Question

Mechanism of a hypothetical reaction $X _{2}+ Y _{2} \rightarrow 2 XY$ is given below: (i) $X_{2} \rightarrow X+X$ (fast) (ii) $X + Y _{2} \rightleftharpoons XY + Y$ ( slow ) (iii) $X + Y \rightarrow XY$ (fast) The overall order of the reaction will be

• A. 2
• B. 0
• C. 1.5
• D. 1

Answer 1

The Correct Option is C

The solution of this question is given by assuming step (i) to be reversible which is not given in question Overall rate $=$ Rate of slowest step (ii) $= k [ X ]\left[ Y _{2}\right]$...(1) $k =$ rate constant of step (ii) Assuming step (i) to be reversible, its equilibrium constant, $k_{e q}=\frac{[X]^{2}}{\left[X_{2}\right]}$ $\Rightarrow[X]=k_{e q}^{\frac{1}{2}}\left[X_{2}\right]^{\frac{1}{2}}$...(2) Put (2) in (1) Rate $= kk _{ eq }^{\frac{1}{2}}\left[ X _{2}\right]^{\frac{1}{2}}\left[ Y _{2}\right]$ Overall $=\frac{1}{2}+1=\frac{3}{2}$