Question

Explain pseudo-first-order reaction with an example.

What is a Pseudo first-order reaction?

Answer 1

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. Let us consider the acid hydrolysis of an ester,

\[\ce{CH3COOCH3_{(aq)} + H2O_{(l)} ->[H^+] CH3COOH_{(aq)} + CH3OH_{(aq)}}\]

Rate = k [CH₃COOCH₃] [H₂O]

If the reaction is carried out with a large excess of water, there is no significant change in the concentration of water during hydrolysis. i.e., the concentration of water remains almost constant.

Now we can define k [H₂O] = k'

∴ The above rate equation becomes
Rate = k' [CH₃COOCH₃]

Thus it follows first order kinetics.