A reaction : A2+B→ Products, involves the following mechanism :
A2⇌ 2A (fast) (A being the intermediate)
A+B→k2(slow).The rate law consistent to this mechanism is :
rate=k[A2]12[B]
From the slow step : rate =k2[A][B] ...............(i)
From fast step : Ke=[A]2[A2]or [A]=K12eA12 ..............(ii)
From (i) and (ii) rate =k2k12e[A2]12[B]=k[A2]12[B]