Introduction

Three important characteristics of a chemical reaction are its extent, its feasibility and its rate. In Standard XI we learnt how equilibrium constants predict the extent of a reaction, and in Unit 3 of this textbook we saw how thermodynamic properties such as the change in entropy or enthalpy tell us whether, under a given set of conditions, the reaction represented by a chemical equation occurs or not. Neither of these, however, says anything about how fast the reaction happens. Chemical kinetics is the branch of chemistry which deals with the rates of chemical reactions and the factors that affect them.

A chemist wants to know the rates of reactions for different reasons. Firstly, the study of reaction rates helps us predict how rapidly a reaction mixture approaches equilibrium. Secondly, it gives information on the mechanism of chemical reactions.

A number of reactions occur as a sequence of elementary steps; this sequence of steps constitutes the mechanism of the reaction.