The reaction $2AX\left(g\right)+2B_2\left(g\right)\to A_2\left(g\right)+2B_{2}X\left(g\right)$ has been studied kinetically and on the basis of the rate law following mechanism has been proposed

I) $2AX\stackrel{ }{\rightleftharpoons }{A}_2{X}_2$                           (fast and reverse)
II) $A_2{X}_2+B_2\to A_{2}X+B_{2}X$      (slow)
III)  $A_{2}X+B_2\to A_2+B_{2}X$         (fast)

Where all the reaction intermediates are gases under ordinary condition.

From the above mechanism in which the steps (elementary) differ considerably in their rates, the rate law is derived using the principle that the slowest step is the rate- determining step (RDS) and the rate of any step varies as the product of the molar concentrations of each reaching species raised to the power equal to their respective stoichiometric coefficients (law of mass action). If a reacting species is solid or pure liquid, its active mass, i.e., molar concentration is taken to be unity, the standard state. In order to find out the final rate law of the reaction, the concentration of any intermediate appearing in the rate law of the RDS is substituted in terms of the concentration of the reactant(s) by means of the law of mass action applied on equilibrium step