CBSE Important Questions Class 12 Chemistry Chapter 11 – Polymers

Class 12 Chemistry Chapter 11 Polymers is part of the CBSE Class 12 Chemistry syllabus. Polymers are very large molecules built from many small repeating units called monomers. Students learn the classification of polymers by source, structure, mode of polymerisation and molecular forces, the two main types of polymerisation, the preparation of important polymers, natural and synthetic rubber, and biodegradable polymers.

These CBSE important questions for Class 12 Chemistry Chapter 11 cover the definition of a polymer and monomer, classification by source and by molecular forces, the difference between addition and condensation polymerisation, the preparation of nylon-6,6 and bakelite, vulcanisation of rubber, copolymers, and biodegradable polymers. Every question has a clear step-by-step solution in simple English, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. What are polymers and monomers? Give one example of each.

Answer: A polymer is a very large molecule made by joining together a great many small units. Its molecular mass runs into thousands.
A monomer is the small repeating unit from which the polymer is built.
Example: Polythene is a polymer; its monomer is ethene (CH₂=CH₂).
Other examples are polyvinyl chloride from vinyl chloride, and nylon-6,6 from hexamethylenediamine and adipic acid.
The process of joining monomers into a polymer is called polymerisation.

Q2. How are polymers classified on the basis of their source? Give one example of each.

Answer: • Natural polymers: found in plants and animals. Examples: starch, cellulose, proteins, natural rubber.
• Semi-synthetic polymers: natural polymers that have been chemically modified. Examples: cellulose acetate (rayon), cellulose nitrate.
• Synthetic polymers: made entirely in the laboratory or factory. Examples: polythene, nylon-6,6, bakelite, PVC, buna-S.

Q3. Classify polymers on the basis of molecular forces. Give one example of each.

Answer: • Elastomers: have weak intermolecular forces, so the chains can be stretched a long way and spring back. Example: buna-S, natural rubber.
• Fibres: have strong forces, mainly hydrogen bonding, so they are tough and thread-like with high tensile strength. Example: nylon-6,6, terylene.
• Thermoplastics: have forces in between. They soften on heating and harden on cooling, and can be remoulded again and again. Example: polythene, polystyrene, PVC.
• Thermosetting polymers: become permanently hard on heating because of cross-linking, and cannot be remoulded. Example: bakelite, melamine.

Q4. What is the difference between addition and condensation polymerisation?

Addition polymerisation Condensation polymerisation
Monomers add on repeatedly, with no loss of any molecule A small molecule such as water is eliminated at each step
Monomers contain a double or triple bond Monomers contain two functional groups
The polymer has the same empirical formula as the monomer The formula is different from that of the monomers
Example: polythene from ethene Example: nylon-6,6 from a diamine and a diacid

Q5. How is nylon-6,6 prepared? What type of polymer is it?

Answer: Preparation: Nylon-6,6 is made by heating hexamethylenediamine with adipic acid under high pressure and at a high temperature.
H₂N(CH₂)₆NH₂ + HOOC(CH₂)₄COOH → nylon-6,6 + H₂O
Type: It is a condensation polymer (water is eliminated) and a polyamide, since the units are joined by amide −CONH− links. It is classed as a fibre.
Why the name: Both monomers have six carbon atoms each, which is where the "6,6" comes from.
Uses: making sheets, bristles for brushes, and textile fabrics.

Q6. How is bakelite prepared? Why is it a thermosetting polymer?

Answer: Preparation: Bakelite is made by the condensation of phenol with formaldehyde in the presence of an acid or base catalyst.
The first product is a linear polymer called novolac. On heating novolac with more formaldehyde, cross-links form between the chains, giving the hard solid bakelite.
Why thermosetting: The chains become joined to one another by strong covalent cross-links in a three-dimensional network. Once this network has formed, heating cannot melt it again — the material simply chars.
So bakelite can be moulded only once, which is why it is called thermosetting.
Uses: electrical switches, plugs, handles of utensils, and combs.

Q7. What is vulcanisation of rubber? Why is it done?

Answer: Vulcanisation is the process of heating natural rubber with sulphur (about 3 to 5 percent) at 373 to 415 K.
Why it is done: Natural rubber by itself has several drawbacks — it is soft and sticky in summer, brittle in winter, has low tensile strength, and swells in organic solvents.
During vulcanisation, sulphur forms cross-links between the polymer chains at the sites of the double bonds. These cross-links stop the chains from sliding past one another.
Result: the rubber becomes harder, stronger, more elastic and is no longer affected much by temperature. Vulcanised rubber is used for tyres, where about 5 percent sulphur is added, while battery cases use as much as 30 percent.

Q8. What are copolymers? Give one example.

Answer: A copolymer is a polymer made from two or more different monomers, so that the chain contains more than one kind of repeating unit.
Example: Buna-S, made by copolymerising 1,3-butadiene with styrene in the ratio 3:1 in the presence of sodium.
Another example is Buna-N, made from 1,3-butadiene and acrylonitrile, which is resistant to oil and petrol.
Why copolymers are useful: By mixing monomers, properties can be tuned to get a material better than either homopolymer alone. Buna-S, for instance, is tougher than polybutadiene and is used in car tyres and floor tiles.

Q9. What are biodegradable polymers? Give two examples.

Answer: Biodegradable polymers are polymers that can be broken down by micro-organisms in the environment within a reasonable time. Ordinary plastics are not biodegradable and stay in the soil for hundreds of years, causing serious pollution.
Two examples:
1. PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate): a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid. It is used in speciality packaging, orthopaedic devices and in the controlled release of drugs.
2. Nylon-2-nylon-6: an alternating polyamide copolymer of glycine and aminocaproic acid, which is also biodegradable.

Q10. Write the monomers of the following polymers: (i) PVC (ii) Teflon (iii) Terylene (iv) Neoprene.

Answer: (i) PVC (polyvinyl chloride): monomer is vinyl chloride, CH₂=CHCl
(ii) Teflon: monomer is tetrafluoroethene, CF₂=CF₂
(iii) Terylene (Dacron): made from two monomers, ethylene glycol and terephthalic acid
(iv) Neoprene: monomer is chloroprene, which is 2-chloro-1,3-butadiene
Note that PVC, Teflon and Neoprene are addition polymers, while Terylene is a condensation polymer (a polyester).

Q11. Distinguish between low density polythene (LDP) and high density polythene (HDP).

Low density polythene (LDP) High density polythene (HDP)
Made at high pressure (1000-2000 atm) and 350-570 K with traces of oxygen Made at low pressure (6-7 atm) and 333-343 K with a Ziegler-Natta catalyst
Chains are highly branched Chains are linear, with little branching
Low density, chemically inert and flexible Higher density, harder and stronger
Uses: squeeze bottles, toys, insulation for electric wires Uses: buckets, dustbins, bottles, pipes

The difference comes down to the branching: branched chains cannot pack closely, so LDP is less dense and more flexible.

Q12. What is the Ziegler-Natta catalyst and what is its role?

Answer: The Ziegler-Natta catalyst is a mixture of triethylaluminium, Al(C₂H₅)₃, and titanium tetrachloride, TiCl₄.
Its role: It allows the polymerisation of ethene to be carried out at low pressure and low temperature, instead of the very high pressure needed otherwise.
More importantly, it controls the shape of the growing chain and gives a linear polymer with almost no branching. The chains can then pack closely together, producing high density polythene, which is much harder and stronger than LDP.
The catalyst is also used to make stereoregular polymers such as isotactic polypropene, which is why its discovery earned Ziegler and Natta the Nobel Prize.

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