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Thursday, May 30, 2019


(1) The basic requirement for a reaction to occur is that the reacting species must collide with one another. This is the basis of collision theory for reactions.
(2) The number of collisions that takes place per second per unit volume of the reaction mixture is known as collision frequency (Z).
(3) Every collision does not bring a chemical change. The collisions that actually produce the product are effective collisions. The effective collisions, which bring chemical change, are few in comparison to the total number of collisions. The collisions that do not form a product are ineffective elastic collisions, i.e., molecules just collide and disperse in different directions with different velocities.
(4) For a collision to be effective, the following two barriers are to be cleared.
(A) Energy barrier: "The minimum amount of energy which the colliding molecules must possess as to make the chemical reaction to occur is known as threshold energy".

(i) In the graph 'E' corresponds to minimum or threshold energy for effective collision.
(ii) There is an energy barrier for each reaction. The reacting species must be provided with sufficient energy to cross the energy barrier
(B) Orientation barrier: The colliding molecules should also have proper orientation so that the
Old bonds may break and new bonds are formed.
During this reaction, the products are formed only when the colliding molecules have proper
Orientation at the time of collisions. These are called effective collisions.
(a) Properly oriented collisions form products

(b) Collisions not properly oriented

(5) Thus, the main points of collision theory are as follows,
(i) For a reaction to occur there must be collisions between the reacting species.
(ii)Only a certain fraction of the total number of collisions is effective in forming the products.
(iii) For effective collisions, the molecules should possess sufficient energy as well as orientation.
(6) The fraction of effective collisions, under ordinary conditions may vary from nearly zero to about one for ordinary reactions. Thus, the rate of reaction is proportional to:
(i) The number of collisions per unit volume per second (Collision frequency, Z) between the
reacting species
(ii) The fraction of effective collisions (Properly oriented and possessing sufficient energy), f
Where f is fraction of effective collision and Z is the collision frequency.
(7) The physical meaning of the activation energy is that it is the minimum relative kinetic energy which the reactant molecules must possess for changing into the products molecules during their collision. This means that the fraction of successful collision is equal to e-Ea /RT called Boltzmann factor.
(8) It may be noted that besides the requirement of sufficient energy, the molecules must be
Properly oriented in space also for a collision to be successful. Thus, if ZAB is the collision frequency, P is the orientation factor (Steric factor) then
If we compare this equation with Arrhenius equation
We know that pre-exponential form 'A' in Arrhenius equation is, A= PZAB

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