Q.Describe the salient features of collision theory as related to the rate of bimolecular reactions. OR State and explain Kohlrausch's law of independent migration of ions.

Collision theory explains reaction rate in terms of the frequency of molecular collisions, the fraction with sufficient energy (≥ activation energy), and a steric/orientation factor.

Salient features of collision theory (for bimolecular gas-phase reactions):

1. Molecules must collide to react. For a reaction to occur between two reactant molecules (e.g. A + B → products), the reactant molecules must first come into contact, i.e., collide with each other. The rate of reaction is proportional to the number of collisions occurring per unit time per unit volume, called the collision frequency (Z).

2. Not all collisions lead to a reaction — only “effective collisions” do. If every collision resulted in a reaction, reaction rates would be far higher than observed experimentally. Only a small fraction of the total collisions are “effective” and actually lead to product formation. An effective collision must satisfy two conditions:

1. Threshold energy condition: The colliding molecules must possess a combined kinetic energy equal to or greater than a certain minimum value called the threshold energy. The energy needed to raise the reactants to this threshold energy from their average energy is the activation energy (Ea). According to the Maxwell–Boltzmann distribution, only a fraction of molecules, given by the Boltzmann factor $e^{-E_a/RT}$, possess energy ≥ threshold energy at a given temperature.
2. Proper orientation condition: The colliding molecules must also collide with the correct spatial orientation, so that the atoms/groups that need to form new bonds are properly aligned at the moment of collision. This is accounted for by a steric factor (or probability factor), P, which typically has a value ≤ 1.

3. Rate expression combining both factors:

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

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