For order and molecularity of a reaction to become equal, the reaction must be elementary reactions. For example, consider the below reaction:
A + B → Products
Rate equation = [A]1 [B]1
The order of reaction is the sum of the coefficient of rate equation.
Therefore, the order of reaction = 2
And molecularity = 2
Few things to know about the elementary reaction:
- The elementary reaction is the sum of the overall reaction equation.
- For elementary reactions, molecularity is derived at every step.