Theories of Chemical Kinetics

Theories of Chemical Kinetics:

Arrhenius Equation [k = Ae^-E_a/RT] is a purely experimental equation that gives a reasonably good representation of temperature depending on the rate constant. There are two common theories of reaction rates leading to the formation of the Arrhenius Equation. They are-

• Collision Theory.
• Transition State Theory.

Collision Theory:

This theory considers that the collision between reactant molecules is responsible for a reaction. The rate of reaction according to this theory depends upon the number of collisions that take place per second per unit volume of the reaction mixture called collision frequency (z) and those collisions which ultimately make the energy of reactant molecules equal to the threshold energy to surmount the energy barrier between reactants and products called effective collision (f). Thus, mathematically,

Further according to the kinetic theory of gases, the fraction of molecules having energy more than a particular value (E_a) [Which is threshold energy] at temperature ‘T’ is given by-

Substituting the value of ‘f’ from equation (ii) in equation (i), we get

As the rate of reaction is directly related to the rate constant k, so equation (iii) can be written as-

Further, it is observed that in some cases it is found that even if a large number of colliding molecules have energy more than the threshold value still reaction is low due to improper orientation of the colliding molecules at the time of the collision. It is called the orientation factor or steric factor (P). Taking that into account in equation (iv), we get-

This equation gives the effect of temperature on rate constant or rate of reaction.

Transition State Theory:

The theory was developed by Eyring in 1935. It is based upon the following fundamental postulates.

• The reacting molecules come close and form an energy-rich intermediate called Activated Complex.
• The energy in the activated complex is higher than that of reacting molecules.
• The activated complex is in equilibrium with the reacting molecules. However, the product may or may not be in equilibrium with the activated complex.
• The activated complex is assumed to be a normal molecule except that it is unstable w.r.t. one special mode of vibration which leads to its dissociation into products. (In simple words the activated complex has a transient existence).