Kinetics and Mechanism of Bromate−Bromide Reaction Catalyzed by Acetate

The initial rate of the bromate−bromide reaction, BrO3- + 5Br- + 6H+ → 3Br2 + 3H2O, has been measured at
constant ionic strength, I = 3.0 mol L-1, and at several initial concentrations of acetate, bromate, bromide, and
perchloric acid. The reaction was followed at the Br2/Br3- isosbestic point (λ = 446 nm) by the stopped-flow
technique. A very complex behavior was found such that the results could be fitted only by a six term rate law, ν
= k1[BrO3-][Br-][H+]2 + k2[BrO3-][Br-]2[H+]2 + k3[BrO3-][H+]2[acetate]2 + k4[BrO3-][Br-]2[H+]2[acetate] +
k5[BrO3-][Br-][H+]3[acetate]2 + k6[BrO3-][Br-][H+]2[acetate], where k1 = 4.12 L3 mol-3 s-1, k2 = 0.810 L4 mol-4
s-1, k3 = 2.80 × 103 L4 mol-4 s-1, k4 = 278 L5 mol-5 s-1, k5 = 5.45 × 107 L6 mol-6 s-1, and k6 = 850 L4 mol-4
s-1. A mechanism, based on elementary steps, is proposed to explain each term of the rate law. This mechanism
considers that when acetate binds to bromate it facilitates its second protonation.