Question

The reaction of hydrogen an iodine monochloride is given as
$H _{2( g )}+2 ICl _{( g )} \rightarrow 2 HCl _{( g )}+ I _{2( g )}$
This reaction is of first order with respect to $H _{2( g )}$ and $ICl _{( g )}$, following mechanisms were proposed
Mechanism $A$:
$H _{2( g )}+2 ICl _{( g )} \rightarrow 2 HCl _{( g )}+ I _{2( g )}$
Mechanism $B$ :
$H _{2( g )}+ ICl _{( g )} \rightarrow HCl _{( g )}+ HI _{( g )} ;$ Slow
$HI _{( g )}+ ICl _{( g )} \rightarrow HCl _{( g )}+ I _{2( g )} ;$ Fast
Which of the above mechanism $(s)$ can be consistent with the given information about the reaction:

• A. Only B
• B. Both A and B
• C. Neither A nor B
• D. Only A

Answer 1

Answer: (A)

In the reactions which take place in a number of steps, the slowest step is known as the rate determining step. Hence, rate of reaction always depends on slow step .$H_2 (g) + ICI (g) \longrightarrow HCI (g) + HI (g)$ is first order reaction with respect to $H_2$ and ICI. Thus, the mechanism B will be more consistent with the given information.