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
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
= [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.