__UNIT 4 CHEMICAL KINETICS__- Chemical kinetics is the study of chemical reactions with respect to reaction rates, effect of various variables, rearrangement of atoms and formation of intermediates.

- The rate of reaction is concerned with decrease in concentration of reactants or increase in the concentration of products per unit time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time.

- A number of factors such as temperature, concentration of reactants, catalyst, etc., affect the rate of a reaction.

- Mathematical representation of the rate of a reaction is called the rate law.

**Order of a reaction**with respect to a reactant is the power of its concentration which appears in the rate law equation. The order of a reaction is the sum of all such powers of concentration of terms for different reactants.

**Rate constant**is the proportionality factor in the rate law or its integrated rate equation.

**Molecularity**is defined only for an elementary reaction. Its values are limited from 1 to 3 whereas order can be 0, 1, 2, 3 or even a fraction.

- Temperature dependence of rate constants is described by Arrhenius equation.

- Activation energy is given by the energy difference between activated complex and the reactant molecules.

- According to collision theory, another factor called steric factor which refers to the orientation of molecules which collide, is important and contributes to effective collisions.