Consider a bimolecular reaction,
A – B + C → A + B – C
(i) Collisions of reactant molecules : The basic requirement for a reaction to occur is reacting species A – B and C must come together and collide. The rate of reaction will depend on the rate and frequency of collisions between them. As the i concentration and temperature increase, rate of collisions increases, hence the rate of reaction increases. But the rate of reaction is low as com-pared to the rate of collisions.
(ii) Energy of activation : For fruitful collisions, the colliding molecules must possess a certain amount of energy called activation energy Ea. Due to collisions between A – B and C, there is a change in electron distribution about three nuclei namely A, B and C so that old A – B bond is weakened while new bond is partially formed between B and C, and results in the formation of an activated complex or a transition state.
Therefore transition state always has higher energy than reactants or products. Due to high energy, activated complex is unstable, short lived and decomposes into the products.
To form activated complex, the reactant molecules have to climb the potential energy barrier i. e., activation energy level, hence molecular collision energy of colliding molecules must be high so that reactant molecules form activated complex and further decompose into products.
The fraction (f) of molecules at temperature T having activation energy Ea is given by f = \(e^{-E_a/RT}\).
If P represents the probability of Z collisions with proper orientation then,
Reaction rate = P x Z x \(e^{-E_a/RT}\),
Hence the rate constant k of the reaction may be represented as,
k = A x \(e^{-E_a/RT}\) where A is called frequency factor or pre-exponential factor and ΔH is the enthalpy change of the reaction. This equation is called Arrhenius equation.
