Consider a simple reaction A → B.

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

             (dx/dt) ∞ CA  or  dx/dt = KCA

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

At a fixed temperature, if CA = 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


Rate of reaction

Reaction rate constant










It is speed with which reactants are converted into products.


It is measured as the rate of decrease of concentration of reactants or the rate of increase of concentration of products with time.


It depends upon the initial concentration of reactants


It is proportionally constant.


It is equal to the rate of reaction when the concentration of each of the reactants in unity.


It is independent of the initial concentration of the reactants. It has a constant value at fixed temperature.



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