Collision Theory of Rate Reaction

Collision theory of rate reaction states that when suitable particles of reactant hit each other, only certain fraction of the collisions cause chemical change.; these successful changes are called successful collisions. The successful collisions must have enough energy, also known as activation energy, at the moment of impact to break the pre-existing bonds and form all new bonds. This results in the products of the reaction. Increasing the concentration of the reactant particles or raising the temperature which brings about more collisions and hence more successful collisions, therefore, increases the rate of a reaction. Collision theory was proposed independently by Max Trautz in 1916 and William Lewis in 1918.

Collision theory of reaction rate can be summarized as:

1. For a chemical reaction to take place collision between the reactant molecules should occur.
2. Only a fraction of molecules have sufficient energy to cross the energy barrier from the products. The minimum amount of energy for a collision to cross the energy barrier is called activation energy.
3. The collision in which molecules are properly arranged is called effective collision and collision in which molecules are not properly arranged is called an ineffective collision. Only effective collisions can give rise to products.
4. There is no direct conversion of reactant into a product and an unstable complex called activated complex is formed between reactant and products. Only the decomposition of activated energy gives rise to products.
5. The rate of collision can be given by r= f Pz (collision equation)
   Where r= rate off collision
   f= fraction of molecules that have sufficient energy to react
   p= probable fraction of collisions with proper orientation
   z= collision frequency
6. The height of the hill (energy of activation) determines the rate of reaction. If the activation energy is high then the reaction will be slow. If the activation energy is low, the reaction will be fast and if activation energy is zero reaction is instantaneous.