A hypothetical reaction A2 + B2 + 2AB follows the mechanism as given below:A2⇌A+A              (fast)A + B2 → AB + B    (slow)A + B → AB            (fast)The order of the overall reaction is

Since (A) is the intermediate reactive species whose concentration is determined from equilibrium step. slow step is:A+B2⟶AB+B (slow) r = k[A][B2] . . . (i)From equilibrium stepA2⇌A+A (fast) keq=[A]2A2∴[A]=keqA21/2substitute the value of [A] in Eq. (i)r=k⋅keq1/2A21/2B2Thus, order of reaction =12+1=112