Question

What is molecularity of a chemical reaction? Explain pseudo unimolecular reaction with an example.

Answer 1

The molecularity of an elementary reaction is the number of reactant molecules taking part in the reaction.
Thus, when only one molecule participates, the reaction is unimolecular.
Similarly, when two molecules participate, the reaction is bimolecular.
A Pseudo unimolecular reaction is a bimolecular reaction that follows first-order kinetics.
Consider the hydrolysis of ethyl acetate.
$C{H}_{3}COO{C}_2{H}_5\left(aq\right)+H_{2}O\left(l\right)\to C{H}_{3}COOH\left(aq\right)+C_2{H}_{5}OH\left(aq\right)$

Water participates in this reaction and appears in the stoichiometric equation. But since water is present in large excess, its concentration remains unchanged during the progress of the reaction. Hence, the hydrolysis of ethyl acetate is an example of a pseudo unimolecular reaction.