Explain pseudo first order reaction with an example.

Explanation

Pseudo first order reaction :
(i) Kinetic study of a higher order reaction is difficult to follow, for example, in a study of a second order reaction involving two different reactants , the simultaneous measurement of change in the concentration of both the reactants is very difficult.
(ii) To overcome such difficulties, A second order reaction can be altered to a first order reaction by taking one of the reactant in large excess, such reaction is called pseudo first order reaction.
(iii) Let us consider the acid hydrolysis of an ester,
`CH_(3)COOCH_(3(aq))+H_(2)O_((l))overset(H^(+))rarr CH_(3)COOH_((aq))+CH_(3)OH_((aq))`
Rate `= k[CH_(3)COOCH_(3)][H_(2)O]`
(iv) If the reaction is carried out with the large excess of water, there is no significant change in the concentration of water during hydrolysis, i.e., concentration of water remains almost a constant.
(v) Now, we can define `k [H_(2)O]=k'`, Therefore the above rate equation becomes
Rate `= k' [CH_(3)COOCH_(3)]`
(vi) Thus it follows first order kinetics.