Cbse Class 12 Chemistry Chapter 15 Ncert Solutions

Exercise 1 ( Page No. : 435 )

Exercise 2 ( Page No. : 438 )

• Q1	Explain the terms polymer and monomer.
  Ans:	Polymers are high molecular mass macromolecules composed of repeating structural units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer, various monomer units are joined by strong covalent bonds. Polymers can be natural as well as synthetic. Polythene, rubber, and nylon 6, 6 are examples of polymers. Monomers are simple, reactive molecules that combine with each other in large numbers through covalent bonds to give rise to polymers. For example: ethene, propene, styrene, vinyl chloride.
  Q2	What are natural and synthetic polymers? Give two examples of each type.
  Ans:	Natural polymers are polymers that are found in nature. They are formed by plants and animals. Examples include protein, cellulose, starch, etc. Synthetic polymers are polymers made by human beings. Examples include plastic (polythene), synthetic fibres (nylon 6, 6), synthetic rubbers (Buna - S).
  Q3	Distinguish between the terms homopolymer and copolymer and give an example of each.
  Ans:	Homopolymer Copolymer The polymers that are formed by the polymerization of a single monomer are known as homopolymers. In other words, the repeating units of homopolymers are derived only from one monomer. For example, polythene is a homopolymers of ethene. The polymers whose repeating units are derived from two types of monomers are known as copolymers. For example, Buna - S is a copolymer of 1, 3-butadiene and styrene.
  Q4	How do you explain the functionality of a monomer?
  Ans:	The functionality of a monomer is the number of binding sites that is/are present in that monomer. For example, the functionality of monomers such as ethene and propene is one and that of 1, 3-butadiene and adipic acid is two.
  Q5	Define the term polymerisation.
  Ans:	Polymerization is the process of forming high molecular mass (103 - 107 u) macromolecules, which consist of repeating structural units derived from monomers. In a polymer, various monomer units are joined by strong covalent bonds.
  Q6	Is ( NH — CHR — CO ) n a homopolymer or copolymer?
  Ans:	( NH — CHR — CO ) n  is a homopolymer because it is obtained from a single monomer unit, NH 2  - CHR - COOH.
  Q7	In which classes, the polymers are classified on the basis of molecular forces?
  Ans:	On the basis of magnitude of intermolecular forces present in polymers, they are classified into the following groups: (i) Elastomers (ii) Fibres (iii) Thermoplastic polymers (iv) Thermosetting polymers
  Q8	How can you differentiate between addition and condensation polymerisation?
  Ans:	Addition polymerization is the process of repeated addition of monomers, possessing double or triple bonds to form polymers. For example, polythene is formed by addition polymerization of ethene. Condensation polymerization is the process of formation of polymers by repeated condensation reactions between two different bi-functional or tri-functional monomers. A small molecule such as water or hydrochloric acid is eliminated in each condensation. For example, nylon 6, 6 is formed by condensation polymerization of hexamethylenediamine and adipic acid.
  Q9	Explain the term copolymerisation and give two examples.
  Ans:	The process of forming polymers from two or more different monomeric units is called copolymerization. Multiple units of each monomer are present in a copolymer. The process of forming polymer Buna-S from 1, 3-butadiene and styrene is an example of copolymerization. Nylon 6, 6 is also a copolymer formed by hexamethylenediamine and adipic acid.
  Q11	Define thermoplastics and thermosetting polymers with two examples of each.
  Ans:	Thermoplastic polymers are linear (slightly branched) long chain polymers, which can be repeatedly softened and hardened on heating. Hence, they can be modified again and again. Examples include polythene, polystyrene. Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.
  Q12	Write the monomers used for getting the following polymers. (i) Polyvinyl chloride (ii) Teflon (iii) Bakelite
  Ans:	1) Polyvinyl chloride (PVC) - The monomeric unit present is vinyl chloride (CH2 =CH –Cl). PVC is used in the manufacture of rain coats, curtain cloths, hand bags, toys, artificial flooring, gramophone records and as a good insulating material in wires and other electrical goods. 2) Teflon (PTFE) - The monomeric unit present is tetrafluoroethene (n F2C = CF2). Teflon is used as a material resistant to heat and chemical attack. It is also used for making gaskets, pump packings, valves, oil seals, non -lubricated bearings. 3) Bakelite - The monomeric unit present is formaldehyde (HCHO) and phenol (C6H5OH). Bakelite is used for making glue for binding laminated wooden planks and in varnishes, combs, fountain pens, electrical switches.
  Q13	Write the name and structure of one of the common initiators used in free radical addition polymerisation.
  Ans:	For polymerization reaction, an initiator is required. The initiator may be an cation, anion or free radical. So the type of polymerization in which free radical is used as an initiator is known as free radical addition polymerization. The common free radical initiator used is benzoyl chloride. The structure of benzoyl chloride is as follows:
  Q14	How does the presence of double bonds in rubber molecules influence their structure and reactivity?
  Ans:	Natural rubber is a linear cis-1,4- polyisoprene in which double bonds are present between C2 and C3 of isoprene units. The cis configuration about double bonds does not allows the chains to come closer for effective intermolecular attraction due to weak intermolecular attractions. Therefore, the natural rubber has coiled structure and shows elasticity and is non crystalline. On the other hand, all trans configurations occur in gutta-percha (synthetic rubber) which is trans-polyisoprene. These zig-zag chains pack more closely in gutta-percha and hence it is highly crystalline, non elastic, more hard and brittle than rubber.
  Q15	Discuss the main purpose of vulcanisation of rubber.
  Ans:	Natural rubber though useful has some problems associated with its use. These limitations are discussed below: 1. Natural rubber is quite soft and sticky at room temperature. At elevated temperatures (> 335 K), it becomes even softer. At low temperatures (< 283 K), it becomes brittle. Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of 283 K-335 K. 2. It has the capacity to absorb large amounts of water. 3. It has low tensile strength and low resistance to abrasion. 4. It is soluble in non-polar solvents. 5. It is easily attacked by oxidizing agents. Vulcanization of natural rubber is done to improve all these properties. In this process, a mixture of raw rubber with sulphur and appropriate additive is heated at a temperature range between 373 K and 415 K. This is a slow process, therefore some additives like zinc oxide etc are used to accelerate the proess. During this process, sulphur cross links are formed which makes rubber hard, tough with greater tensile strength .The vulcanized rubber has excellent elasticity, low water absorption, resistance to oxidation and organic solvents.
  Q16	What are the monomeric repeating units of Nylon-6 and Nylon-6, 6?
  Ans:	The monomeric repeating unit of nylon 6 is [NH – (CH2)5 – CO], which is derived from Caprolactam. The monomeric repeating unit of nylon 6, 6 is [NH – (CH2)6 - NH  – CO – (CH2)4 – CO], which is derived from hexamethylene diamine and adipic acid.
  Q17	Write the names and structures of the monomers of the following polymers: (i) Buna-S  (ii) Buna-N (iii) Dacron  (iv) Neoprene
  Ans:	Polymer Monomer Structure of monomer i Buna-S 1, 3-butadiene CH2 = CH – CH = CH2 Styrene C6H5CH = CH2 ii Buna-N 1, 3-butadiene CH2 = CH – CH = CH2 Acrylonitrile CH2 = CH – CN iii Neoprene Chloroprene iv Dacron Ethylene glycol HOH2C –CH2OH Terephthalic acid 1) Buna-S (styrene butadiene rubber) - It is obtained by the polymerization of Buta-1, 3 -diene and styrene in the ratio of 3:1 in the presence of sodium. It is used for making automobile tyres and footwear.  2) Buna-N (Nitrile rubber)- It is obtained by polymerization of 1,3-butadiene and acrylonitrile in the presence of a peroxide catalyst. It is used for making oil seals, manufacture of hoses and tank linings. 3) Dacron – It is obtained by the polymerization of ethylene glycol and terphthalic acid. 4) Neoprene –It is prepared by the free radical polymerization of chloroprene. It is used as insulator, for making belts, gaskets, hoses etc.
  Q18	Identify the monomer in the following polymeric structures. (i) (ii)
  Ans:	(i) The monomers of the given polymeric structure are decanoic acid [HOOC –(CH2)8 – COOH] and hexamethylene diamine [H2N(CH2)6 NH2 ] . (ii) The given structure is the polymerization product of N2-methyl-1,3,5-triazine-2,4,6-triamine & formaldehyde (HCHO)
  Q19	How is dacron obtained from ethylene glycol and terephthalic acid?
  Ans:	Dacron (terylene) is obtained by the polymerization of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst. The reaction is as follows: It is used in blending with cotton or wool fibres, as glass reinforcing materials in safety helmets, for making magnetic recording tapes.
  Q20	What is a biodegradable polymer? Give an example of a biodegradable aliphatic polyester.
  Ans:	A polymer that can be decomposed by bacteria is called a biodegradable polymer. The biodegradable polymer are the polymers which are degraded by the micro-organism within a suitable period so that biodegradable polymers and their degraded products do not cause any serious effects on the environment. They degrade by enzymatic hydrolysis and oxidation. The decomposition reactions involves hydrolysis (either enzymatically induced or by non –enzymatic mechanism) to non- toxic small molecules which can be metabolized or excreted from the body. The common examples of aliphatic biodegradable polymers are polyglycolic acid(PGA), Polyhydroxy butyrate (PHB), Polyhydroxy butyrates-co-beta hydroxyl valerate( PHBV), Polycaprolactone(pcl), Nylon-2-nylon-6. These polymers are used mainly for medical goods such as surgical sutures, tissues in growth materials, for controlled drug release, plasma substitutes etc. They are also used in agriculture materials, such as films, seed coatings, fast food wrappers, personal hygiene products etc. Poly-β-hydroxybutyrate-CO-β-hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.

Cbse Class 12 Chemistry Chapter 15 Ncert Solutions

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