The order of reaction refers to the relationship between the rate of a chemical reaction and the concentration of the species taking part in it. In order to obtain the reaction order, the rate expression of the reaction in question must be obtained.
The molecularity of a reaction is defined as the number of reacting molecules which collide simultaneously to bring about a chemical reaction. In other words, the molecularity of an elementary reaction is defined as the number of reactant molecules taking part in the reaction.
Condition for which order and molecularity of a reaction are equal
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.