In the context of chemical kinetics, the terms “order of reaction” and “molecularity of reaction” refer to different aspects of the rate equation that describes the rate at which a chemical reaction occurs.

1. Order of Reaction: The order of reaction represents the mathematical relationship between the concentration of reactants and the rate of the reaction. It determines how the rate of a reaction is influenced by changes in the concentration of the reactants. The order of reaction is determined experimentally and can be an integer, fractional, or even zero.

For a generic chemical reaction: A + B → Products

The rate equation for this reaction can be expressed as: rate = k[A]^m[B]ⁿ

In this equation, [A] and [B] represent the concentrations of the reactants A and B, respectively. The exponents ‘m’ and ‘n’ represent the orders of reaction with respect to A and B, respectively. The sum of the exponents (m + n) gives the overall order of the reaction.

2. Molecularity of Reaction: The molecularity of a reaction refers to the number of reactant particles that are involved in the elementary step of a chemical reaction. It provides information about the complexity of the reaction and helps classify reactions based on the number of molecules colliding in a single step.

Molecularity can have three categories:

a) Unimolecular Reaction: A reaction in which a single reactant molecule undergoes a transformation in an elementary step. For example: A → Products

b) Bimolecular Reaction: A reaction involving the collision of two reactant molecules leading to a chemical transformation. For example: A + B → Products

c) Termolecular Reaction: A reaction involving the simultaneous collision of three reactant molecules leading to a chemical transformation. Termolecular reactions are relatively rare due to the low probability of three particles simultaneously colliding. For example: A + B + C → Products

It’s important to note that the molecularity of a reaction is a theoretical concept that describes the elementary steps of a reaction mechanism, while the order of reaction is determined experimentally and reflects the dependence of the reaction rate on the reactant concentrations.