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Consider the reaction: Cl₂(aq) + H₂S(aq) → S(s) + 2H⁺(aq) + 2Cl⁻(aq)
The rate equation for this reaction is rate = k[Cl₂][H₂S]
Which of these mechanisms is/are consistent with this rate equation?
A. Cl₂ + H₂S → H⁺ + Cl⁻ + Cl⁺ + HS⁻ (slow)
Cl⁺ + HS⁻ → H⁺ + Cl⁻ + S (fast)
B. H₂S ⇌ H⁺ + HS⁻ (fast equilibrium)
Cl₂ + HS⁻ → 2Cl⁻ + H⁺ + S (slow)
- A only
- Both A and B
- Neither A nor B
- B only
Correct answer: A only
Solution
In A the slow step is Cl2 + H2S, giving rate = k[Cl2][H2S], which matches. In B the pre-equilibrium gives [HS-] = K[H2S]/[H+], so the slow step gives rate = k'[Cl2][H2S]/[H+] (an extra 1/[H+] dependence), which does not match. Hence only A is consistent.
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