The branch of chemistry which deals with the rate of chemical reactions, the factors affecting the rate of reactions and the mechanism of the reaction is called chemical kinetics.
Chemical reaction which completes in less than Ips (10-12s) time is known as fast reaction. It is practically impossible to measure the speed of such reactions, e.g., ionic reactions, organic substitution reactions.
2. Slow reactions Chemical reactions which completes in a long time from some minutes to some years are called slow reactions. e.g., rusting of iron. transformation of diamond etc.
3. Moderately slow reactions
Chemical reactions which are intermediate between slow and fast reactions are called moderately slow reactions.
Rate of a chemical reaction is the change in the concentration of any one of the reactants or products per unit time. It is expressed in mol L-1 s-1 or Ms-1 or atm time-1 units.
Rate of reaction= (decrease/increase in the concentration of reactant/product/time taken)
This rate of reaction is known as average rate of reaction (rav). rav can be calculated by dividing the concentration difference by the time interval.
Rate of a chemical reaction at a particular moment of time is known as instantaneous rate of reaction.
(i) pH measurement,
(ii) change in optical activity,
(iii)change in pressure,
(iv) change in conductance.
Slowest step of a reaction is called rate determining step by van't Hoff.