Elementary and Complex Reactions

Elementary Reactions

Simple reactions are those, which involve a single elementary reaction. The rate law can be written from the stoichiometry of the chemical equation.

Elementary reactions are classified according to their molecularity.

1. Unimolecural
   
2. Bimolecular
   

Complex Reaction

For complex reactions, to account for the kinetic data, reaction mechanisms are formulated. Formulation of complex mechanism involves intermediates. Therefore, the mechanism formulated can be tested with further observations of the intermediates. Then, the rate law is predicted from the mechanism. If the predicted rate law matches the experimental rate law, then the hypothesized mechanism can be accepted as the most probable mechanism of reaction.

Reactions Involving Two or Three Steps Only

Series reaction

A complex reaction involving two series reactions can be of the type,

A reacts to give B, which is then transformed to C. Both the elementary reactions (1) and (2) exhibit first - order kinetics, that is,

For this reaction,

[A] = [A]_o exp (-k₁t)

It is difficult to derive the concentrations of [B] and [C] and will not be discussed.

If k₁ is very much less than k₂ that is k₁ << k₂ then the intermediate B is transformed into the product C as soon as it is formed. In such case,

Parallel Reaction

A parallel reaction is of the following type in which a reactant A transforms itself to two different products B and C.

The rate expressions for the reactant and the products are,

The solution to these equations are as follows

This shows that B and C are formed in a constant ratio