The reaction of bromate and bromide ions in acid solution is represented by the equation, 5Br–(aq) + — Kinetics Chemistry Question
Problem Context
The reaction of bromate and bromide ions in acid solution is represented by the equation,
5Br–(aq) + BrO3 –(aq) + 6H3O+(aq) → 3Br2(aq) + 9H2O(l)
In order to measure the rate of the reaction, stock solutions were prepared as shown in the table:
Reaction mixtures were prepared by mixing the volumes of solutions listed below, and the initial rate of disappearance of bromate ion was measured.
Calculate the rate of appearance of Br2(aq) in experiment one.
Model Answer
= 3(5.63×10–6) = 1.69×10–5 mol L–1 s–1
Write the rate law for this reaction and give the value of the specific rate constant, k.
Model Answer
Looking at experiments 2 and 1, the volume of Br– is doubled and the rate increases by 1.94 (essentially doubles) so the reaction is first order in Br–.
Looking at experiments 3 and 1, the volume of BrO3– is doubled and the rate doubles so the reaction is first order in BrO3–.
Looking at experiments 2 and 4, the volume ratio of H3O+ is (1.00/0.700 = 1.4) and the rate ratio is (1.09 / 0.55 = 1.98) so the reaction is second order in H3O+ because (1.4)^2 = 1.96
Thus, the rate law is : rate = k[Br–][BrO3–][H3O+]2 and
= 2.86 L3 mol–3 s–1
The following mechanism is proposed for the reaction:
(I) BrO3 –(aq) + H3O+(aq) → HBrO3(aq) + H2O(l)
(II) HBrO3(aq) + H3O+(aq) → H2BrO3 +(aq) + H2O(l)
(III) H2BrO3 +(aq) → BrO2 +(aq) + H2O(l)
(IV) BrO2 +(aq) + Br–(aq) → BrOBrO(aq)
(V) BrOBrO(aq) + Br–(aq) → Br2 (aq) + BrO2 –(aq)
Subsequent reactions of BrO2 -(aq) are fast.
i. Draw a Lewis structure of BrO2+ and predict its geometry.
Model Answer
One resonance structure is shown below, and the shape determined by VSEPR is bent.
ii. Given the rate law you determined in b, which of the steps (I-(V) could potentially be rate-limiting? Justify your answer.
Model Answer
To have the rate law determined in part (b), the rate limiting step of the mechanism must depend on the concentrations of [Br–], [BrO3–], and [H3O+]2. The only way for this to be true is for Step IV to be the rate limiting step. One way to confirm this is to determine what the rate law would be for each step as the rate limiting step. If Step I is limiting, the rate would vary with [BrO3–][H3O+]. If Step II is limiting, the rate would vary with [BrO3–][H3O+]2. If Step III is limiting, the rate would vary with [BrO3–][H3O+]2. If Step IV is limiting, the rate would vary with [BrO3–][Br–][H3O+]2. If Step V is limiting, the rate would vary with [BrO3–][Br–]2[H3O+]2.