Question

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 the rate equation? A. Cl₂ + H₂S → H⁺ + Cl⁻ + Cl⁺ + HS⁻ (slow) Cl⁺ + HS⁻ → H⁺ + Cl⁻ + S (fast) B. H₂S rightleftharpoons H⁺ + HS⁻ (fast equilibrium) Cl₂ + HS⁻ → 2Cl⁻ + H⁺ + S (slow)

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

Hint:

The rate law must be derivable directly from the slow step of the mechanism without involving intermediates.

Answer 1

The Correct Option is D

Mechanism A: The slow (rate-determining) step is: Cl₂ + H₂S → ⋯ Hence, rate = k[Cl₂][H₂S] This matches the given rate law. Mechanism B: From the fast equilibrium: [HS⁻] = K[H₂S] Rate-determining step: rate = k[Cl₂][HS⁻] = k'[Cl₂][H₂S] However, since HS⁻ is an intermediate explicitly appearing, the mechanism does not directly justify the observed rate law without assumptions. Thus, only mechanism A is consistent.