Kinetics and mechanism of bromate-bromide reaction catalyzed by acetate

The initial rate of the bromate-bromide reaction, BrO3(-) + 5Br(-) + 6H(+) --> 3Br(2) + 3H(2)O, has been measured at constant ionic strength, l = 3.0 mol L-1, and at several initial concentrations of acetate, bromate, bromide, and perchloric acid. The reaction was followed at the Br-2/Br-3(-) isosbestic point (lambda = 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, nu = k(1)[BrO3-][Br-][H+](2) + k(2)[BrO3-][Br-](2)[H+](2) + k(3)[BrO3-][H+](2)[acetate](2) + k(4)[BrO3-][Br-](2)[H+](2)[acetate] + k(5)[BrO3-][Br-][H+](3)[acetate](2) + k(6)[BrO3-][Br-][H+](2)[acetate], where k(1) = 4.12 L-3 mol(-3) s(-1), k(2) = 0.810 L-4 mol(-4) s(-1), k(3) = 2.80 x 10(3) L-4 mol(-4) s(-1), k(4) = 278 L-5 mol(-5) s(-1), k(5) = 5.45 x 10(7) L-6 mol(-6) s(-1), and k(6) = 850 L-4 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.