Collision Theory | Chemical Kinetics Class 12

Collision Theory of Chemical Reactions

Molecular Collisions & Reaction Rates | Chemical Kinetics

1. Basic Principles

According to Collision Theory (developed by Max Trautz and William Lewis):

Principle: For a reaction to occur, the reactant molecules must collide with each other. However, not all collisions result in product formation.

• Collision Frequency ($Z$): The number of collisions per second per unit volume of the reaction mixture.
• Effective Collisions: Collisions that actually lead to the formation of products.

2. Criteria for Effective Collisions

For a collision to be effective, it must overcome two main barriers:

A. Energy Barrier (Activation Energy)

The colliding molecules must possess a minimum amount of energy called Threshold Energy ($E_T$).

If molecules have lower energy, they just bounce off. The extra energy required by reactant molecules to reach the threshold is the Activation Energy ($E_a$).

$$ E_T = E_{reactants} + E_a $$

The fraction of molecules having energy $\ge E_a$ is given by the Boltzmann factor: $e^{-E_a/RT}$.

B. Orientation Barrier (Steric Factor)

Molecules must collide with the Proper Orientation to facilitate the breaking of old bonds and formation of new bonds.

Example: In the reaction of $CH_3Br + OH^- \rightarrow CH_3OH + Br^-$, the $OH^-$ ion must attack the Carbon atom from the side opposite to $Br$.

3. Rate Expression

Combining the factors of Collision Frequency ($Z_{AB}$) and Activation Energy:

$$ \text{Rate} = Z_{AB} \times e^{-E_a/RT} $$

However, to account for the Orientation Factor (Probability Factor, $P$), the equation is modified:

$$ \text{Rate} = P \times Z_{AB} \times e^{-E_a/RT} $$

Comparing this with the Arrhenius Equation ($k = A e^{-E_a/RT}$), we see that the Arrhenius Factor ($A$) is related to collision frequency and orientation:

$$ A = P \times Z_{AB} $$

4. Limitations of Collision Theory

• It assumes atoms/molecules to be Hard Spheres.
• It ignores the structural aspect of molecules (though the P factor attempts to correct this).
• It does not fully explain reactions in solution or complex chain reactions.

Practice Quiz

Test your understanding of Reaction Dynamics.

Your Score: 0 / 10

© 2026 chemca-chemistry made easy

Quiz Revision Notes Downloads Class Notes Video Lectures