Consecutive Reactions

The reactions in which the final product is formed through one or more intermediate steps are called consecutive reactions. These are also known as sequential reactions. In such reactions the product formed in one of the elementary reactions acts as the reactant for some other elementary reaction. Various step reactions can be written for the overall reaction as shown below

In the above reaction the product C is formed from the reactant A through intermediate B. In this reaction each stage has its own different rate constants k₁ for the first step and k₂ for the second step.

The net or overall rate of reaction depends upon the magnitude of these two rate constants. The initial concentration and concentration after time tare shown below each species in above reaction under consideration. It is clear that

During the course of the reaction the concentration of A, B and C vary as shown in the Fig. 1.2

From the Fig 1.2 it is clear that the concentration of A decreases exponentially, the concentration of B first increases and then decreases and that of C increases (from zero) with time and finally attains the value equal to [A]₀ (initial concentration A) when all A has changed into the final product C.