RESUMO
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(I)]/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 HPO(4)(2)(-) (k(B) = 0.05 M(-)(2) s(-)(1)) and by CO(3)(2)(-) (k(B) = 0.33 M(-)(2) s(-)(1)). Above pH 8, the first observable product is BrO(2)(-) (initially n = 2). Below pH 4, n = 5 due to Br(2) and BrO(3)(-) formation. From pH 4 to 7, n varies from 5 to 3. A detailed mechanism is presented.
RESUMO
Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Brønsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).