With the help of an example,explain what is meant by a pseudo first order reaction.

(N/A) reaction that is actually of higher order but follows first order kinetics under certain conditions is called a pseudo first order reaction.
This usually happens when one of the reactants is present in a large excess.
Example: The acid-catalyzed hydrolysis of ethyl acetate $(CH_3COOC_2H_5 + H_2O \xrightarrow{H^+} CH_3COOH + C_2H_5OH)$.
In this reaction,water is present in a large excess,so its concentration remains practically constant throughout the reaction.
The rate law is given by: $Rate = k[CH_3COOC_2H_5][H_2O]$.
Since $[H_2O]$ is constant,the rate becomes $Rate = k'[CH_3COOC_2H_5]$,where $k' = k[H_2O]$.
Thus,the reaction behaves as a first order reaction.