Question

The reaction of hydrogen and iodine monochloride is given
as:
H₂ (g) + 2ICl(g) → 2HCl(g) + I₂ (g)
The reaction is of first order with respect to H₂(g) and ICI(g),
following mechanisms were proposed.
Mechanism A:
H₂ (g) + 2ICl(g) → 2HCl(g) + I₂ (g)
Mechanism B:
H₂ (g)+ ICl(g) → HI(g);slow
HI(g) + ICl(g) → HCl(g) + I₂ (g);fast
Which of the above mechanism(s) can be consistent with
the given information about the reaction?
(a) A and B both (b) neither A nor B
(c) A only (d) B only

Answer 1

The mechanism that can be consistent with  the given information about the reaction is B only

- We are given that the reaction is first order with respect to both H₂(g) and ICI(g)

-this means that only 1 molecule of both H₂(g) and ICI(g) must be present in the rate determining step

-this condition is fulfilled only by mechanism B in whose slow step (rate determining step) it involves 1 molecule of H2(g) and 1 molecule of ICl(g)

- so the correct answer is (d)