Question

Write two main differences between order of reaction and molecularity of reaction.

Answer 1

First we need to know about what is an elemental and a complex reaction. So, elementary reaction is the one which takes place in one step only whereas a complex reaction is the one which takes place in two or more steps. The sequence of steps in a complex reaction are termed as the mechanism of the reaction therefore each step in a mechanism is an elemental reaction.

Complete answer:
Two differences between order of reaction and molecularity of reaction are:

Order of reaction	Molecularity of reaction
1. The order of reaction is defined as the sum of the powers to which the reactant concentrations terms are raised in the rate law. It can be a whole number, a fraction or even zero.	1. The molecularity of an elementary reaction is defined as the number of reacting molecules taking part in the reaction or it is equal to the sum of the reacting molecules in a reaction.it is always a whole number can never be zero.
2. It is an experimental property and cannot be determined by just looking at the reaction.	2. The molecularity of the reaction can be determined by just looking at the reaction as it is a theoretical concept.

Note:
The molecularity of the reaction is always determined from the rate determining step. The slowest step in the reaction is the rate determining step. Molecularity of a reaction cannot be greater than three as it is difficult for three molecules to mutually combine with each other.