AIEEE 2010 Solved Paper

Since the slow step is the rate determining step hence-
if we consider option (1) we find
Rate =k[Cl2][H2S]
Now if we consider option (2) we find
Rate =k[Cl2][HS−]...(1)
From equation (i)
k=‌

[H+][HS−]

H2S

or
[HS−]=‌

k[H2S]

H+

Substituting this value in equation (1) we find
Rate =k[Cl2]K‌

[H2S]

H+

=k′‌

[Cl2][H2S]

[H+]

hence only, mechanism (1) is consistent with the given rate equation.