But experimentally, it is observed that the rate of reaction is found to depend upon‘ α’ concentration terms of A and‘ β’ concentration terms of B Then,
Rate α [ A ] α [ B ] β = k [ A ] α [ B ] β
where, [ A ] and [ B ] molar concentrations of A and B respectively and k is the velocity constant or rate constant. The above expression is known as rate law.
Rate Constant
In the above expression, k is called rate constant or velocity constant.
Rate constant may be defined as the specific rate of reaction when the molar concentrations of the reactants is taken to be unity, i. e.,
Rate = k, if [ A ] = [ B ] = 1
Units of rate constant or specific reaction rate for a nth order reaction is given as
K =( 1 / Time) x( 1 /[ Conc.] n – 1)