Mechanism of a hypothetical reaction $X_2+Y_2\to 2XY$ is given below:(i) $X_2\to X+X$ (fast)(ii) $X+Y_2\rightleftharpoons XY+Y$ (slow)(iii) $X+Y\to XY$ (fast)The overall order of the reaction will be:

The rate of the overall reaction is determined by the slowest step (Rate Determining Step).From step (ii): $\text{Rate}=k[X][Y_2]$ However, $X$ is an intermediate. From the first (fast) equilibrium step (assuming it to be reversible):$K_{eq}=\frac{[X{]}^2}{[X_2]}⟹[X]=(K_{eq}[X_2]{)}^{1/2}$ Substitute $[X]$ into the rate law:$\text{Rate}=k(K_{eq}[X_2]{)}^{1/2}[Y_2]=k^{\prime }[X_2{]}^{1/2}[Y_2{]}^1$ Overall order = $\frac{1}{2}+1=1.5$.