The order of a reaction is occasionally changed by conditions. Consider a chemical reaction between two substances where one reactant is there in large excess. During the hydrolysis of 0.01 mol of ethyl acetate with 10 ml of water, amounts of the various constituents at the beginning (t = 0) and complexion (t) of the reaction are given as under
                               
 

The concentration of water does not get altered much during the course of the reaction.

So, in the rate equation

Rate = k’ [CH₃COOC₂H₅] [H₂O]

The term [H₂O] can be taken as constant. The equation, thus, becomes

Rate = k [CH₃COOC₂H₅]

Where k = k’ [H₂O]

And the reaction behaves as first order reaction. Such reactions are called pseudo first order reaction.

Conversion of cane sugar is one more pseudo first order reaction.

 

Rate = k [C₁₂H₂₂O₁₁]