The reaction $\text{A}_2 + \text{B}_2 \rightarrow 2\text{AB}$ follows the mechanism
$\text{A}_2 \overset{k_1}{\underset{k_{-1}}{\rightleftharpoons}} \text{A} + \text{A} \quad \text{(fast)}$
$\text{A} + \text{B}_2 \overset{k_2}{\longrightarrow} \text{AB} + \text{B} \quad \text{(slow)}$
$\text{A} + \text{B} \rightarrow \text{AB} \quad \text{(fast)}$
The overall order of the reaction is :
