Pseudo Order Reactions

What is pseudo-order reaction? Give one example of pseudo first order reaction.

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 → Product
in which the reactant B is present in a large excess. Since it is an elementary 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.

Hydrolysis of an ester is an example of pseudo order reaction. Ethyl acetate upon hydrolysis in aqueous solution using a mineral acid as catalyst forms acetic acid and ethyl alcohol.
CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
excess

Here a large excess of water is used and the rate law can be written as
rate = k [CH₃COOH] [H₂O] = k′[CH₃COOH]
The reaction is actually second-order but in practice it is found to be first-order. Thus it is a pseudo-first order reaction.