Question

Define pseudo first order reaction.

Answer 1

The reactions that have higher order true rate law but are found to behave as first order are called pseudo first order reactions.

eg - $$\underset{(\mathrm{aq})}{\mathrm{CH}{\phantom{A}}_3\mathrm{COOCH}{\phantom{A}}_3}+\underset{(\mathrm{l})}{\mathrm{H}{\phantom{A}}_2\mathrm{O}}-\underset{(\mathrm{aq})}{\mathrm{CH}{\phantom{A}}_3\mathrm{COOH}}+\underset{(\mathrm{aq})}{\mathrm{CH}{\phantom{A}}_3\mathrm{OH}}$$

In the above example of acid hydrolysis of methyl acetate, the reaction is expected to have second order kinetics (according to the rate law equation). However, experimentally the reaction is found to follow first order kinetics because water as a solvent is present in such a large excess that its concentration remains constant.

rate = k [CH₃COOCH₃] × [H₂O]

∴ rate = k [CH₃COOCH₃]  .......(∵ [H₂O] = constant)

Thus, this bimolecular reaction which appears of second order is actually pseudo first order reaction.