Theories of reaction rate

(1) Collision theory

(i) The basic requirement for a reaction to occur is that the reacting species must collide with one another. This is support the collision theory for reactions.

(ii) The number of collisions which takes place per second per unit volume of the reaction mixture is called ascollision frequency (Z). The value of the collision frequency is quite high of the order of 10²⁵ to 10²⁸ in case of the binary collisions.

(iii) Every collision does not bring the chemical change. The collisions which in reality produce the product areeffective collisions. The collisions which are effective bring chemical change, are less in the comparison to the total number of collisions. The collisions which do not result in product are ineffective elastic collisions, that is molecules simply collide and disperse in different directions with different velocities.

(iv) For the collision to be effective enough, under given are the two barriers which 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.

·         In graph given to us E corresponds to minimum or the threshold energy for effective collision.

· There is an energy barrier for each reaction. The reacting species should be provided sufficient energy to cross 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. For example,     during the reaction, products are formed at the time when colliding molecules have proper orientation at the time of collisions. These are usually known as as effective collisions.

(v) Therefore, the main points of the collision theory are given as follows,

         (a) If the reaction is to take place there should be collisions among the reacting species.

         (b) The particular fraction of total the number of collisions is effective in forming products.

         (c) For effective collisions, molecules should posses' sufficient energy as well as orientation.

(vi) The fraction of effective collisions, under the normal conditions can vary from nearly zero to about one for ordinary reactions. Hence, the rate of reaction is proportional to :

(a) The number of collisions per unit volume per second (Collision frequency, Z) among the reacting species

(b) The fraction of effective collisions (Properly oriented and possessing sufficient energy), f that is   Rate=-dx/dt=f*Z

Here f is the fraction of effective collision and Z is the collision frequency of it.

(vii) The physical meaning of activation energy is the minimum relative kinetic energy which the reactant molecules must possess for changing into the products molecules during their collision. Which means that the fraction of successful collision is equal to     termed as Boltzmann factor.

(viii) It can be noted that besides the need of sufficient energy, the molecules should be properly oriented in space also for the collision to be successful. Hence, if Z_AB is collision frequency, P is the orientation factor (or Steric factor) then,   .  When we compare this equation with the Arrhenius equation   .

         We already know that pre-exponential form 'A' in the Arrhenius equation is, A=PZ_AB.

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