Question

In what condition order and molecularity of the reaction becomes equal?

Answer 1

Order of the reaction and the molecularity of the reaction are two separate terms but they both depend directly or indirectly on the reactant species of a chemical equation. Number of coefficients of the reacting species and the number of reacting species itself is taken into consideration.

Complete answer:
So, for the better understanding of this question, first we need to familiarise ourselves with both the terms i.e. ‘Order of the reaction’ and the ‘Molecularity of the reaction’.
So, generally Order of the reaction can be defined as the power dependence of the rate on the concentration of the reactants. That is the rate of reaction is determined by the concentrations of the reactant raised to the power of their coefficients.
So, now let’s know the condition in which order and molecularity of the reaction becomes equal.
So, for the order of the reaction and the molecularity of the reaction to become equal, the reaction must be an elementary reaction.
Let’s take an example:
$A + B \to \text{Product}$
Then the rate of equation: $k = {\left[ A \right]^1}{\left[ B \right]^1}$
Order of the reaction is sum of the coefficients that will be $2$
Also, the number of reacting species i.e. the molecularity of the reaction is $2$
So, in this case both the order of the reaction and the molecularity of the reaction is the same.

Note:
On the other hand the molecularity of the reaction can be defined as the number of reacting species in the chemical reaction i.e. the number of reactants that interact with each other simultaneously to bring about the chemical reaction.