PSEUDO–ORDER REACTIONS

A reaction in which one of the reactants is present in a large excess shows an order different from the actual order. The experimental order which is not the actual one is referred to as the pseudo order. Since for elementary reactions molecularity and order are identical, pseudo-order reactions may also be called pseudo molecular reactions.

Let us consider a reaction

A + B ⎯⎯→ products

in which the reactant B is present in a large excess. Since it is an elementar reaction, its rate law can be written as

rate = k[A] [B]

As B is present in large excess, its concentration remains practically constant in the course of reaction. Thus the rate law can be written as  Rate = k′[A] where the new rate constant k′= k[B]. Thus the actual order of the reaction is second-order but in practice it will be first-order. Therefore, the reaction is said to have a pseudo-first order.