It is applied for these reactions which involve more than one reactant. In this method, the concentration of all reactants are taken in large excess than one. The changes in the concentration of the reactant that is not taken in excess will influence the rate of reaction.
The concentrations of all other reactants practically remain same during the course of reaction.
Consider the reaction.
m1A + m2B + m3C → product
Rate = k[A]α [B]β [C]γ
When B and C are taken in excess, the rate law will reduce to
rate = k’ [A]α
where k’ = k[B]β [C]γ
Two sets are taken in which the concentration of A is [A1] and [A2] while B and C are present in large excess,
r1 = [A1]α
r2 = [A2]α
\(\frac{r_1}{r_2} = \frac{[A_1]^\alpha}{[A_2]^\alpha}\)
From this, the value of α, i.e., the order with respect to A is obtained. The process is repeated one by one and order with respect to others is determined.
Total order of reaction = α + β + γ
