Consider the reaction : $$ \mathrm{Cl}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{~S}(\mathrm{aq}) \rightarrow…
Consider the reaction :
$$
\mathrm{Cl}_2(\mathrm{aq})+\mathrm{H}_2 \mathrm{~S}(\mathrm{aq}) \rightarrow \mathrm{S}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{Cl}^{-}(\mathrm{aq})
$$
The rate equation for this reaction is rate $=\mathrm{k}\left[\mathrm{Cl}_2\right]\left[\mathrm{H}_2 \mathrm{~S}\right]$ Which of these mechanisms is/are consistent with this rate equation?
(A) $\mathrm{Cl}_2+\mathrm{H}_2 \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}+\mathrm{Cl}^{+}+\mathrm{HS}^{-} \quad$ (slow)
$$
\mathrm{Cl}^{+}+\mathrm{HS}^{-} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}+\mathrm{S} \text { (fast) }
$$
(B) $\mathrm{H}_2 \mathrm{~S} \Leftrightarrow \mathrm{H}^{+}+\mathrm{HS}^{-} \quad$ (fast equilibrium) $\mathrm{Cl}_2+\mathrm{HS}^{-} \rightarrow 2 \mathrm{Cl}^{-}+\mathrm{H}^{+}+\mathrm{S} \quad$ (slow)
B only
Both A and B
Neither A nor B
A only
Solution
Rate equation is to be derived wrt slow
Step $\therefore$ from mechanism (A)
$
\text { Rate }=\mathrm{k}\left[\mathrm{Cl}_2\right]\left[\mathrm{H}_2 \mathrm{~S}\right]
$