what is pseudo first order reaction? give one example it.

These are the reactions that appear to be second order in nature but are approximated as a first-order of reaction on close analysis. For example, a second-order of reaction is given by the equation,

A + B  —-> C + D

This reaction is dependent upon the concentrations of both A and B but one of the components is present in large excess and thus its concentration hardly changes as the reaction proceeds. So, if component B is in large excess and the concentration of B is very high as compared to that of A, the reaction is considered to be pseudo-first order reaction with respect to A and if component A is in large excess and the concentration of A is very high as compared to that of B, the reaction is considered to be pseudo first order with respect to B. We can understand this with the help of the following example.

Consider the hydrolysis of ethyl acetate, during the hydrolysis, the concentration of ethyl acetate is 0.02 mol/L whereas the amount of water is 20 mol/L as the process of hydrolysis involves a large amount of water. Let us say, the process of hydrolysis attains completion in time t. The reaction can be represented as

  CH₃COOC₂H₅    +     H₂O  —–>    CH₃COOH    +       C₂H₅OH