Chlorine reacts with hydrogen sulfide in aqueous solution Cl_2 (aq) + H_2S (aq)

Question

Chlorine reacts with hydrogen sulfide in aqueous solution Cl_2 (aq) + H_2S (aq) rightarrow S (s) + 2 H^+ (aq) + 2 Cl^- (aq) in a second-order reaction that follows the rate expression: rate = k[Cl_2)[H_2S] Which, if any, of the following mechanisms are consistent with the observed rate expression? Mechanism A i. Cl_2 + H_2S rightarrow h^+ + Cl^- + Cl^+ + HS^- (slow) ii. Cl^+ + HS^- rightarrow H^+ + Cl^- + S (fast) Mechanism B i. H_2S HS^- + H^+ (fast equilibrium) ii. HS^- + Cl_2 rightarrow 2 Cl^- + S + H+ (slow) Mechanism C i. H_2S HS^- + H^+ (fast equilibrium) ii. H^+ + Cl_2 H^+ + Cl^- + Cl^+ (fast equilibrium) iii. Cl^+ + HS^- rightarrow H^+ + Cl^- + S (slow)

Explanation / Answer

The mechanism A only is consistent with the observed rate expression.

As the rate expression is written from the slow and rate determining step of the reaction, mechanism A has slow step as step i, which has the rate law as

rate = k [Cl2][H2s}

For mechanism B

From the slow step

rate = k [Hs-][Cl2]

= k.Kc [H2S] [Cl2]/[H+]

so it is not consistent with the rate law.

Mechnism C also does not give the same expression.

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