Collision Theory

According to this theory, the reactant molecules are assumed to be hard spheres and the reaction occurs when molecules collide with each other.

The number of collisions between the reacting molecules taking place per second per unit volume is known as collision frequency (Z_AB).

But only those collisions in which the colliding species have certain minimum amount of energy and collide in proper orientation, result in the product formation. Such collisions are called fruitful collisions or effective collisions.

Here, rate = –

= collision frequency × fraction of effective collision

= Z_AB × f

= Z_AB ×

where, Z_AB represents the collision frequency of reactants, A and B,  represents the fraction of molecules with energies equal to or greater than E_a.

The proper orientation of reactant molecules leads to bond formation whereas improper orientation makes them simply bounce back and no products are formed.

Example

So, to account for effective collisions, another factor, P called the probability or steric factor is introduced.

Rate = PZ_AB ×

Drawback of collision theory:

It considers the atoms/molecules to be hard spheres and ignores their structural aspect.