RATE CONSTANT

Consider a simple reaction A → B.

If C₄ is the molar concentration or active mass of A at a particular instant, then

             (dx/dt) ∞ C_A  or  dx/dt = KC_A

where k is a proportionality constant, called velocity constant or rate constant or specific reaction rate.

At a fixed temperature, if C_A = 1, then

             Rate  =  dx/dt = k              ....(i)

Let us consider a general reaction.

             αA + bB → product

             Rate = (dx/dt) ∞ [A]^α [B]^b

             Rate = k [A]a[B]b               ....(ii)

When    [A]=[B] = 1 mol/litre, then

             Rate = k

Rate of a reaction at unit concentration of reactants is called rate constant.

The volume of rate constant depends on:

(i)    Nature of reactant

(ii)    Temperature

(iii)   Catalyst

Unit of rate constant

 Rate constant has different units for reactions of different order. General rule for rate of reaction may be given as:

Unit of rate constant = [1/(unit of concentration)]^n-1 × time^-1

           = [1/(mol/litre)]^n-1×sec^-1

           = [litre/mol]^n-1 × sec^-1

where n = order of reaction