A pseudo first order reaction is a reaction that is truly second order but can be approximated to be first order under special circumstances

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

For the above-mentioned reaction, the rate equation can be given as,

Rate = k [CH3COOC2H5] [H2O]

Here we see that the concentration ofwateris very high and thus does not change much during the course of the reaction. Thus the rate of the reaction can be said to be independent of the change in the concentration of H2O. Putting the term for change of concentration of water in the above reaction to be zero we can write the effective rate equation as,

Rate = k [CH3COOC2H5]

Here, the term kt takes into account the value of the constant concentration of water.

where K = K’ [H2O]

We see that the reaction behaves as a first-order reaction. Such reactions are termed as pseudo-first order reactions.