Kinetics of hypobromous acid disproportionation

The kinetics of aqueous hypobromous acid disproportionation are measured at 25.0 degrees C from p[H+] 0.2, to 10.2. The reactions are second order in HOBr with a maximum rate at pH 3-8. The rate of disproportionation decreases significantly above pH 8 as OBr- forms. Another suppression observed below pH 3 is attributed to the reversibility of initial steps in the decomposition. The rate expression is given by -d[Br(T)]/dt = n((c/(c + [H+)k(1a) + k(B)-[B])[HOBr](2) + k(1b)[OBr-](2)), where k(1a) = 2 x 10(-3) M-1 s(-1), k(B)[B] is a general-base-assisted pathway, k(1b) = 6 x 10(-7) M-1 s(-1), n is a stoichiometric factor that ranges from 2 to 5, and c is a ratio of rate constants that is equal to 0.03 M. Decomposition is catalyzed by HPO42-(k(B) = 0.05 M-2 s(-1)) and by CO32-(k(B) = 0.33 M-2 s(-1)). formation. From pH 4 to 7, n varies from 5 to 3. A detailed mechanism is presented.