Reactions I through III represent the elementary reactions proposed as the mechanism for the overall reaction:
H ₃ASO₄(aq) + 2 I - (aq) + H₃O + (aq) H₂ AsO₃ - (aq) + I₂ (aq) + 2 H₂ O Mechanism
I. H ₃ASO₄(aq) + H₃O + (aq) H₂ AsO (aq) + 2 H₂ O
II. H₂ AsO + (aq) + I - (aq) H₂ AsO (aq) + I + (aq)
III. I + (aq) + I - (aq) I₂ (aq) The observed rate law for this reaction is Rate = k H ₃ASO₄H₃O + I - . Which reaction in the mechanism must be the slow step, to obtain this rate law?

Question

Reactions I through III represent the elementary reactions proposed as the mechanism for the overall reaction:
H ₃ASO₄(aq) + 2 I - (aq) + H₃O + (aq) H₂ AsO₃ - (aq) + I₂ (aq) + 2 H₂ O Mechanism
I. H ₃ASO₄(aq) + H₃O + (aq)

Reactions I through III represent the elementary reactions proposed as the mechanism for the overall reaction: H ₃ASO₄(aq) + 2 I - (aq) + H₃O + (aq) H₂ AsO<sub>3</sub> - (aq) + I<sub>2</sub> (aq) + 2 H₂ O Mechanism I. H ₃ASO₄(aq) + H₃O + (aq) H₂ AsO (aq) + 2 H₂ O II. H₂ AsO + (aq) + I - (aq) H₂ AsO (aq) + I + (aq) III. I + (aq) + I - (aq) I<sub>2</sub> (aq) The observed rate law for this reaction is Rate = k H ₃ASO₄H₃O + I - . Which reaction in the mechanism must be the slow step, to obtain this rate law? A) I B) II C) III D) both II and III E) cannot be determined from the information given

H₂ AsO (aq) + 2 H₂ O
II. H₂ AsO + (aq) + I - (aq) H₂ AsO (aq) + I + (aq)
III. I + (aq) + I - (aq) I₂ (aq) The observed rate law for this reaction is Rate = k H ₃ASO₄H₃O + I - . Which reaction in the mechanism must be the slow step, to obtain this rate law?

Quizplus · Accepted Answer

The rate law includes the concentration of reactants H3ASO4, H3O+ and I-. Only reaction I includes H3ASO4 and H3O+. Therefore, reaction I should be fast, and reaction II should be slow. If reaction III were slow, the rate law would have to include two I- ions instead of just one.

Reactions I Through III Represent the Elementary Reactions Proposed as the Mechanism