TABLE OF CONTENTS Chemical kinetics is the branch of physical chemistry that deals with a study of the speed of chemical reactions. Such studies also enable us to understand the mechanism by which the reactions occur. This reveals not only the influence of different factors on the progress of the reaction, but it also throws light on the mechanism through which reactant molecules are transformed into products. The knowledge of the rate of reactions is very valuable for the success of an industrial process to give maximum yield.
The rate of reaction, that is the velocity of a reaction, is the amount of chemical change occurring per unit time. The rate is generally defined as the decrease in the concentration of the reactant or an increase in the concentration of the products per unit time. Mathematically, the rate of reaction can be expressed as
Consider a simple hypothetical reaction of the type
The rate of the reaction at any given time will be given by
where [A] and [B] indicate the concentration of A and B, respectively.
The rate of reaction is invariably a positive quantity. The negative sign merely indicates that the concentration of the reactant is falling with the increase in time, whereas the positive sign indicates the increase in concentration of the product with the increase in time.
The numerical value of 
can be obtained from a plot of the concentration of a reactant (or product) versus time (Fig. 5.1).
