Question

Define and explain the term 'molecularity of a reaction' with suitable example.

Answer 1

Molecularity of a reaction is defined as the number of reactant molecules (or atoms or ions) taking part in an elementary reaction.
(a) Molecularity of a reaction is always an integer.
(b) It doesn't change with experimental conditions.
(c) The molecularity of a reaction cannot have a fractional or zero values.
(d) The minimum value of the molecularity is one.
The molecularity of a reaction can be explained by considering the following example :
Consider a reaction, i.e.,
$N_2{O}_5\left(g\right)⟶2N{O}_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)$
In this reaction, only one molecule is taking part in the reaction, therefore the rate law expression for this reaction is :
Rate $=k\left[N_2{O}_5\right]$
Hence, the reaction is unimolecular and first order.
Similarly, a reaction can be bimolecular or trimolecular depending on the number of molecule taking part in the reaction.