Question

Explain pseudo first order reaction by taking the example of hydrolysis of ethyl acetate.

Answer 1

Hydrolysis of ethyl acetate is an example of psuedo first order reaction.
$C{H}_{3}COO{C}_2{H}_5+H_{2}O\to C{H}_{3}COOH+C_2{H}_5-OH$
The rate of the reaction is given by the expression
rate $=k\left[C{H}_{3}COO{C}_2{H}_5\right]\left[H_{2}O\right]$
But water is present in large excess so its concentration remains constant over a period of time.
$k\left[H_{2}O\right]=k^{\prime }=$ constant.
The rate of the reaction is given by the expression
rate $=k^{\prime }\left[C{H}_{3}COO{C}_2{H}_5\right]$
Thus when one of the reactants is taken in large excess, a second order reaction will behave as a first order reaction. Such reactions are known as psuedo first order reactions.