Collision theory of Reaction rates

Collision theory of Reaction rates

– According to collision theory, a chemical reaction takes place only by collisions between the reacting molecules. However, not all collisions are effective.

– Only a small fraction of the collisions produce a reaction.

– The two main conditions for a collision between the reacting molecules to be productive are:

(1) The colliding molecules must possess sufficient kinetic energy to cause a reaction.

(2) The reacting molecules must collide with proper orientation.

– Now let us have a closer look at these two postulates of the collision theory.

(1) The molecules must collide with sufficient kinetic energy

– Let us consider a reaction:

A – A + B – B → 2A – B

– A chemical reaction occurs by breaking bonds between the atoms of the reacting molecules and forming new bonds in the product molecules.

– The energy for the breaking of bonds comes from the kinetic energy possessed by the reacting molecules before the collision.

– The followimg Figure shows the energy of molecules A₂ and B₂ as the reaction A₂ + B₂ → 2AB progresses.

– The previous Figure also shows the activation energy, E_a, which is the minimum energy necessary to cause a reaction between the colliding molecules.

– Only the molecules that collide with a kinetic energy greater than E_a, are able to get over the barrier and react.

– The molecules colliding with kinetic energies less than E_a fail to surmount the barrier.

– The collisions between them are unproductive and the molecules simply bounce off one another.

(2) The molecules must collide with the correct orientation

– The reactant molecules must collide with favorable orientation (relative position).

– The correct orientation is that which ensures direct contact between the atoms involved in the breaking and forming of bonds. 

– From the above discussion, it is clear that: Only the molecules colliding with kinetic energy greater than E_a and with correct orientation can cause a reaction.

– The following figure shows the Orientations of reacting molecules A and B which lead to an effective and ineffective collision.

Collision Theory and Reaction Rate Expression

– Taking into account the two postulates of the collision theory, the reaction rate for the elementary process.

A + B ⎯⎯→ C + D

is given by the expression:

rate = f × p × z

– where:

f = fraction of molecules that possess sufficient energy to react

p = probable fraction of collisions with effective orientations

z = collision frequency.