Question

Explain briefly the collision theory of bimolecular reactions.

Answer 1

The Collision Theory of Bimolecular Reactions deals with the energetic and mechanistic aspects of reactions. According to the theory, reactant molecules are assumed to be hard spheres. A reaction can occur only when these molecules collide with one another to bring about a chemical change.

The average number of collisions per second per unit volume of the reaction mixture is known as the collision frequency (Z_AB). All the collisions do not result in a chemical reaction; only those result in a chemical reaction in which the colliding molecules possess a certain amount of energy known as the threshold energy.

For collisions to be effective, it is essential that the reactant molecules collide with proper orientation. Colliding molecules having sufficient energy but no proper orientation. They just bounce back without any chemical reaction. If Z_AB is the collision frequency and f is the fraction of molecules possessing sufficient energy to overcome the energy barrier, then the rate constant for the reaction will be
$k=Z_{AB}f$
By Maxwell's law of distribution, $f=e^{-\frac{E_a}{\mathrm{R}T}}$ , all collisions do not result in a chemical change because of improper orientation; therefore, to incorporate the orientation probability, factor P is introduced into the equation.

Rate constant

$k=P{Z}_{AB}{e}^{\frac{-E_{\mathrm{a}}}{RT}}$

On comparing this equation with the Arrhenius equation, we get

Rate Constant, $k=A{e}^{\frac{-E_{\mathrm{a}}}{RT}}$

A (frequency factor) is equal to PZ_AB .
⇒A = PZ_AB