Collision theory of reaction rate

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Collision theory of reaction rate



The main points of this theory is as follows:-

1.For reaction to occur there must be collision between the reacting species.

2.Only fraction of total number of collision are effective in forming the products.

3.For effective collision,the necessary requirement are:-

i.The colliding species must posses certain minimum amount of energy called Threshold energy.

ii.The colliding species must be properly oriented at the time of collision,so that the old bonds breaks and new bonds are formed.
Thus according to collision theory-

Rate of reaction is directly proportional to the Collision frequency(Number of collision per unit volume per second) and Collision efficiency(fraction of effective collision).


According to collision theory of reaction rate,the reacting species must posses certain minimum amount of energy called “Threshold Energy”.But at room temperature molecule have energy less than threshold energy.Thus some energy is to be supplied.So that their energy becomes equal to the threshold energy.This extra energy supplied (over or about) the average energy of the reactant for collision to become effective is called “Activation Energy”.

Activation energy = Threshold energy – Activation energy of reactant reactant

Ea = Et – Er

Each reaction has a definite value of activation energy.If activation energy is low,greater the number of molecules can have this energy,fraction of the effective collision is large.Hence,rate of reaction is large.

For example-:

If the rate of reaction is fast,then the activation energy is high for the reaction.

NO(g) + 1/2 O2(g)—–>NO2(g)        {Fast}

If the rate of reaction is slow,then the activation energy is low for the reaction.

                                             PROGRESS OF THE REACTION

According to the concept of activation energy,reactant does not change directly into the products.There is an energy barrier between reactant and product.It is called Activation Energy Barrier.

Let us consider an exothermic reaction:-

A2+B2——–>2AB               ΔH=-ve

In the beginning of reaction,the collision between A2 and B2 are effective.They absorb the required activation energy reach to the top of the energy barrier forming an activation state,transition state or activation complex.This represent short life configuration of the atoms in which old bonds break and new bonds are formed.This immediately changes its product.

In case of Reversible Reaction:-

Activation energy is required for both backward and forward reaction.In such cases,Activation state for backward and forward reaction is same but fave different Activation Energy.

In case of endothermic reaction

Ea(f) > Ea(b)                  ΔH = +ve


In case of exothermic reaction

Ea(f)<Ea(b)                   ΔH = -ve


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