cheaper and easy from bauxite. Hence, ore of aluminium is bauxite. Similarly, minerals of copper are copper glance (Cu₂S), cuprite (CU₂O), malachite
(CuCO₃. Cu(OH)2), copper pyrites (CuFeS₂), etc., but the ore of copper is copper pyrites. Thus, it can be concluded that all the ores are minerals but all the minerals are not ores.

Examples of some important ores are given in Table 50.3.

Table 50.3. Metals and Their Ores

Metal	Ore		Composition
Potassium	Sylvine		KCI
Sodium	Rock-salt		NaCI
Calcium	Limestone		CaCO3
	Gypsum		CaSO4, 2H20
Magnesium	Magnesite		MgCO3
	Dolomite		MgCO3, CaCO3
Aluminium	Bauxite		AI2O3, 2H2O
	Diaspore		AL2O3‘ H2O
	Kaolinite		AL2(OH)4 . Si2O5
	(a form of clay)
Titanium	Rutile		TiO2
	Ilmenite		FeTiO3
Zinc	Zinc blende blende		ZnS
	or Sphalerite
	Calamine		ZnCO3
	Zincite		ZnO
Iron	Haematite		Fe2O3, xH2O
	Iron pyrite		FeS2
	(Fool’s gold)
	Magnetite		Fe3O4
Tin	Cassiterite or	SnO2
	Tin stone
Chromium	Chromite		FeCr2O4
Manganese	Pyrolusite		MnO2
Lead	Galena		PbS.
Copper	Chalcopyrite	CuFeS2
	or Copper pyrites
	Malachite		CuCO3, Cu(OH)2
	Cuprite		Cu2O
	Copper glance	Cu2S
Mercury	Cinnabar		HgS
Silver	Argentite		Ag2S
Gold	Native metal		Au

 It may be noted that the metals at top tend to occur more commonly as chlorides, carbonates and sulphates. Metals in the middle commonly occur as oxides and those which are
lower in the series occur as sulphides. The metals at the bottom such as silver, platinum and gold exist in their native state.

A few metals in the decreasing order of their abundance in the earth’s crust are listed as follows:

Aluminium, Iron, Calcium, Sodium, Potassium, Magnesium, Titanium

DECREASING ABUNDANCE IN EARTH’S CRUST

SOLVED EXAMPLES

 Example 50.1 What type of metals are likely to exist in native state in nature? Give some examples of such metals.

Solution. The less reactive metals are likely to exist in nature state. These metals lie below hydrogen in the activity series. Some examples are gold, silver, platinum, copper, etc.

Example 50.2 Differentiate between “minerals” and “ores”.

Solution. Minerals are the naturally occuring chemical substances in the earth’s crust which can be extracted by mining. The minerals from which a metal can be extracted economically and conveniently
are. called ores. For example, minerals of copper are copper glance (Cu₂S), cuprite (Cu₂0) and copper pyrites (CuFeS₂) but its ore is copper pyrites. All ores are minerals but all minerals are not ores.

Example 50.3 Predict the modes of occurrence of the following three types of metals:

(i) Highly reactive (such as sodium).

(ii) Moderately reactive (such as iron).

 (iii) Noble metal (such as gold, platinum).

Solution.(i) Highly reactive metals occur in combined stare generally as their chlorides, sulphates or carbonates.

(ii) Moderately reactive metals generally occur as their oxides.

(iii) Noble metals exist in free state or native state.

Utilization of Ghana’s mineral resources is managed _ Mineral Commission of Ghana. Important mineral deposits .in Ghana and their locations in Ghana are discussed as follows:

1. METALS

The major metallic minerals of Ghana are gold, bauxite, iron and manganese. At present, gold, bauxite and manganese are being mined in Ghana.

Gold: There is great potential of gold exploration in Ghana. It is the main focus of majority of foreign and local exploration companies in the country. The main gold deposits in Ghana are at Obuasi, Tarkwa, Pres tea, Damang, Bogoso, Aboso, Bibiani Kenyase, and Ntronang-Abrem.

Bauxite: Ghana has large deposits of good quality bauxite in the Western, Eastern and Ashanti regions. The four major bauxite deposits in Ghana are; Sefwi-Bekwai (Awaso), Kibi, Ejuanema and Aya-Nyinahin deposits.

Iron: There are three main iron ore deposits in Ghana. These are: Shieni, Oppong Mansi and Pudo.

Manganese: There are two types of manganese ores which occur in Ghana. These are oxides and carbonate. The major deposits are at Nsuta in the Western region.

Manganese occurs in seven of the ten regions of Ghana, in the Central, Eastern, Western, Ashanti, Northern, Upper- west and Upper-east.

2. PRECIOUS STONES

Diamonds: Ghana’s diamond mining industry has produced industrial grade gems since the 1920′s. These are mainly mined from the Birim and the Bonza diamond fields. Diamonds found in Ghana are found in alluvial gravel. More than 11 million carats of proven and probable reserves are located about seventy miles North-west of Accra. Currently one million carats of diamonds is produced in a year. Ghana is 9th largest producer of diamonds in the world.

3. INDUSTRIAL MINERALS

Limestone (CaCO₃). There are four limestone deposits in Ghana. These are Buipe, Nauli Oterkpolu and Bongo-Da deposits.

Dolomite (CaCO₃ MgCO₃).The main dolomite deposit is located at Buipe-Baka area in the Northern region.

Solar Salt. Ghana possesses one of the largest proven renewable solar salt production potential along the entire coastline stretching over a distance of over 500 km. Effective exploitation would enable the country to meet the needs of the entire sub-region.

At present Ghana’s total annual production of salt is about 250,000 tonnes.

A massive development of the salt industry within the catchment area of the Songhor Lagoon, in the Ada Traditional Area, is to take off soon.

As already mentioned only a few most unreactive metals occur in nature in free state. Most of the metals occur in nature in oxidized form as their compounds. Therefore, metals are
generally extracted by subjecting their ores to reduction by chemical methods or by electrolytic methods.

The process of extracting pure metal from its ore is known as metallurgy.       .

Since, the nature of the ore and also the properties of different metals are different, therefore, it is not possible to have the universal scheme which may be applicable to all the metals. However, some common steps involved in the metallurgical operations are:

I     Crushing and grinding of the ore.

II.   Concentration or benefication of the ore.

III. Preliminary treatment of the concentrated ore.

IV.  Reduction.

V.   Purification or refining of crude metal.

I. CRUSHING AND GRINDING OF THE ORE

Most of the ores are obtained from the crust of the earth in the form of huge lumps. These lumps have to be converted. into powdered form so that the chemical changes which have
to take place at the later stages may become convenient. The huge lumps are broken into small pieces in the jaw crushers They are further pulverised in stamp mill or ball mill.

II. CONCENTRATION OR BENEFICATION OF THE ORE

The ore obtained from the earth’s crust is associated with rocky and silicious impurities. It is quite essential to get rid of these impurities so that they may not cause any interference.
in the process of extraction. The removal of unwanted materials such as sand, clays, etc., from the pulverised ore called concentration, dressing or benefication of the ore
The benefication of the ore is carried out by any of the following methods depending upon the nature of the ore an also the impurities present in the ore:

1. Hydraulic Washing, Levigation or Gravit Separation Method. This method is usually applicable to oxide ores in which the ore particles are heavier than the
impurities. The powdered ore is washed with running stream of water. The lighter impurities are washed away leaving behind the heavier ore particles.

Native metals such as gold can be separated from sand and gravel by shaking and washing of earth with water in pan or sieve. The process is known as panning.

2. Froth Floatation Process. This process is generally used for the concentration of sulphide ores. The finely powdered ore and water are taken in a tank. Additional reagents such as pine oils, fatty acids, etc., are added to the mixture. These reagents increase the nonwettability of the mineral particles and are known as collectors. The contents are kept agitated by the blast of air. As a result of agitation
the froth is produced.

The.ore particles are preferentially wetted by the oil and are carried to the surface by the foam. The gangue material, which is preferentially wetted by water sinks to the bottom of the tank. The foam at the surface of the tank is transferred to the other tank where it is washed with water to recover the
ore particles.

3. Magnetic Separation of Impurities: This method is usually employed when either the ore or the gangue is capable of being attracted by the magnetic field. For example, tungstates of iron and manganese from tin stone are separated by this method. The powdered ore is dropped over the belt revolving around the rollers, one of which is magnetic. The magnetic roller attracts the magnetic part of
the ore and they are collected in the form of a heap near it. The non-magnetic part of the ore flies off and forms a heap away from the impurities.

4. Leaching: It is a chemical method for the concentration of the ore. In this process the powdered ore is dissolved selectively in acids, bases or other suitable reagents. The impurities remain undissolvedas sludge. The solution of ore is filtered and the ore is. recovered by precipitation or
crystallisation.

III. PRELIMINARY TREATMENT OF THE CONCENTRATED ORE

The process of extraction of metal from the concentrated ore depends upon the nature of the ore as well as the nature of impurities present in the ore. Before the concentrated ore is subjected to final metallurgical operations in order to get the metal in the free state, the preliminary chemical treatment may be necessary. The objective of this preliminary chemical treatment is:

(a) to get rid of impurities which would cause difficulties in the later stages; and

(b) to convert the ore into oxide of the metal because it is easier to reduce an oxide than the carbonate or sulphide.

The processes employed for preliminary treatment are calcination and roasting.

1. Calcination

It is a process of heating the ore in a limited supply of air below its melting point. The process involves:
–  the removal of volatile impurities,

–  the removal of moisture,

–  the decomposition of any carbonate ore into oxide.

Fe₂O_3.xH₂O        →       Fe₂O_{3 +} xH₂O (g)

ZnC0_{3             →}        ZnO + CO₂

     CuC0₃ .Cu(OH)₂        →       2CuO + H₂O + CO₂

1. 2.     Roasting

It is the process of heating the ore in the excess supply of air below its melting point. This process is employed when oxidation of the ore is required. As a result of roasting,
–  moisture is driven away,

–  volatile impurities are removed,

–  the impurities like sulphur, phosphorus, arsenic are removed as their oxides,

–  the ore undergoes oxidation to form metal oxide or sulphate.

2PbS + 3O₂        →                   2PbO + 2SO₂

              2ZnS + 30₂        →                   2ZnO + S0₂

It is advantageous to roast a sulphide ore to the oxide before reduction because metal oxides can be reduced to metal by carbon and hydrogen much more easily than sulphides.

IV. REDUCTION

After the preliminary treatment, the ore may be subjected to reduction process by one of the following methods depending upon its nature:

1. Smelting or Reduction with Carbon. In this process, the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which act as reducing agent) and is heated
to a high temperature above its melting point. During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional
reagent is called flux. Flux combines with the impurities to form a fusible product called slag.

Flux + Impurities → Slag

The selection of flux depends upon the nature of impurities. If purities are acidic in nature, the flux is basic, lime (CaO). On the otherhand, for basic impurities, are acidic flux such as silica (SiO₂) is used.

CaO    +    SiO₂ → CaSiO₃

                                                             (Basic           (Acidic      (Slag)

impurity)                  flux)

2. Reduction with Aluminium. Certain metal oxides such as CR₂O₃ and Mn₃O₄ are not easily reduced with carbon. In such cases is used as reducing agent because it is more electropositive than chromium or manganese. The process of reduction of oxides with aluminium is called aluminothermy. Some examples are:

Cr₂ + 2A1     →        Al₂O₃ + 2Cr

3Mn₃O₄ + 8Al          →        4Al₂O₃ + 9Mn

The chemical methods are suitable for reduction of compounds of metals which are in the middle of the activity series.

3. Auto-reduction. Certain metals are obtained from their ores roasting without using any reducing agent. For example, mercury is directly obtained by roasting its ore cinnabar (HgS) in air.

HgS + O₂                       →        Hg(l) + SO₂(g)

or           2Hgs(s)  + 3O₂(g)          →        2HgO(s) + 2SO₂g)

                                              2HgO(s) + HgS(s)         →        3Hg(l) + SO₂(g)

4. Electrolytic Reduction. The highly electropositive elements such as alkali metals, alkaline earth metals and aluminium cannot be extracted by carbon reduction methods. They are extracted by the electrolysis of their fused salts. The process of extraction of metals by the use of electrolysis phenomenon is called electrometallurgy.

For example, sodium metal is extracted by electrolysis of molten sodium chloride containing other salts as impurities.

NaCI(l)                        →                    Na⁺ + Cl^–

At cathode                    Na⁺ + e^–                        →                    Na

At anode                      2Cl^‑                    →                    Cl₂ + 2e^–

5. Displacement Method (Hydrometallurgy). Some metals like gold and silver are extracted from their concentrated ores by leaching. They are dissolved in suitable reagents like acids or bases leaving behind insoluble impurities. The metal is recovered from the solution by displacement with some more electropositive metal such as zinc. For example, silver ore is leached with dilute solution of sodium cyanide. Silver dissolves forming a complex, sodium dicyanoargentate (I). The solution is further treated with scrap zinc which displaces silver from complex.

Ag₂S + 4NaCN                       2 Na[Ag(CN)₂]             + Na₂S

Sod. dicyanoargentate (I)

2Na[Ag(CN)₂] + Zn         →        Na₂ [Zn(CN)₄] + 2Ag ↓

Sod. tetracyanozincate (II)

On the basis of reactivity we can group the metals into the following three categories:

(i) Metals of low reactivity.

(ii) Metals of medium reactivity.
(iii) Metals of high reactivity.

Different reduction processes are to be used for obtaining the metals falling in each category.

The relation between the reduction process employed and the position of the metal in the activity series is depicted .

 Position of the Metal in the Activity Series
and the Related Reduction Process

V. REFINING OF METALS

The process of purification of impure metals is known as refining.

Depending upon the nature of the metal and the nature of the impurities present different methods are applied for the refining of metals. Some of the commonly used methods are discussed below:

1. Distillation. Volatile metals like zinc and mercury are purified by this method. The pure metal distils over and is condensed in a receiver. The non-volatile impurities are left behind in the retort.

2. Liquation. The method is used for easily fusible metals like bismuth, tin and lead. The crude metal is placed on the sloping hearth of a furnace and heated gently when the metal melts and flows down, leaving behind the infusible impuritieswhich remain sticking to the floor of the hearth.

3. Electrolytic Refining- or Electro-refining. This method is based on the phenomenon of electrolysis. Many metals like copper, silver, tin, gold, zinc, nickel and chromium are purified by electro-refining.

The impure metal is made anode whereas the thin sheet of pure metal is made cathode. Electrolyte is the solution of some salt of the metal. On passing electricity, the metal from the anode goes into solution as ions due to oxidation while pure metal gets deposited at the cathode due to the reduction
of metal ions. -The-insoluble impurities settle down below the anode as anode mud whereas the soluble impurities go into solution. The reactions taking place at the two electrodes may
be represented as:

                                                At cathode:     Mⁿ⁺ + ne^‑          →        M

At anode:                     M         →        Mⁿ⁺ + ne^–

The various steps involved in the extraction of pure metals from their respective ores are summarized in Fig. 50.2.

Mining of gold in Ghana began long before the arrival of the Europeans, mainly by panning by hand. The metal was extracted from alluvial sand and gravel deposited on the banks and beds of rivers.

The use of machines in the mining of the metal started in about 1880 in Tarkwa area and extended to Obuasi in 1898. Ghana’s pre-independence name of ‘Gold Coast’ shows the importance of the metal in the economy and history of the country.

OCCURRENCE

Since gold is very unreactive it occurs in free state in nature. The primary sources of gold are:

(i) Alluvial gravel. It is found in the beds of certain rivers. It is referred to as alluvial gold.

(ii) Auriferous quartz. Gold also occurs in the veins of quartz. This is called relf gold.

EXTRACTION

The various steps involved in the extraction of gold are:

1. The concentrated ore is roasted to remove all the oxidisable impurities.

2. Treatment with KCN

The roasted ore is then treated with a solution of sodium cyanide or potassium cyanide for some days. In the presence of atmospheric oxygen, gold dissolves in the form of a complex.

4Au + 8NaCN + 2H₂0 + O₂      →        4Na[Au(CN)₂] + 4NaOH

Sodium dicyanoaurate (1)

3.Precipitation of Gold. From the solution the metal is precipitated by adding zinc shavings.

2Na[Au(CN)₂]+Zn                   →        Na₂[Zn(CN)₄] +2Au ↓

Sodium tetracyanozincate (IT)

The process is called Mac Arthur Forest Cyanide Process.

4. Purification. Gold obtained in the above process is impure and contains metals such as silver and copper as impurities, Removal of silver and copper impurities from impure gold is known as parting. It is achieved by boiling impure gold with cone. H₂SO₄ Silver and copper dissolve in hot cone. H₂SO₄ while the gold remains as such. Parting can also be done with cone. HNO₃. Purification of gold can also

be done by electrolysis.

The anode is impure gold. The cathode is a strip of pure gold coated with a thin layer of graphite. The electrolyte is an aqueous solution of gold (III) chloride or gold (III) trixonitrate(V) and trioxonitrate(V) acid. The gold ions are the cathode on electrolysis:

Au+³ + 3e-       →        Au

Crude mining and extraction of gold and diamond is on a small scale in Ghana. It is popularly called ‘Galamsey’ Gold bearing ores or tailings, i.e., waste from previous ore treatment, is first concentrated by washing several times with water in a pan or other suitable container

to remove mud and other fifth.

The gold bearing product is then poured on a jute bag or any wide mesh cloth laid on an inclined table. It is then repeatedly washed with water to remove the lighter gangue. The heavier gold material, trapped in the. sack or cloth, is washed off into a pan with water and then decanted. The solid gold product is rubbed well with mercury to form an amalgam leaving the remaining gangue.

In the final stage, the gold amalgam is put in a white cloth and squeezed hard to let out as much mercury as possible. gold product thus obtained is white due to mercury contamination. This is washed with water in a pan. The heavier mercury falls to the bottom and the gold remains on top. The gold is harvested (collected) ready to be sold. The gold may also be recovered by distillation.

The gold produced by both Ashanti Goldfields Company Limited and ‘Galamsey’ operators is not very pure. Pure gold is obtained by refining their products outside Ghana.

USES OF GOLD

1. For preparing ornaments.
2. For electrogilding (i.e., gold plating by electrolysis).
3. For filling teeth, and in switch contacts.
4. For making compounds of gold.
5. For preparing alloys.

Do You Know?

Pure gold is soft and is generally hardened by adding silver or copper to it for the purpose of
making ornaments. The weight of gold in gold ornaments is expressed in terms of ‘carats’. Pure
gold is taken as 24 carats. 18 carat gold means that it contains 18 parts by weight of gold in 24
parts by weight of the given alloy. The percentage of gold in this sample may be calculated as:

Percentage of gold in 18 carat gold

Most jewellery is made out of 22 carat gold.

SODIUM

Sodium is an alkali metal. It is a very reactive metal, hence it does not occur freely. It is found as NaCl, NaBr and NaI in the sea.

It is found also in deposit as NaCl (also known as rock salt); NaNO₃ (also known as chile salt petre); and Na₂CO₃. It is found in borax and complex silicates found in clay soil.

Sodium dissolves readily in cold water to give an alkaline solution.

Sodium Hydroxide, NaOH

 Sodium hydroxide is an important compound of sodium with a wide range of usage.

Production of NaOH by Electrolysis of Brine:

This process is used for the industrial production of NaOH

Action of NaOH On Aluminium:

Aluminium readily dissolves in sodium hydroxide solution to form sodium aluminate and hydrogen.

2Al_(s) + 2NaOH_(aq) + 6H₂O_(l) → 2NaAl(OH)_4(aq) + 3H_2(g)

On Zinc: zinc also dissolves readily in sodium hydroxide solution to form sodium zincate and hydrogen.

Zn_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Zn(OH)_4(aq) + H_2(g)

On Lead: lead as well dissolves in sodium hydroxide solution, forming sodium leadate and hydrogen.

Pb_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Pb(OH)_4(aq) + H_2(g)

On Tin: tin dissolves in conc. sodium hydroxide solution to form trioxostannate(IV) salts and hydrogen.

Sn_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂SnO_3(aq) + 2H_2(g)

Uses of NaOH

1. To precipitate insoluble metallic hydroxides – certain metallic hydroxides which are insoluble in water are precipitated from their salt solutions by NaOH .

Example, ZnSO_4(aq) + 2NaOH_(aq) → Zn(OH)_2(s) + Na₂SO_4(aq)

Pb(NO₃)_2(aq) + 2NaOH_(aq) → Pb(OH)_2(s) + 2NaNO_3(aq)

AlCl_3(aq) + 3NaOH_(aq) → Al(OH)_3(s) + 3NaCl_(aq)

FeSO_4(aq) + 2NaOH_(aq) → Fe(OH)_2(s) + Na₂SO_4(aq)

FeCl_3(aq) + 3NaOH_(aq) → Fe(OH)_3(s) + 3NaCl_(aq)

Note: the hydroxides of zinc, aluminium and lead are amphoteric. They will dissolve in excess of sodium hydroxide solution to form complex salts. Example,

Zn(OH)_2(s) + 2NaOH_(aq) → Na₂Zn(OH)_4(aq)

2. For absorbing carbon(IV) oxide in the laboratory.

3. For the manufacture of soap, paper, rayon (artificial silk), and for the manufacture of different other chemical compounds, such as sodium chlorate(V), sodium mathanoate and phosphine.

4. In the purification of bauxite.

5. In the refining of petroleum.

6. In the bleaching of cotton and textiles.

Sodium Trioxocarbonate(IV), Na₂CO₃  

Sodium trioxocarbonate(IV), Na₂CO₃ can exist in different forms: in anhydrous state, i.e., dry or powdery (this is called soda ash) and in crystalline state.

There are two forms of crystal: a. Sodium trioxocarbonate(IV) decahydrate (washing soda), Na₂CO₃ . 10H₂O. This may lose 9 molecules of water when exposed to the atmosphere to give the second form.

b. Sodium trioxocarbonate(IV) monohydrate, Na₂CO₃.H₂O.

Industrial Preparation of Na₂CO₃

Sodium trioxocarbonate(IV) is manufactured industrially by a process called “the Solvay process”.

Ammonical brine, which is made by dissolving ammonia gas in brine (25% NaCl solution) is made to run down a solvay tower, up which carbon(IV) oxide is forced.

The towers are fitted at intervals with perforated mushroom-shaped baffle-plates. These plates slow down the flow of liquid and gas, as well as present a larger surface area for the reaction.

The chemical processes for the production of Na₂CO₃ is thus:

(1). The CO₂ reacts with the NH₃ in the ammonical brine to form ammonium hydrogen trioxocarbonate(IV).

NH3(g) + CO2(g) + H2O(l)

NH4HCO3(aq)

(2). The NH₄HCO₃ reacts with NaCl to form sodium hydrogen trioxocarbonate(IV).

NaCl_(aq) + NH₄HCO_3(aq) → NaHCO_3(s) + NH₄Cl(aq)

The formed NaHCO₃ is insoluble and is filtered off. It is washed and heated to obtain the anhydrous Na₂CO₃, together with water and CO₂ (this is passed back into the tower for further use).

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(g) + CO_2(g)

If the crystals are required, the soda ash is dissolved in hot water.

Na₂CO_3(s) + 10H₂O_(l) → Na₂CO₃.10H₂O_(aq)

Efficiency of the Process (1). The major raw materials, NaCl and CaCO₃ are cheap and easily obtainable. The required CO₂ is obtained by heating limestone (i.e. CaCO₃), also about half of it is recovered from heating NaHCO₃.

Most of the ammonia used are recovered from NH₄Cl (by heating and washing with Ca(OH)₂).

It is then returned to the ammoniating tower.

2NH₄Cl_(aq) + Ca(OH)_2(aq) → CaCl_2(aq) + 2H₂O_(l) + 2NH_3(g)

(2). The process is a continuous flow – it uses minimum labour.

Properties of Na₂CO₃

(1). Both the anhydrous (soda ash) and crystalline (washing soda) form alkaline solution in water.

(2). Na₂CO₃ does not decompose by heat. (3). Both the anhydrous and crystalline forms react with acids to liberate carbon(IV) oxide.

Uses of Sodium Trioxocarbonate(IV), Na₂CO₃

(1). In the manufacture of glass – ordinary glass is made by fusing together Na₂CO₃, CaCO₃, SiO₂ and a little carbon (as a reducing agent).

To facilitate fusion, broken glass (cullet) is added. Na₂CO_3(s) + SiO_2(s) → Na₂SiO_3(s) + CO_2(g)

Na₂SiO_3(s) – Sodium trioxosilicate(IV) is the glass.

(2). In domestic water softening process, Na₂CO₃ precipitates the Ca²⁺ ions which cause hardness in water.

(3). In the manufacture of water glass.

(4). In the manufacture of NaOH, sodium silicate(VI) and borax.

(5). In the manufacture of soap and paper.

(6). Used in standardizing acids for titrimetry analysis in the laboratory.

Sodium Hydrogen Trioxocarbonate(IV)

Sodium hydrogen trioxocarbonate(IV) is also known as baking soda, and has the molecular formula NaHCO₃.

Industrial Preparation of NaHCO₃

The salt is manufactured industrially by the solvay process by saturating a wet mush of Na₂CO₃ obtained from the solvay process with CO₂.

Na₂CO_3(aq) + H₂O_(l) + CO_2(g) → 2NaHCO_3(s)

Properties of NaHCO₃

Sodium hydrogen trioxocarbonate(IV) shows the following properties:

(1). It is a white crystalline solid.

(2). It is slightly soluble in water.

(3). It hydrolysis in water to give alkaline solution (even though it is an acid salt), hence its solution turns red litmus paper blu

(4). On heating, it decomposes to release CO₂

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(l) + CO_2(g)

(5). With acids, NaHCO₃ produces water, CO₂ and a salt.

NaHCO_3(aq) + HCl_(aq) → NaCl_(aq) + H₂O(l) + CO₂

Uses of NaHCO₃

It is used in the manufacture of baking powder. As we have stated, it produces CO₂ when heated. The CO₂ causes the cake to rise. This is the reason it is called ‘baking soda’.

To Distinguish Between Na₂CO₃ and NaHCO₃

The following test can be used to identify either of Na₂CO₃ or NaHCO₃ from the other:

(1). Application of heat

When both substances are heat only NaHCO₃ decomposes. It produces Na₂CO₃, H₂O and CO₂.

(2). Addition of magnesium sulphate solution

 In the presence of magnesium sulphate solution only Na₂CO₃ reacts to form a white precipitate.

MgSO_4(aq) + Na₂CO_3(aq) → MgCO_3(s) + Na₂SO_4(aq) 

MgCO_3(s) is the white precipitate

(3). Addition of a solution of a metallic salt (other than that of magnesium or aluminium)

Only NaHCO₃ solution reacts to liberate CO₂ Example,

ZnSO4(aq) + 2NaHCO₃ → ZnCO_3(s) + Na₂SO_4(aq) + H₂O_(l) + CO_2(g)

Sodium Chloride, NaCl

Sodium chloride is found to the extent of about 3% by mass in sea water. It can be obtained by the vaporization of sea water by the sun – this is suitable in the part of the world where the sun’s heat energy is intense.

Uses of NaCl

(1). It is converted into a number of useful compounds, such as NaOH, Na₂CO₃, NaHCO₃, Na₂SO₄, Cl₂, HCl and NaOCl.

(2). It is used to ‘salt-out’ soap in the manufacture of soap.

(3). As a food preservative.

(4). In regenerating water softeners.

(5). In glazing earth ware

EVALUATION

1. The impurities associated with’ the ore after mining are collectively called

  (a) flux                            (b) slag

(c) minerals                  (d) gangue.

2. An ore after levigation is found to have acidic impurities. Which of the following can be used as flux during smelting operation?

  (a) H₂SO₄                     (b) CaCO₃

(c) SiO₂                        (d) Both CaCO₃ and SiO₂.

3. Which of the following metals can be extracted by smelting?

(a) Aluminium             (b) Magnesium

(c) Iron                        (d) None of these.

4. In the froth-floatation process for benefication of the ores, the ore particles float because

(a)       they are light

(b)      their surface is not easily wetted by water

(c)       they bear electrostatic charge

(d)      they are insoluble.

5.     (a) Name the three most abundant metals in the decreasing order of abundance.

6.  Name any two metals which exist in native state.
7. Define the following:

(i) mineral                (ii) ore

(iii) gangue                (iv) metallurgy.

Lessons on Chemistry SS3 –Edudelight.com

WEEK 8

 Calcium is an alkaline earth metal. It does not occur freely in nature due to its high reactivity. It is found in the following minerals as CaCO₃ in limestone, marble, aragonite, calcite, chalk, coral and sea shells.

It is found as CaSO₄ in gypsum and anhydrite. It is also found as a double carbonate, such as CaCO₃.MgCO₃ in dolomite; as calcium floride in fluorspar; as several silicate(IV) in the soil; and as phosphate(V) in bones and teeth.

Preparation

Calcium is prepared by the Electrolysis of fused anhydrous calcium chloride and calcium fluoride at 700 degC.

Occurrence

• Calcium is widely found as its carbonate being a constituent of rocks.
• Chalk, limestone and marble are different crystalline forms of calcium carbonate which occur in mountain ranges. The mixed carbonate, Dolomite, also occurs in mountain rocks.
• Other important minerals include
  • Gypsum, MgSO4.2H2O,
  • Fluorospar, CaF2,
  • Apatite, 3Ca3(PO4)2CaF2,
  • Diopside, CaMg(SiO3)2, and
  • Lime Felspar, CaAl3Si2O8.
• Calcium also occurs as soluble salts in natural waters and is responsible for its hardness.

Calcium Oxide

Calcium oxide, CaO, is also known as quicklime and can be prepared from sea shells.

Sea shells are composed of calcium trioxocarbonate(IV), CaCO₃.The action of strong heat upon CaCO₃ produces calcium oxide, together with carbon(IV) oxide.

CaCO_3(s) → CaO_(s) + CO_2(g)

Properties of Calcium oxide

Here are properties shown by calcium oxide.

1. It is a white solid.

2. It has a high melting point (about 2600^oC).

3. It is hygroscopic and is used to dry ammonia gas.

4. When water is added drop wise onto CaO, it cracks with a hissing sound and breaks up into a powdery form, with the liberation of enormous heat.

The product formed is Ca(OH)₂ and is known as slaked lime and the reaction process is called slaking.

CaO(s) + H₂O_(l) → Ca(OH)_2(s)

5. It is a strong base. It reacts with acids to form salts and water only. It displaces ammonia from ammonium salts.

CaO_(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l)

CaO_(s) + 2NH₄Cl_(aq) → CaCl_2(aq) +H₂O_(l) + 2NH_3(g)

Uses of Calcium oxide, CaO

CaO can be used in the following ways:

(1). In the manufacture of slaked lime, Ca(OH)₂

(2). In the building industry for the preparation of mortar and for the manufacture of cement.

(3). In the manufacture of calcium carbide.

(4). In smelting processes.

(5). In the manufacture of refractory furnace linings.

(6). In the manufacture of glass.

(7). For drying ammonia in the laboratory.

The Chemical Composition of Cement

Cement is composed of the following materials:

(1). Lime or CaO – from limestone, chalk, shells, calcareous rock.

(2). Silica, SiO₂ – from sand, old bottles, clay or argillaceous rock.

(3). Alumina, Al₂O₃ – from bauxite, recycled aluminium, clay. 

(4). Iron, Fe₂O₃ – from clay, iron ore, scrap iron and fly ash.

(5). Gypsum, CaSO₄ . 2H₂O. Gypsum is found together with limestone.

In the manufacture of cement, the above materials are crushed, milled and proportioned.

The materials, without the gypsum are proportioned to produce a mixture with the desire chemical composition and then ground and blended by one or two processes – dry or wet process.

The materials are then fed through a kiln at 2,600^oF to produce grayish-black pellets known as Clinker.

The cement clinker formed has the following typical composition:

Tricalcium aluminate – Ca₃Al₂O₆

Sodium oxide, Na₂O

Tetracalcium alumino ferrite, Ca₄Al₂Fe₂O₁₀

Potassium oxide – K₂O

Belite or dicalcium silicate, Ca₂SiO₅

Alite or tricacium silicate, Ca₃SiO₄

These compounds contribute to the properties of cement in different ways. By mixing them appropriately, manufacturers can produce different types of cement to suit several construction environments.

An example is the common Portland cement. The alumina and iron act as fluxing agents, which lower the melting point of silica from 3000 to 2600^oF.

The Clinker is cooled, pulverized and gypsum is added (gypsum is added last) to regulate setting time. It is then thoroughly ground to produce cement.

Cement gets its strength from chemical reactions between the cement and water. These reactions, that is, hydration and hydrolysis continue for many years.

Setting Mortars

Mortar is the mixture of lime (CaO or Ca(OH)₂ and sand and water to set bricks, stones, ceramics, tiles to walls, e.t.c., in building.

Mortars have plastic and hardening properties. The first step in the “setting” of mortars is the loss of water by evaporation and by absorption into the bricks.

The ultimate hardening is due to the action of the atmospheric carbon(IV) oxide, producing insoluble calcium carbonate.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Note: lime (CaO or Ca(OH)₂) is a natural plasticiser and 100% cementitious. That is, it sets things firmly.

Calcium Hydroxide (slaked lime), Ca(OH)₂

As has been stated, Ca(OH)₂ is made by adding water to quicklime (CaO).

CaO_(s) + H₂O_(l) → Ca(OH)_2(s)

When excess water is added, a suspension, known as the milk of lime or whitewash is obtained. On filtration, the filtrate (saturated solution) obtained is known as limewater.

Note: slaked lime, milk of lime and lime water are all composed of calcium hydroxide.

Properties of Calcium hydroxide

Calcium hydroxide shows the following properties:

1. Calcium hydroxide dissolves in water to give an alkaline solution.

2. It absorbs CO₂.

Uses of Calcium hydroxide

Calcium hydroxide can be used as follows:

(1). Used in treating acidic soils.

(2). Used in softening water.

(3). In the manufacture of bleaching powder.

(4). In the manufacture of mortar.

Calcium Trioxocarbonate(IV), CaCO₃

Calcium trioxocarbonate(IV) occurs abundantly in nature as limestone, chalk and marble. It is also found in coastal caves as stalactites (deposits of CaCO₃ which grow downwards from the top of the cave to its floor) and stalagmites (deposits of CaCO₃ which grow upwards from the floor of the cave to its top).

It is also found in the bones of animals, and in the external skeleton of marine organisms; and in natural ores, such as calcite, dolomite and Iceland spar.

CaCO₃ can be produced in the laboratory as a precipitate when Na₂CO₃ solution is added to a solution of CaCl₂.

Na₂CO_3(aq) + CaCl_2(aq) → CaCO_3(s) + 2NaCl_(aq)

Properties of Calcium Trioxocarbonate(IV)

Below are properties shown by calcium trioxocarbonate(IV)

1. It is a white solid which is insoluble in pure water.

2. It dissolves in water that contains dissolved CO₂ , to form calcium hydrogen trioxocarbonate(IV)

CaCO3(s) + H2O(l) + CO2(g)

Ca(HCO3)2(aq)

Recall that the presence of calcium hydrogen trioxocarbonate(IV), Ca(HCO₃)₂, is the cause of temporary hardness in water.

3. CaCO₃ decomposes into CaO and CO₂ when strongly heated.

4. CaCO₃ is attacked by dilute acids to liberate CO₂

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

Uses of Trioxocarbonate(IV)

Trioxocarbonate(IV) is useful in the following ways:

1. In the manufacture of sodium carbonate, quicklime, cement, glass and steel.

2. For neutralizing acids in acidic soils.

3. In the extraction of iron.

4. As building materials.

5. In the manufacture of pigments, putty and paper.

Test for Ca²⁺ Ion

(1). Flame test

Calcium compounds, when heated in a non-luminous flame, produce a brick-red colour. The compound under investigation is prepared for the flame test by moistening it with some concentrated hydrochloric acid.

A reaction occurs, leading to the formation of the chloride of the metal. It is necessary to convert the sample compound (i.e. the compound under investigation) to a chloride because chlorides are more volatile, and would therefore give a satisfactory result on heating.

To confirm the presence of calcium ions, the brick red colour produced will appear green when viewed through a blue glass.

(2). With sodium hydroxide

A solution of calcium salt gives a white precipitate with NaOH solution. The precipitate is insoluble in excess sodium hydroxide solution. The white precipitate is calcium hydroxide.

2NaOH(aq) + Ca²⁺_(aq) → Ca(OH)_2(s) + 2Na+(aq)

Note: NaOH is a stronger alkaline than Ca(OH)₂, hence it displaces it from solutions of its salts.

(3). With ammonium ethanedioate (ammonium oxalate)

A calcium salt solution produces a white precipitate when reacted with ammonium oxalate solution. The precipitate is calcium ethanedioate, which is soluble in dilute HCl, but insoluble in ethanoic acid (this indicates the presence of Ca²⁺ ion).

(NH₄)₂C₂O_4(aq) + Ca²⁺_(aq) → CaC₂O_4(s) + 2NH₄⁺_(aq)

Note: this test is also used to identify barium and strontium.

(4) With ammonium trioxocarbonate(IV)

A white precipitate is also produced when a solution of calcium salt is reacted with that of ammonium trioxocarbonate(IV). This indicates the presence of the Ca²⁺ ion.

(NH₄)₂CO_3(aq) + Ca²⁺_(aq) → CaCO_3(s) + 2NH₄⁺_(aq)

Note: this test also indicates the presence of barium and strontium ions. Calcium salts may be distinguished from barium and strontium salts by the following test:

1. With potassium chromate(V) solution – no precipitate is produced from solutions of calcium salts, while barium salt solutions form precipitates.

2. With saturated CaSO₄ solution – no precipitate is produced from solutions of calcium salts, while barium and strontium salt solutions produce precipitate.  

 

Aluminiumis one of the elements of Group 13 of the periodic table. Although aluminium is a reactive metal according to the electrochemical series, it is rendered unreactive due to the formation of a oxide film on its surface.

OCCURRENCE

Aluminium is quite reactive element and hence does not occur in nature in the native form. Aluminium is the most abundant metal and the third most abundant element in the earth’s crust. The important ores of aluminium are:

(i) Bauxite, Al₂O_3.2H₂O

(ii) Cryolite, Na₃ AlF₆

(iii) Feldspar, KAlSiO₃O₈

(iv) Mica, KAISiO₁₀ (OH)₂

 EXTRACTION (From bauxite)

Aluminium metal is extracted from bauxite is a two stage process.

Stage 1. Involves extraction of alumina (AIP3) from bauxite.

Stage 2. Involves extraction of pure aluminium from (Al₂O₃) by its electrolysis in molten cryolite [Na₃AlF₆J]

1. Purification of Bauxite. Bauxite contains SiO₂, iron oxides and titanium (IV) oxide as impurities.Bauxite is digested with a hot concentrated (45%) solution of sodium hydroxide at about 473-523 K and 35-36 bar pressure.

Alumina dissolves to form sodium tetrahydroxoaluminate (III), Na[Al(OH)4^J leaving behind iron oxide and TiO₂.

AlO₃ + 2NaOH + 3H₂O   →        2Na[Al(OH)₄

Silica (SiO₂) also dissolves in sodium hydroxide to form soluble sodium trioxosilicate(IV), Na₂SiO₃.

SiO₂ + 2NaOH                 →        2Na₂SiO₃ + H₂O

The impurities are filtered out and CO₂ is bubbled. Through the filtrate containing sodium tetrahydroxoaluminate(IlI) and sodium trioxosilicate(IV). At this stage the solution may be seeded with freshly precipitated aluminium hydroxide. Aluminium hydroxide precipitates leaving behind

sodium trioxosilicate(IV) in solution. This is filtered and the precipitate of AI(OH)3 is heated at 1473 K to obtain pure alumina.

Na[AI(OH)₄ + CO₂       →        NaHCO₃ + Al(OH)3 ↓

1473 K

2Al(OH)3                   →        Al₂O₃ + 3H₂O

2. Electrolysis of Pure Alumina (Hall-Heroult Process).

Aluminium is obtained from alumina by the process of electrolysis. This method is known as HaU-Heroult process. Purified alumina is dissolved in molten cryolite (Na₃AlF₆) and is electrolysed in an iron tank lined inside with carbon (Fig. 50.3). Carbon lining serves as cathode while a number

of carbon rods dipping in the fused electrolyte serve as anode. Cryolite improves the electrical conductivity of the cell as alumina is a poor conductor. Moreover, cryolite lowers the
melting point of the mixture to about 1250 K. Other impurities such as CaF₂, NaF and AIF₃ may also be added. The temperature of the electrolyte is maintained between 1200-1300 K.

On passing electric current, aluminium is liberated at the cathode and gets collected at the bottom of the tank from where it is removed. Oxygen liberated at the anode combines with the carbon of the anode to produce carbon monoxide which either burns or escapes out. The reactions taking place

during electrolysis are:

At cathode:      A1³⁺ (melt) + 3e^–                 →        7 Al (l)

At anode:            C(s) + 0^2- (melt)                    →    CO(g) + 2e^–

C(s) + 20^2- (melt)                   →          CO₂(g) + 4e^–

Since during electrolysis, the carbon electrodes get consumed, they have to be replaced periodically. For each kg of aluminium about 0.5 kg carbon is burnt away.

The extraction of aluminium requires large amount of electrical energy. About 15,000 kWh are needed to produce 1 tonne of aluminium. For example, the smelter at Tema, Ghana, owned by the Volta Aluminium Company, uses about

2,760,000 MWh electricity to produce nearly 200,000 tonnes of aluminium annually. The company receives power from the Akosombo power station of the Volta River hydroelectric dam. Due to its proximity to the power station, the company receives sufficient electrical energy at relatively cheap rate.

USES OF ALUMINIUM

Aluminium is a soft metal with a density of 2.7 g cm^-3 Aluminium can be easily extruded through dies to form various shapes.

1. It is used for making angles used in windows.

2. It is a good conductor of electricity. Since it is not as good conductor as copper, thicker cables of
aluminium are used for transmission of electricity.

3. Aluminium forms many useful alloys e.g., magnalium (AI and Mg), duralumin (AI, Cu, Mg and Mn). Aluminium alloys are used in aircraft and other transportation vehicles.

4. In the form of finely-divided powder, aluminium is used in antirust paints.

5.Aluminium foil is used for wrapping cigarettes, confectionery, etc.

6.Aluminium is used to produce metals such as chromium and manganese from their ores (aluminothermic process)

Cr₂O₃ + 2Al                 →                    Al₂O₃ + 2Cr

3MnO₂ + 4Al               →                    2Al₂O₃ + 3Mn

• Aluminium utensils are extensively used for household purposes.

EVALUATION

1. Describe how pure aluminium oxide is obtained from its ore.

2. Explain why aluminium, though an electropositive metal, finds extensive use as a structural material.

3. Give important uses of aluminium.

 4.What is magnalium?

5. The process of reduction of oxides by aluminium is known         as…..

6. The process of removal of gaugue from ore is known       as…..

7. Aluminium is obtained from Al₂O₃ by…..reduction.

WEEK  9

TIN AND ITS COMPOUNDS

Tin is a chemical element with the symbol Sn (for Latin: stannum) and atomic number 50, is a post-transition metal in group 14 of the periodic table. It is obtained chiefly from the mineral cassiterite, which contains tin dioxide, SnO₂. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, and has two main oxidation states, +2 and the slightly more stable +4. Tin is the 49th most abundant element and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, thanks to its magic number of protons. It has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures it transforms into the less dense grey α-tin, which has the diamond cubic structure. Metallic tin is not easily oxidized in air.

The first alloy used on a large scale was bronze, made of tin and copper, from as early as 3000 BC. After 600 BC, pure metallic tin was produced. Pewter, which is an alloy of 85–90% tin with the remainder commonly consisting of copper, antimony, and lead, was used for flatware from the Bronze Age until the 20th century. In modern times, tin is used in many alloys, most notably tin/lead soft solders, which are typically 60% or more tin. Another large application for tin is corrosion-resistant tin plating of steel. Inorganic tin compounds are rather non-toxic. Because of its low toxicity, tin-plated metal was used for food packaging as tin cans, which are now made mostly of steel, even though the name is kept in English. However, overexposure to tin may cause problems with metabolizing essential trace elements such as copper and zinc, and some organotin compounds can be almost as toxic as cyanide.

Characteristics

Physical properties

Droplet of solidified molten tin

The most common allotrope of tin is a silver-white metallic-looking solid known as the β-form (or “beta-form”). Allotropes are forms of an element with different physical and chemical properties. This “white tin” has a melting point of 232°C (450°F), a boiling point of 2,260°C (4,100°F), and a density of 7.31 grams per cubic centimeter.

One of tin’s most interesting properties is its tendency to give off a strange screeching sound when it is bent. This sound is sometimes known as “tin cry.” β-tin is both malleable and ductile. Malleable means capable of being hammered into thin sheets. Ductile means capable of being drawn into a thin wire. At temperatures greater than 200°C, tin becomes very brittle.

A second form of tin is α-tin (or “alpha-tin”), also known as “gray tin.” Gray tin forms when white tin is cooled to temperatures less than about 13°C. Gray tin is a gray amorphous (lacking a crystalline shape) powder. The change from white tin to gray tin takes place rather slowly. This change is responsible for some peculiar and amazing changes in objects made from the element For example, tin and its alloys are used in jewelry, kitchenware, serving cups, and other metallic objects. When these objects are cooled below 13°C for long periods of time, the tin changes from a silvery, metallic material to a crumbly powder.

In the late nineteenth century, organ pipes in many cathedrals of Northern Europe were made of tin alloys. During the coldest winters, these pipes began to crumble as tin changed from one allotropic form to the other. The change was known as “tin disease.” At the time, no one knew why this change occurred.

One of tin’s most interesting properties is its tendency to give off a strange screeching sound when it is bent. This sound is sometimes known as “tin cry.”

Chemical properties

Tin is relatively unaffected by both water and oxygen at room temperatures. It does not rust, corrode, or react in any other way. This explains one of its major uses: as a coating to protect other metals. At higher temperatures, however, the metal reacts with both water (as steam) and oxygen to form tin oxide.

Similarly, tin is attacked only slowly by dilute acids such as hydrochloric acid (HCl) and sulfuric acid (H ₂ SO ₄ ). Dilute acids are mixtures that contain small amounts of acid dissolved in large amounts of water. This property also makes tin a good protective covering. It does not react with acids as rapidly as do many other kinds of metals, such as iron, and can be used, therefore, as a covering for those metals.

Tin dissolves easily in concentrated acids, however, and in hot alkaline solutions, such as hot, concentrated potassium hydroxide (KOH). The metal also reacts with the halogens to form compounds such as tin chloride and tin bromide. It also forms compounds with sulfur, selenium, and tellurium.

Occurrence in nature

Tin is not very abundant in nature. It ranks about 50th on the list of elements most commonly found in the Earth’s crust. Estimates are that the crust contains about 1 to 2 parts per million of tin.

By far the most common ore of tin is cassiterite, a form of tin oxide (SnO ₂ ). An ore is a compound or mixture from which an element can be extracted for commercial profit. Cassiterite has been mined for thousands of years as a source of tin. During ancient times, Europe obtained most of its tin from the British Isles. Today, the major producers of tin are China, Indonesia, Peru, Brazil, and Bolivia. The United States produces almost no tin of its own although it is the major consumer of the metal.

Isotopes

Tin has ten naturally occurring isotopes. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element’s name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Fifteen radioactive isotopes have also been discovered. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

None of the radioactive isotopes of tin have any commercial applications.

EXTRACTION

Tin can be produced easily by heating cassiterite with charcoal (nearly pure carbon). In this reaction, the carbon reacts with and removes oxygen from the cassiterite, leaving pure tin behind.

This reaction occurs so easily that people knew of the reaction thousands of years ago.

In order to obtain very pure tin, however, one problem must be solved. Iron often occurs in very small amounts along with tin oxide in cassiterite. Unless the iron is removed during the extraction process, a very hard, virtually unusable form of tin is produced. Modern systems of tin production, therefore, involve two steps. In one of those steps, impure tin is heated in the presence of oxygen to oxidize any iron in the mixture. In this reaction, iron is converted to iron(III) oxide, and metallic tin is left behind:

USES

The largest amount of tin used in the United States goes to the production of solder. Solder is an alloy, usually made of tin and lead, with a low melting point. It is used to join two metals to each other. For example, metal wires are attached to electrical devices by means of solder. Solder is also used by plumbers to seal the joint between two metal pipes.

The largest amount of tin used in the United States goes to the production of solder.

Solder is often applied by means of a soldering iron. A soldering iron consists of a steel bar through which an electric current runs. The electric current heats the bar as it passes through it. When a small piece of solder is placed on the tip of the soldering iron, it melts. The solder is then applied to the joint between two metals. When it cools, the bond is strong. In 1996, 15,600 metric tons of tin were used in the production of solder.

Tin is also used in the manufacture of other alloys. Bronze, for example, is an alloy of tin and copper. In 1996, more than 2,750 metric tons of bronze were produced in the United States. It is used in a wide variety of industrial products, such as spark-resistant tools, springs, wire, electrical devices, water gauges, and valves.

One application of tin that was once important is in the manufacture of “tin foil.” Tin foil is a very thin sheet of tin used to wrap candies, tobacco, and other products. The tin protected the products from spoiling by exposure to air. Today, most tin foil is actually thin sheets of aluminum because aluminum is less expensive.

A very important application of tin is tinplating. Tinplating is the process by which a thin coat of tin is placed on the surface of steel, iron, or another metal. Tin is not affected by air, oxygen, water, acids, and bases to the extent that steel, iron, and other metals are. So the tin coating acts as a protective layer.

Perhaps the best known example of tin plating is in the production of food cans. Tin cans are made of steel and are covered with a thin layer of tin. Most food and drink cans today are made out of aluminum because it is cheaper.

Metals can be plated with tin in one of two ways. First, the metal to be plated can simply be dipped in molten (liquid) tin and then pulled out. A thin layer of liquid tin sticks to the base metal and then cools to form a thin coating. The second method is electroplating. In the process of electroplating, the base metal is suspended in a solution of tin sulfate, or a similar compound. An electric current passes through the solution, causing the tin in the solution to be deposited on the surface of the base metal.

Tin was once important in the manufacture of “tin foil.” Now, aluminum is used because it is less expensive.

Another tin alloy is Babbitt metal. Babbitt metal is a soft alloy made of any number of metals, including arsenic, cadmium, lead, or tin. Babbitt metal is used to make ball bearings for large industrial machinery. The Babbitt metal is laid down as a thin coating on heavier metal, such as iron or steel. The Babbitt metal retains a thin layer of lubricating oil more efficiently than iron or steel.

COMPOUNDS

About a sixth of all tin consumed in the United States is used in the production of tin compounds. Some of the most important of those compounds and their uses are as follows:

 tin chloride (SnCl ₂ ): used in the manufacture of dyes, polymers, and textiles; in the silvering of mirrors; as a food preservative; as an additive in perfumes used in soaps; and as an anti-gumming agent in lubricating oils

tin oxide (SnO ₂ ): used in the manufacture of special kinds of glass, ceramic glazes and colors, perfumes and cosmetics, and textiles; and as a polishing material for steel, glass, and other materials

tin chromate (SnCrO ₄ or Sn(CrO ₄ ) ₂ ): brown or yellowish-brown compounds used as a coloring agent for porcelain and china

tin fluoride (SnF ₂ ) and tin pyrophosphate (Sne₂ P ₂ O ₇ ): used as toothpaste additives to help protect against cavitiTY

Etymology

The word tin is shared among Germanic languages and can be traced back to reconstructed Proto-Germanic *tin-om; cognates include German Zinn, Swedish tenn and Dutch tin. It is not found in other branches of Indo-European, except by borrowing from Germanic (e.g. Irish tinne from English).

The Latin name stannum originally meant an alloy of silver and lead, and came to mean ‘tin’ in the 4th century BCE—the earlier Latin word for it was plumbum candidum, or “white lead”. Stannum apparently came from an earlier stāgnum (meaning the same substance),^[17] the origin of the Romance and Celtic terms for ‘tin’. The origin of stannum/stāgnum is unknown; it may be pre-Indo-European.

The Meyers Konversationslexikon speculates on the contrary that stannum is derived from (the ancestor of) Cornish stean, and is proof that Cornwall in the first centuries AD was the main source of tin.

History

 Tin sources and trade in ancient times

Ceremonial giant bronze dirk of the Plougrescant-Ommerschans type, Plougrescant, France, 1500–1300 BC.

Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BC, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties. The earliest bronze objects had a tin or arsenic content of less than 2% and are therefore believed to be the result of unintentional alloying due to trace metal content in the copper ore. The addition of a second metal to copper increases its hardness, lowers the melting temperature, and improves the casting process by producing a more fluid melt that cools to a denser, less spongy metal. This was an important innovation that allowed for the much more complex shapes cast in closed moulds of the Bronze Age. Arsenical bronze objects appear first in the Near East where arsenic is commonly found in association with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age. This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures.

Cassiterite (SnO₂), the tin oxide form of tin, was most likely the original source of tin in ancient times. Other forms of tin ores are less abundant sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits because it is harder, heavier, and more chemically resistant than the accompanying granite. Cassiterite is usually black, purple or otherwise dark in color, and these deposits can be easily seen in river banks. Alluvial (placer) deposits could be easily collected and separated by methods similar to gold panning.

Compounds and chemistry

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In the great majority of its compounds, tin has the oxidation state II or IV.

Inorganic compounds

Halide compounds are known for both oxidation states. For Sn(IV), all four halides are well known: SnF₄, SnCl₄, SnBr₄, and SnI₄. The three heavier members are volatile molecular compounds, whereas the tetrafluoride is polymeric. All four halides are known for Sn(II) also: SnF₂, SnCl₂, SnBr₂, and SnI₂. All are polymeric solids. Of these eight compounds, only the iodides are colored.

Tin(II) chloride (also known as stannous chloride) is the most important tin halide in a commercial sense. Illustrating the routes to such compounds, chlorine reacts with tin metal to give SnCl₄ whereas the reaction of hydrochloric acid and tin produces SnCl₂ and hydrogen gas. Alternatively SnCl₄ and Sn combine to stannous chloride by a process called comproportionation:

SnCl₄ + Sn → 2 SnCl₂

Tin can form many oxides, sulfides, and other chalcogenide derivatives. The dioxide SnO₂ (cassiterite) forms when tin is heated in the presence of air. SnO₂ is amphoteric, which means that it dissolves in both acidic and basic solutions. Stannates with the structure [Sn(OH)₆]²⁻, like K₂[Sn(OH)₆], are also known, though the free stannic acid H₂[Sn(OH)₆] is unknown.

sulfides of tin exist in both the +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold).

Ball-and-stick models of the structure of solid stannous chloride (SnCl₂).

Hydrides

Stannane (SnH₄), with tin in the +4 oxidation state, is unstable. Organotin hydrides are however well known, e.g. tributyltin hydride (Sn(C₄H₉)₃H). These compound release transient tributyl tin radicals, which are rare examples of compounds of tin(III).

Organotin compounds

Organotin compounds, sometimes called stannanes, are chemical compounds with tin–carbon bonds. Of the compounds of tin, the organic derivatives are the most useful commercially. Some organotin compounds are highly toxic and have been used as biocides. The first organotin compound to be reported was diethyltin diiodide ((C₂H₅)₂SnI₂), reported by Edward Frankland in 1849.

Most organotin compounds are colorless liquids or solids that are stable to air and water. They adopt tetrahedral geometry. Tetraalkyl- and tetraaryltin compounds can be prepared using Grignard reagents:

SnCl
4 + 4 RMgBr → R
4Sn + 4 MgBrCl

The mixed halide-alkyls, which are more common and more important commercially than the tetraorgano derivatives, are prepared by redistribution reactions:

SnCl
4 + R
4Sn → 2 SnCl₂R₂

Divalent organotin compounds are uncommon, although more common than related divalent organogermanium and organosilicon compounds. The greater stabilization enjoyed by Sn(II) is attributed to the “inert pair effect“. Organotin(II) compounds include both stannylenes (formula: R₂Sn, as seen for singlet carbenes) and distannylenes (R₄Sn₂), which are roughly equivalent to alkenes. Both classes exhibit unusual reactions.

Occurrence

See also: Category:Tin minerals.

Sample of cassiterite, the main ore of tin.

Granular pieces of cassiterite, collected by placer mining

Tin is generated via the long S-process in low-to-medium mass stars (with masses of 0.6 to 10 times that of Sun), and finally by beta decay of the heavy isotopes of indium.

Tin is the 49th most abundant element in Earth‘s crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead.

Tin does not occur as the native element but must be extracted from various ores. Cassiterite (SnO₂) is the only commercially important source of tin, although small quantities of tin are recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always associated with granite rock, usually at a level of 1% tin oxide content.

Because of the higher specific gravity of tin dioxide, about 80% of mined tin is from secondary deposits found downstream from the primary lodes. Tin is often recovered from granules washed downstream in the past and deposited in valleys or the sea. The most economical ways of mining tin are by dredging, hydraulicking, or open pits. Most of the world’s tin is produced from placer deposits, which may contain as little as 0.015% tin.

TRANSITION METALS

The elements in the periodic table are often divided into four categories: (1) main group elements, (2) transition metals, (3) lanthanides, and (4) actinides. The main group elements include the active metals in the two columns on the extreme left of the periodic table and the metals, semimetals, and nonmetals in the six columns on the far right. The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table. The lanthanides and the actinides at the bottom of the table are sometimes known as the inner transition metals because they have atomic numbers that fall between the first and second elements in the last two rows of the transition metals.

Transition Metals vs. Main-Group Elements

There is some controversy about the classification of the elements on the boundary between the main group and transition-metal elements on the right side of the table. The elements in question are zinc (Zn), cadmium (Cd), and mercury (Hg).

The disagreement about whether these elements should be classified as main group elements or transition metals suggests that the differences between these categories are not clear. Transition metals are like main group metals in many ways: They look like metals, they are malleable and ductile, they conduct heat and electricity, and they form positive ions. The fact the two best conductors of electricity are a transition metal (copper) and a main group metal (aluminum) shows the extent to which the physical properties of main group metals and transition metals overlap.

There are also differences between these metals. The transition metals are more electronegative than the main group metals, for example, and are therefore more likely to form covalent compounds.

Another difference between the main group metals and transition metals can be seen in the formulas of the compounds they form. The main group metals tend to form salts (such as NaCl, Mg₃N₂, and CaS) in which there are just enough negative ions to balance the charge on the positive ions. The transition metals form similar compounds [such as FeCl₃, HgI₂, or Cd(OH)₂], but they are more likely than main group metals to form complexes, such as the FeCl₄^–, HgI₄^2-, and Cd(OH)₄^2- ions, that have an excess number of negative ions.

A third difference between main group and transition-metal ions is the ease with which they form stable compounds with neutral molecules, such as water or ammonia. Salts of main group metal ions dissolve in water to form aqueous solutions.

	H2O
NaCl(s)	The Electron Configuration of Transition-Metal Ions The relationship between the electron configurations of transition-metal elements and their ions is complex. Example: Let’s consider the chemistry of cobalt which forms complexes that contain either Co2+ or Co3+ ions. The electron configuration of a neutral cobalt atom is written as follows. Co: [Ar] 4s2 3d7 The discussion of the relative energies of the atomic orbitals suggests that the 4s orbital has a lower energy than the 3d orbitals. Thus, we might expect cobalt to lose electrons from the higher energy 3d orbitals, but this is not what is observed. The Co2+ and Co3+ ions have the following electron configurations. Co2+: [Ar] 3d7 Co3+: [Ar] 3d6 In general, electrons are removed from the valence-shell s orbitals before they are removed from valence d orbitals when transition metals are ionized. Because the valence electrons in transition-metal ions are concentrated in d orbitals, these ions are often described as having dn configurations. The Co3+ and Fe2+ ions, for example, are said to have a d6 configuration. Co3+: [Ar] 3d6 Fe2+: [Ar] 3d6 Oxidation States of the Transition Metals Most transition metals form more than one oxidation state. Some oxidation states, however, are more common than others. The most common oxidation states of the first series of transition metals are given in the table below. Efforts to explain the apparent pattern in this table ultimately fail for a combination of reasons. Some of these oxidation states are common because they are relatively stable. Others describe compounds that are not necessarily stable but which react slowly. Still others are common only from a historic perspective. Common Oxidation States of the First Series of Transition Metals     Sc   Ti   V   Cr   Mn   Fe   Co   Ni   Cu   Zn +1                                   d10     +2           d3       d5   d6   d7   d8   d9   d10 +3   d0           d3       d5   d6             +4       d0           d3                     +5           d0                             +6               d0                         +7                   d0                     One point about the oxidation states of transition metals deserves particular attention: Transition-metal ions with charges larger than +3 cannot exist in aqueous solution. Consider the following reaction in which manganese is oxidized from the +2 to the +7 oxidation state. Mn2+(aq) + 4 H2O(l)   H2O       NaCl(s)   Extracting copper from other ores Copper can be extracted from non-sulphide ores by a different process involving three separate stages: Reaction of the ore (over quite a long time and on a huge scale) with a dilute acid such as dilute sulphuric acid to produce a very dilute copper(II) sulphate solution. Concentration of the copper(II) sulphate solution by solvent extraction. The very dilute solution is brought into contact with a relatively small amount of an organic solvent containing something which will bind with copper(II) ions so that they are removed from the dilute solution. The solvent mustn’t mix with the water. The copper(II) ions are removed again from the organic solvent by reaction with fresh sulphuric acid, producing a much more concentrated copper(II) sulphate solution than before. Electrolysis of the new solution. Copper(II) ions are deposited as copper on the cathode (for the electrode equation, see under the purification of copper below). The anodes for this process were traditionally lead-based alloys, but newer methods use titanium or stainless steel. The cathode is either a strip of very pure copper which the new copper plates on to, or stainless steel which it has to be removed from later.   Purification of copper When copper is made from sulphide ores by the first method above, it is impure. The blister copper is first treated to remove any remaining sulphur (trapped as bubbles of sulphur dioxide in the copper – hence “blister copper”) and then cast into anodes for refining using electrolysis.   Electrolytic refining The purification uses an electrolyte of copper(II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes. The diagram shows a very simplified view of a cell. At the cathode, copper(II) ions are deposited as copper. At the anode, copper goes into solution as copper(II) ions. For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same. All that happens is that there is a transfer of copper from the anode to the cathode. The cathode gets bigger as more and more pure copper is deposited; the anode gradually disappears. In practice, it isn’t quite as simple as that because of the impurities involved.   What happens to the impurities? Any metal in the impure anode which is below copper in the electrochemical series (reactivity series) doesn’t go into solution as ions. It stays as a metal and falls to the bottom of the cell as an “anode sludge” together with any unreactive material left over from the ore. The anode sludge will contain valuable metals such as silver and gold. Metals above copper in the electrochemical series (like zinc) will form ions at the anode and go into solution. However, they won’t get discharged at the cathode provided their concentration doesn’t get too high. The concentration of ions like zinc will increase with time, and the concentration of the copper(II) ions in the solution will fall. For every zinc ion going into solution there will obviously be one fewer copper ion formed. (See the next note if you aren’t sure about this.) The copper(II) sulphate solution has to be continuously purified to make up for this. EVALUATION 1.Describe transition metals ,their members(first transition series). 2.Describe  the extraction of tin. Lessons on Chemistry SS3 –Edudelight.com WEEK 10 Iron is the second most abundant metal occurring in the earth’s crust. It is an element of the first transition series. OCCURRENCE The important ores of iron are: 1.      Haematite, Fe2O3 (red oxide of iron) 2.      Magnetite, Fe3O4 (magnetic oxide of iron) 3.      Limonite, Fe2O3.3H20 (hydrated oxide of iron) 4.      Iron pyrites, FeS2 5.      Siderite or Spathic ore, FeCO3. The cast iron is generally extracted from haematite (Fe2O3). The various steps involved in the process are as  follows: 1.      Concentration. The are is crushed with the help of jaw crushers into small pieces of about 5 cm size. The crushed ore is washed with a stream of water whereby lighter sand particles are washed away and the heavier are particles settle down. 2.      Calcination. The concentrated are is heated strongly in the presence of air. This is called calcination. During calcination following changes take place: Moisture is driven out. Sulphur, arsenic and phosphorus impurities are expelled as their volatile oxides. Carbonate are changes into oxide are. FeCO3             →          FeO + CO2 Ferrous oxide changes into ferric oxide. 4FeO + O2    →               2Fe2O3 3. Smelting. After calcination the are is subjected to reduction with carbon in a blast furnace (Fig. 50.4). The process is called smelting. Blast furnace is made up of steel, lined inside with fire resistant bricks. It has a cup and cone arrangement for the introduction of charge at the top, and at the base it has: (i)  a ‘tapping hole for removing molten iron from time to time. (ii)   an arrangement for the introduction of hot air. (iii)  an outlet for slag. The calcined ore, coke and lime stone are mixed in the ratio 8: 4 : 1 and are fed into the furnace. through cup and cone arrangement. Blast of hot air at about 1000 K is blown in through narrow pipes (tuyeres) at the base of the furnace. Coke here serves as a fuel as well as a reducing agent while lime is a flux. Reactions which take place in the furnace are as follows: (i)                 Coke bums at the base to produce CO2 which rises up. C + O2             →             CO2 , H = – 406 kJ mol-1 The reaction is exothermic and temperature here is upto about 2173 K. This region is called combustion zone. (ii)               As CO2 rises up it is reduced to CO with the coke. CO2 + C         →       2CO, H = + 173 kJmol-1 Due to the endothermic nature of this reaction, the temperature in this region falls to 1475-1575 K. In this region Fe2O3, if present, gets reduced to iron by hot coke and the spongy iron produced in the upper region gets melted. This is known as fusion zone. (iii)             Near the top of the furnace, where temperature is about 873 K, the oxides of iron (Fe2O3 and Fe3O4) are reduced to iron and Fe by carbon (II) oxide Fe2O3 + CO              →                    2FeO + CO2 3Fe2O3 + CO                        →                    2Fe3O4 + CO2 Fe3O4 + 4CO                        →                    3Fe + 4CO2 (iv)              In the middle portion of the furnace where temperature is about 1073 to 1273 K, FeO is reduced to iron while coke is oxidized to CO. CO2 + C                   →                    2CO FeO + CO                →                    Fe + CO2 FeO + C                   →                    Fe + CO Lime stone decomposes to produce CaO which combines with silica (impurity) to form slag. CaCO3                            →                             CaO + CO2 CaO + SiO2                  →                             CaSiO3 Molten slag is lighter than molten iron and forms the upper layer. This region is called slag formation zone. Iron formed moves down and melts in the fusion zone. Molten iron dissolves in it some carbon, silicon, and phosphorus and forms the lower layer at the base of the furnace. Iron thus formed is called pig iron. Pig iron contains about 4% of carbon and many other impurities such as phosphorus, silicon, sulphur and manganese. Iron works are located close to coal fields so that plentiful of coal is available at cheap rate. Coal is used as reducing agent and as fuel. Pig iron is converted into cast iron by heating molten pig iron with scrap iron and coke in specially designed furnaces. A blast of hot air is blown through the mixture. Cast iron contains about 3% carbon. It slightly expands on solidifying and hence reproduces the shape of the mould. Cast iron is very hard and brittle. Its melting point is about 1473 K CAST IRON FROM IRON PYRITES Iron pyrites (FeS2) is not directly used as an ore for the extraction of iron because it contains quite high percentage of sulphur. Due to high content of sulphur it is primarily us for the manufacture of H2SO4, For this purpose it is burnt in air to get SO2: 4FeS2 + 11O2                    →2Fe2O3 + 8SO2 ↑ SO2 is converted into SO3 which is absorbed in water get H2SO4. The residual FeO3 formed during burning of FeS2 is reduced to iron in blast furnace as already discussed. COMMERCIAL FORMS OF IRON There are three commercial forms of iron which differ from each other mainly in their carbon content. Cast Iron or Pig Iron. It contains about 2 to 5% of carbon. along with impurities such as sulphur, silicon, phosphorus. manganese, etc. It is the least pure form of iron. It is brittle and is very difficult to weld. It is resistant to corrosion and is used for sewage pipes. Steel. It contains 0.1 to 1.5% carbon and other impurities. Wrought Iron. It is the purest form of iron and contains carbon and other impurities less than 0.2%. It is malleable and can be easily welded. This type of iron is structurally weak and hence cannot be magnetised permanently. It is used for making wires, chains, electromagnets, etc. 4. CONVERSION OF CAST IRON INTO WROUGHT IRON Wrought iron which is comparatively pure form of is obtained by refining cast iron. The cast iron is heated haematite (Fe2O3) so that the impurities are oxidized. Fe2O3 + 3C                   →                    2Fe + 3CO CO escapes while MnO present as an impurity, combines with SiO2 to form slag. MnO + SiO2                   →                             MnSiO3 (slag) Wrought iron thus formed contains about 0.2 – 0.5% carbon and traces of P and Si in the form of slag.  Wrought iron ismalleable and ductile. Its melting point ut 1400°C. STEEL The amount of carbon present in steel is nearly intermediate between that contained in cast and wrought iron. The various steels contain carbon from 0.1 to 1.5%. The increase in carbon content in the steel decreases its ductility and increases its tensile strength. The following methods commonly used for the production of steel:       Bessemer Process       The Basic Oxygen Process       Open Hearth Process       The Electric Arc Process      The High Frequency Induction Process. All these methods are based on the oxidation of impurities pig iron, Two of these methods are discussed below: 1. Bessemer Process. The Bessemer process involves blowing a strong blast of air through the molten pig iron. The Bessemer process is accomplished in large pear-shaped iron vessels called converters which are lined on the inside with silica bricks and can hold upto 50 tons of pig iron at a time. The converter is mounted on horizontal pivots around which it can be tilted. The converter is filled with molten pig iron at a temperature of about 1473 K and air is blown into the chamber. Oxygen reacts with impurities and raises the temperature to 2173 K. The impurities are oxidised by air. Si + O2                     →                    SiO2 2Mn + O2                     →                    2MnO C + O2                     →                    CO2 SiO2 is slagged off with lime and MnO with silica in the form of their respective silicates. CaO + SiO2                            →                             CaSiO3 SiO2 + MnO                     →                    MnSiO3 The slag floats on the surface whereas purified iron being than impure iron sinks to the bottom. The Bessemer process produces iron containing less than 0.3 per cent carbon. If it is desired to obtain steel, the air blast is either shut off before all the carbon has burnt out, or a definite amount of pig iron rich in carbon is added to the iron produced in the converter. Air is again blown through it for a short period in order to mix the ingredients. 2. The Basic Oxygen Process. The Basic Oxygen process is a development of the Bessemer Process. In this process pure oxygen is used instead of air. Initially, the furnace is charged with hot molten pig iron from the blast furnace lime and scrap steel. A jet of oxygen is blown onto the surface of the metal at a great speed through a water cooled pipe. The oxygen oxidizes impurities rapidly. Since the reactions taking place in (he furnace are highly exothermic, the heat evolved keeps the contents of the furnace in the molten state. When steel of desired composition is obtained, the oxygen is turned off and molten steel is poured by tilting the furnace. ALLOY STEELS Alloy Steel Special Components Chief Properties Uses 1. Nickle Ni 3.5% Hard, flexible, rust resistant For cables armour plates 2. Stainless steel Cr 18%, Ni 8% Does not rust or corrode For household utensils, shaving, blades, watch cases 3. Chrome-vanadium steel Cr 1% V 0.15% Tenacious and load bearing For axles, springs and cog-wheels 4. Manganese steel Mn 12 to 15% Extremely hard and high melting For rock crusher, burglar proof safes 5. Tungsten steel W 14 to 20% Cr 3 to 8% Very hard and strong For cutting tools, springs 6. Invar Ni36% Extremely low expansion on heating For clock pendulums 7. Alnico Al 12%, Ni 20%, Co 5% Highly magnetic For permanent magnets Steel and alloy steels are preferred to the pure iron because they are stronger and resistant to corrosion. Iron undergoes corrosion (rusting). SOLVED EXAMPLES Example 50.4 Giving examples, differentiate between ‘roasting’ and ‘calcination’. Solution. Roasting is the process of heating the ore in the excess supply of air below its melting point. This process is applied for sulphide ores. As a result sulphide ore changes into oxide. Calcination is the process of heating the ore in limited supply of air below its melting point. This process is applied for oxide and carbonate ores. For example, zinc blende (sulphide ore of zinc) is roasted whereas haematite (oxide ore of iron) is subjected to calcination. Heat 2ZnS + 3O2      →        2ZnO + SO2                                         … Roasting Heat Fe2O3.xH2O     →        Fe2O3 + xH2O                                       …Calcination Example 50.5 How is ‘cast iron’ different from ‘pig iron? Solution. Cast iron contains about 3% carbon whereas pig iron contains about 4% carbon. Pig iron is harder than cast iron. LOCAL EXTRACTION OF IRON Arc Method. This is the modern method used in GHANA. This method involves smelting scrap iron at a high temperature. Scrap iron is any discarded iron material. The high temperature is provided by an electric arc produced when a pair of high voltage oppositely charged electrodes are brought near each other. 2. In a typical local production of iron, a mixture of iron ore, oyster shells and charcoal or palm kernel shells is loaded into a kiln or furnace. Fire is then set at the bottom of the furnace. Bellows are used to blow air continuously through the lower parts of the furnace. The charcoal burns to produce CO2. C(s) + O2(g) → CO2(g) As the CO2 rises up the heap in the furnace, it reacts with more charcoal to produce carbon (II) oxide, CO. CO2(g) + C(s) → 2CO(g) CO reduces iron ore, either FeO or Fe2O3, to iron metal. FeO(s) + CO(g) → Fe(l) + CO2(g) Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2 (g) The oyster shells contain limestone which is a source of CaCO3. When heated, CaCO3 decomposes producing calcium oxide and CO2. CaCO3(s) → CaO(s) + CO2(g) The calcium oxide combines with silica (SiO2) removing this impurity in the form of slag. CaO(s) + SiO2 (s) → CaSiO3(s) Calcium trioxosilicate (lV) The molten iron flows to the bottom of the furnace where it is collected. The process of slow conversion of metals into their undesirable compounds (usually oxides) by reaction with moisture and other gases present in the atmosphere is called corrosion of metals. For example, (i) Iron when exposed to moist air, gets corroded and a layer of reddish brown flaky substance, known as rust, is formed on its surface. Rust is chemically hydrated iron (ill) oxide, Fe2O3.xH2O. (ii) Copper metal on exposure to moist air gets coated with greenish white powdery substance which is basic copper carbonate, CuCO3,Cu(OH)2 (iii) Silver on exposure to air, loses it shine and its surface becomes black due to formation of silver sulphide, Ag2S. The metals occupying higher position in the activity series undergo corrosion rapidly. Only a few metals such as gold, platinum, palladium, etc., which are at the bottom of the activity series, are resistant to corrosion. Sometimes corrosion of metals is an advantage because it prevents the metal underneath from further corrosion. For example, aluminum when exposed to air becomes coated With a layer of aluminium oxide which protects the metal underneath from further corrosion. Now let us study corrosion of iron (rusting) in somewhat detail: RUSTING OF IRON The corrosion of iron with moist air is known as rusting. When an iron object is exposed to air in the presence of moisture, it gets corroded and reddish brown flaky substance, which is known as rust, is formed. Rust is hydrated iron (III) oxide Fe2O3. xH2O. Unlike the oxide layers of other metals such as aluminium and chromium, rust does not stick to the surface and does not protect the metal from further corrosion. Rust once formed, causes more and more rusting until the whole of the metal is eaten up. Thus, rusting weakens the structures that are made up of iron. Moisture 4Fe + 3O2 → 2Fe2O3 xH2O Hydrated iron (III) oxide (Rust) The two conditions necessary for the rusting of iron to 1. Presence of moisture, and 2. Presence of oxygen. Conditions for rusting can be investigated by the following simple experiment. Investigation of Conditions for Rusting Take three test tubes and label them as A, B and C. Add a few clean iron nails to each of them. To the tube A add .tap water. To the tube B add anhydrous calcium chloride and cork it. To the tube C add boiled distilled water and then add about 1 cm3 oil and cork it. Leave the test tubes as such for a few days. Investigation of conditions for rusting. What do you abserve? After a few days it is observed that the nails in the tube A rust but the nails in the tubes B and C do not rust. In tube A both air and water are present. In tube B nails are exposed only to air while in the tube C nails are exposed to water but the layer of oil prevents the air from dissolving in water. Thus, presence of water and air both is essential for rusting. The following factors further catalyse the process of rusting. 1. Presence of carbon dioxide, 2. Presence of acids, 3. Presence of impurities in the iron. Due to rusting, iron object loses its strength. Rusting can be prevented by alloying iron with other metals and non-metals. For example, stainless steel is an alloy of iron which contains about 0.05% carbon, 8% nickel and 18% chromium. It is quite hard and is resistant to corrosion. Add To Your Knowledge Anodising is a process of forming a thick layer of aluminium oxide on the surface of aluminium so as to protect it from corrosion. Aluminium which has been exposed to atmosphere is covered with a thin layer of aluminium oxide, Al2O3 which protects the metal from further corrosion. In order to protect the aluminium even more, it is possible to increase the thickness of the oxide layer to about 10-5 m by anodising. The aluminium is anodised by making it the anode during electrolysis of dilute sulphuric acid. Oxygen, released at the anode combines with the aluminium and thickens the oxide layer. The oxide layer at this stage readily absorbs dyes. By carrying out the electrolytic anodising process in the presence of dyes, the anodised material can be coloured attractively. Native state. An element is said to exist in native state, if it is found in nature in its elementary form. Less reactive elements such as gold, platinum, etc., are found in native state. Combined state. An element is said to exist in combined state if it exists in nature in the form of its compounds. Reactive elements occur in nature in combined state. Minerals. A mineral is a naturally occurring material obtained by mining from the earth’s crust that contains metal in its native state or combined state. Ore. A mineral from which metal can be extracted conveniently and economically called an ore. Gangue or Matrix. The earthly and silicious impurities associated with the ore are known as gangue or matrix. Metallurgy. The process of extraction of pure metal from one of its ores is known as metallurgy. Concentration. The process of removal of earthly impurities from the pulverized ore is called concentration or benefication of ore. This is also sometimes called dressing of ore. Calcination. It is the process of heating the ore in a limited supply of air below its melting point. This process is employed for oxide ores and carbonate ores. Roasting. It is the process of heating the ore in the excess supply of air below its melting point. This process is employed for sulphide ores. Smelting. In this process, the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which act as reducing agent) and is heated to a high temperature above its melting point. During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional reagent is called flux. Flux combines with the impurities to form a fusible product called slag. Reduction with aluminium. The process of reduction of oxides with aluminium is called aluminothermy. Electrolytic reduction. The highly electropositive. metals such as alkali metals, alkaline earth metals and aluminium are extracted by electrolysis of their fused salts. Liquation. This process is applied for purification of metals having low melting points, such as tin and lead. Distillation. Metals having low boiling points, such as mercury and zinc, can be purified by distillation. Electro-refining. Metals such as copper, silver, aluminium and gold are refined by process of electrolysis. In this method impure metal is made anode, while a thin strip of pure metal is made cathode and solution of some salt of the metal is used as electrolyte. Gold occurs in native form in alluvial sand or gravel. Gold is extracted by hydrometallurgy. Aluminium is the most abundant metal in the earth’s crust. Its most important ore is bauxite (Al2O3 . 2H2O) Bauxite is concentrated by leaching process. Aluminium is obtained by electrolysis of pure alumina dissolved in molten cryolite. The process is known as Hall-Heroult process. Iron is the second most abundant metal in the earth’s crust The important ores of iron are: (i) Haematite, Fe2O3 (red oxide of iron) (ii) Magnetite, Fe3O4 (Magnetic oxide of iron) ALLOYS You might see the word alloy described as a “mixture of metals”, but that’s a little bit misleading because some alloys contain only one metal and it’s mixed in with other substances that are nonmetals (cast iron, for example, is an alloy made of just one metal, iron, mixed with one nonmetal, carbon). The best way to think of an alloy is as a material that’s made up of at least two different chemical elements, one of which is a metal. The most important metallic component of an alloy (often representing 90 percent or more of the material) is called the main metal, the parent metal, or the base metal. The other components of an alloy (which are called alloying agents) can be either metals or nonmetals and they’re present in much smaller quantities (sometimes less than 1 percent of the total). Although an alloy can sometimes be a compound (the elements it’s made from are chemically bonded together), it’s usually a solid solution (atoms of the elements are simply intermixed, like salt mixed with water). The structure of alloys If you look at a metal through a powerful electron microscope, you can see the atoms inside arranged in a regular structure called a crystalline lattice. Imagine a small cardboard box full of marbles and that’s pretty much what you’d see. In an alloy, apart from the atoms of the main metal, there are also atoms of the alloying agents dotted throughout the structure. (Imagine dropping a few plastic balls into the cardboard box so they arrange themselves randomly among the marbles.) Substitution alloys If the atoms of the alloying agent replace atoms of the main metal, we get what’s called a substitution alloy. An alloy like this will form only if the atoms of the base metal and those of the alloying agent are of roughly similar size. In most substitution alloys, the constituent elements are quite near one another in the periodic table. Brass, for example, is a substitution alloy based on copper in which atoms of zinc replace 10–35 percent of the atoms that would normally be in copper. Brass works as an alloy because copper and zinc are close to one another in the periodic table and have atoms of roughly similar size. Interstitial alloys Alloys can also form if the alloying agent or agents have atoms that are very much smaller than those of the main metal. In that case, the agent atoms slip in between the main metal atoms (in the gaps or “interstices”), giving what’s called an interstitial alloy. Steel is an example of an interstitial alloy in which a relatively small number of carbon atoms slip in the gaps between the huge atoms in a crystalline lattice of iron. The alloy of tin known as bronze was probably produced even earlier than the pure metal. An alloy is made by melting and mixing two or more metals. The mixture has properties that are different than any of the metals alone. The Egyptians, Mesopotamians, Babylonians, and Peruvians were producing bronze as far back as 2000 B.C. The alloy was probably discovered accidentally when copper and tin compounds were heated together. Over time, a method for producing consistent bronze was developed. Bronze is a much better replacement for copper in tools, eating utensils, and weapons. Bronze marked a significant advance in human civilization. This strong alloy improved transportation methods, food preparation, and quality of life during a period now known as the Bronze 4000 Bronze became popular among ancient peoples because it was harder and tougher than copper. Before the discovery of bronze, many metal items were made out of copper. But copper is soft and bends easily. Some common alloys and what we use them for There are zillions of different alloys used for zillions of different purposes. We’ve listed 20 of the more common (or otherwise interesting) ones in the table below. There are lots of different variations on most alloys and the precise mixture can vary widely, so the percentage figures you see quoted in different books will often not agree exactly. Alloy Components Typical uses Alnico Iron (50%+), aluminum (8–12%), nickel (15–25%), cobalt (5–40%), plus other metals such as copper and titanium. Magnets in loudspeakers and pickups in electric guitars. Amalgam Mercury (45–55%), plus silver, tin, copper, and zinc. Dental fillings. Babbitt metal (“white metal”) Tin (90%), antimony (7–15%), copper (4–10%). Friction-reducing coating in machine bearings. Brass Copper (65–90%), zinc (10–35%). Door locks and bolts, brass musical instruments, central heating pipes. Bronze Copper (78–95%), tin (5–22%), plus manganese, phosphorus, aluminum, or silicon. Decorative statues, musical instruments. Cast iron Iron (96–98%), carbon (2–4%), plus silicon. Metal structures such as bridges and heavy-duty cookware. Cupro-nickel (copper nickel) Copper (75%), nickel (25%), plus small amounts of manganese. Coins. Duralumin Aluminum (94%), copper (4.5–5%), magnesium (0.5–1.5%), manganese (0.5–1.5%). Automobile and aircraft body parts, military equipment. Gunmetal Copper (80–90%), tin (3–10%), zinc (2–3%), and phosphorus. Guns, decorative items. Magnox Magnesium, aluminum. Nuclear reactors. Nichrome Nickel (80%), chromium (20%). Firework ignition devices, heating elements in electrical appliances. Nitinol Nickel (50–55%), titanium (45–50%). Shape-memory alloy used in medical items, spectacle frames that spring back to shape, and temperature switches. Pewter Tin (80–99%) with copper, lead, and antimony. Ornaments, used to make tableware before glass became more common. Solder Varies. Old-fashioned solders contain a mixture of tin (50-70%), lead (30-50%), copper, antimony, and other metals. Newer solders dispense with lead for health reasons. A typical modern solder has 99.25 percent tin and 0.75 percent copper. Connecting electrical components into circuits. Steel (general) Iron (80–98%), carbon (0.2–2%), plus other metals such as chromium, manganese, and vanadium. Metal structures, car and airplane parts, and many other uses. Steel (stainless) Iron (50%+), chromium (10–30%), plus smaller amounts of carbon, nickel, manganese, molybdenum, and other metals. Jewelry, medical tools, tableware. Stellite Cobalt (67%), chromium (28%), tungsten (4%), nickel (1%). Coating for cutting tools such as saw teeth, lathes, and chainsaws. Sterling silver Silver (92.5%), copper (7.5%). Cutlery, jewelry, medical tools, musical instruments. White gold (18 carat) Gold (75%), palladium (17%), silver (4%), copper (4%) Jewelry. Wood’s metal Bismuth (50%), lead (26.7%), tin (13.3%), cadmium (10%). Solder, melting element in fire sprinkler systems.               EVALUATION 1. Give name and formula of the flux used during extraction of iron. 2. What is difference between pig iron and cast iron? How is pig iron converted into cast iron? – 3. Name the most pure form of iron. What is the percentage of carbon in it? 4. Give equations for the reactions taking place in different regions of the blast furnace during extraction of iron from haematite. 5. Name any two ores of iron. 6What are the components of stainless steel? 7.What is rust? List two ways by which rusting of iron can be prevented. 8.Explain why the surface of sonie metals acquires a dull appearance when exposed to air for a long time. 9.What are the factors which catalyse the process of rusting? Explain, why rusting is considered harmful? 10.Which metals do not corrode easily? 11.Define corrosion. Name three metals which when exposed to air get corroded. Identify the substance formed after corrosion of these metals. Answers I.                   Objective Type Questions                         1. (d)                            2. (b)                            3. (a)                         4. (b)                            5. (c)                            6. (b)                         7. (b)                            8. (d) II.                Objective Type Questions 9.   (i) sulphide                  (ii)aluminium      (iii) refining                 (iv) aluminothermy      (v) concentration          (vi) electrolytic      (vii) impure                  (viii) flux 2 hours read Sharing is caring Facebook Messenger Messenger WhatsApp Telegram Line Sharing is caring Facebook LinkedIn WhatsApp Telegram Line Share via Email Read Next Uncategorized Lesson Note on Basic Technology JSS 2 Third Term Uncategorized Business Studies Lesson JSS 3 Second Term Uncategorized Lesson Notes on Social Studies JSS3 (Basic 9) First Term Uncategorized FIRST TERM SCHEME OF WORK FOR COMMERCE SS2 Lesson Note on Basic Technology JSS 2 Third Term Business Studies Lesson JSS 3 Second Term Lesson Notes on Social Studies JSS3 (Basic 9) First Term FIRST TERM SCHEME OF WORK FOR COMMERCE SS2 Read Also PARENT MANAGEMENT GUIDE SECOND TERM SCHEME OF… Lesson Note on Social… Economics Lesson Note for… Leave a Reply Cancel reply Your email address will not be published. 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