POLYMERS

Polymers are macro-sized molecules with relatively high molecular mass. They are formed by joining together large number of small molecules. The process of formation of polymers from their starting materials is called polymerisation and the small molecules which combine with each other are termed as monomers.

Polymers formed from one kind of monomers is called homopolymer while those formed from more than one kind of monomer units is called copolymer or mixed polymer.

Ethylene or ethene is the monomer unit of polyethylene. It is a homopolymer.
n CH2= CH2 = (-CH2CH2-)n

Nylon-66 is a copolymer made from Hexamethylene diamine and Adipic acid.
nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH =
- [NH-(CH2)6-NH-CO-(CH2)4-CO-]n + nH2O


Classification of Polymers.
Polymers are classified in different ways.

1. Classification based on source.On the basis of source polymers are classified into two types. They are natural polymers and synthetic polymers.

Natural polymers are obtained from nature. They are cellulose, starch, natural rubbers, proteins etc. Natural rubber is a polymer of isoprene. It is obtained from the latex of rubber tree. Polymers such as polysaccharides (starch, cellulose), proteins and nucleic acid which control various life processes in plants and animals are also called as biopolymers.

Synthetic polymers are artificially prepared in the laboratory. They are man-made polymers. Examples are polyethene, polyvinyl chloride (PVC), nylon, terylene, bakelite etc. They find diverse applications in man's daily life.

2. Classification based on Structure. Bases on structure, polymers are classified as:

Linear polymers.In these, the monomeric units are linked together to form long straight chains. They are closely packed and hence have high densities, high melting point and high tensile strength. eg; nylon, polyethene.

Branched chain polymers. The monomer units are linked together to form a main chain. Side chains of different lengths arises from the main chain thus forming branches. These polymers are irregularly packed and have low tensile strength and melting points. eg; amylopectin, glycogen.

Cross Linked polymers.In these the monomer units are linked together to form a three dimensional network. These cross-linked polymers are hard, rigid and brittle. eg; melamine, bakelite.

3. Classification Based on Synthesis. On this basis, polymers can be classified as addition polymers and condensation polymers.

Addition polymers.It is formed by the direct addition of repeated monomers. eg; polyethylene, polypropylene, PVC etc.

Condensation polymers.They are formed from two or more kind of monomers. Each monomer usually contains two functional groups. During the formation of condensation polymers simple molecules such as H2O is lost. eg; Nylon 66, Dacron.

THERMOCHEMISTRY

Thermochemistry is a branch of physical chemistry which deals with the energy changes accompanying chemical transformations. This is also known as chemical energetics and is based on the first law of thermodynamics.


Chemical reactions are accompanied by the evolution or absorption of heat.

Endothermic Reactions.

In these chemical reactions, heat is absorbed by the reactants. It is possible when the bond energy of reactants is greater than that of the products.
At constant pressure, Hp >HR
At constant volume, Ep>ER

Chemical equation can be written as;
C(s) + H2O(g) = CO(g) + H2O(g) ; Enthalpy change= +31.4 kcal


Exothermic Reactions.

In these chemical reactions heat is evolved as a product of the reaction. It occurs when the bond energy of the reactants is less than that of the products.
At constant pressure, Hp < HR
At constant volume, Ep< ER

Chemical equation can be written as;
NaOH(aq) +HCl (aq) = NaCl(aq) +H 2O(l); Enthalpy change = -13.7 kcal

ENTROPY

Entropy is a thermodynamic state quantity which is a measure of randomness or disorder of the molecules of the system.
Entropy is represented by the symbol 'S'. The entropy is a state function. It depends only on the initial and final states of the system. It is expresed in terms of calories per degree, i.e; cal K^-1

Change in entropy = S final - S initial.
When S final > S initial, the change in entropy is positive.

For a chemical reaction, S = Sproducts - Sreactants

Spontaneity in terms of Entropy Change.

For a spontaneous process in an isolated system, the change in entropy is positive. If the system is not isolated, the entropy change of both the system and the surroundings are taken into consideration. The total entropy change (S total) is the sum of the change in entropy of the system (S system) and the change in entropy of the surroundings (S surroundings).

S total = S system + S surroundings

For a spontaneous process, S total must be positive. During spontaneous process, the entropy of the system goes on increasing and becomes a maximum till no more increase in entropy of the system is possible. The system attains an equilibrium. i.e; Change in entropy = 0 (at equilibrium for an isolated system)
If S total is negative, the direct process is non-spontaneous and the reverse process is spontaneous.