Explain pseudo first order reaction with an example.

Explanation

(i) Kinetic study of a a higher order reaction is difficult to follow, for example, in a study , of a second order reaction a study of a second order reaction involving two different reactants , the simultaneous measurement of change in the concentration of both 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 is consider the acid hydrolysis of an ester,
` CH_(3)COOCH_(3(aq)) + H_(2)O_(l) overset(H^(+)) to 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 concenration 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 is follows first order kinetics.