In chemical kinetics, molecularity refers to the number of reactant molecules involved in an elementary step of a reaction. While simple reactions may involve only one or two molecules, complex reactions consist of multiple steps, each with its own molecularity. Understanding molecularity is crucial in predicting reaction rates and understanding reaction mechanisms.
This topic explains the molecularity of complex reactions, its significance, and how it differs from the overall reaction order.
What Is Molecularity?
Molecularity is defined as the number of reactant molecules that collide in a single elementary step of a reaction. It can be:
- Unimolecular (1 molecule) – Example: Decomposition of ozone (O₃ → O₂ + O)
- Bimolecular (2 molecules) – Example: H₂ + I₂ → 2HI
- Termolecular (3 molecules) – Example: 2NO + O₂ → 2NO₂
Difference Between Molecularity and Reaction Order
| Feature | Molecularity | Reaction Order |
|---|---|---|
| Definition | Number of molecules in an elementary step | Sum of exponents in the rate law equation |
| Type of reaction | Applies only to elementary steps | Applies to the overall reaction |
| Values | Always an integer (1, 2, or 3) | Can be fractional or zero |
Molecularity of a Complex Reaction
A complex reaction consists of multiple elementary steps. Since molecularity applies only to elementary reactions, we must analyze each step separately.
Example of a Complex Reaction
Consider the reaction:
This reaction occurs in two steps:
- Step 1: NO_2 + F_2 rightarrow NO_2F + F (Bimolecular)
- Step 2: NO_2 + F rightarrow NO_2F (Bimolecular)
Each step has molecularity = 2, but the overall reaction does not have a defined molecularity. Instead, the slowest step determines the reaction rate.
How to Determine Molecularity?
- Break the reaction into elementary steps.
- Identify the number of reactant molecules involved in each step.
- Label each step as unimolecular, bimolecular, or termolecular.
Example Problem
Given the reaction:
If it occurs in two steps:
- Step 1: A + B rightarrow X (Bimolecular)
- Step 2: X + A rightarrow C + D (Bimolecular)
Each step is bimolecular, meaning the reaction proceeds through two elementary steps, each involving two reactant molecules.
Why Molecularity Matters
- Predicts reaction mechanisms
- Helps derive rate laws
- Explains why some reactions occur faster than others
The molecularity of a complex reaction refers to the number of reactant molecules in each elementary step. Since complex reactions consist of multiple steps, each step has its own molecularity. Understanding this concept helps in analyzing reaction mechanisms and predicting reaction rates accurately.