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Collision Theory of Reaction Rates

Collision theory, was developed by Max Trautz and William Lewis in 1916-18 and, provides a greater insight into the energetic and mechanistic aspects of reactions and their rates. According to this theory, the reactant molecules are assumed to be hard spheres and reaction can occur when molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z). The more frequent are collisions, the faster is the reaction.

 

POSTULATES OF COLLISION THEORY

(i) Reactions occur due to approach and collisions of reactant particles (atoms, molecules or ions) .

(ii) A successful collisions, the one required for changing reactants to products, can occur between two molecules only if they possess a certain minimum amount of energy in excess of the normal energy of molecules.

(iii) Only a small fraction of collisions are successful in producing a reaction. These collisions are called effective collisions.

(iv) The minimum energy which molecules must possess before collision should be equal to or greater than the activation energy.

(v) The rate of reaction is proportional to the frequency of effective collisions per second.

 

It must be noted here that the collisions between reactant molecules will not lead to reaction even if the energy requirement is satisfied. It is because the colliding molecules should also have proper orientation.

 

Proper Orientation:

Improper Orientation:

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

A pictorial representation (Fig. 20.11(n) .and 20.11 (b)) is shown, to illustrate the above idea.

An extension of the collision theory is the activated complex or transition state theory. In the activated. complex theory:

1. When two molecules or species approach each other, their electron clouds repel and distort each other. The potential energy of the reactants rises and reaches a maximum value, forming a combined and highly unstable molecule called the activated complex.

2. When the activated complex loses energy it can either form products or break down to form the original reactants. The activated complex is described as being in the transition state.

3. The energy required to from the activated complex is known as the activated energy barrier, and it is only reacting molecules which have greater energy than the activation energy barrier which ‘climb’ over to form the products (Fig. 20.11).