The kinetics of the Dushman reaction, 5I -+ IO 3 -+ 6H + f 3I 2 + 3H 2 O, is considerably more complex in aqueous, acidic media than is the behavior of the analogous reaction of Br -with BrO 3 -. The latter reaction is first order in both [Br -] and [BrO 3 -] and is believed to proceed via a series of simple oxygen-atom transfers, there being no evidence for the intermediacy of polybromine species such as H 2 Br 2 O 3 . In contrast, the [I -] order in the Dushman reaction is reported variously to be one or two, depending upon experimental conditions, especially [IO 3 -] 0 /[I -] 0 , and the kinetics of this reaction is generally very complex with the intermediacy of H 2 I 2 O 3 having been suggested by several authors. Stopped-flow kinetic measurements of the Dushman reaction are reported here with excess [KIO 3 ] 0 ≈ 40[KI] 0 /5 and [HClO 4 ] 0 g 140 × 6[KI] 0 /5 in which the observed [I -] order is cleanly two, but saturation kinetics is observed for the pseudo-second-order rate constant as [KIO 3 ] 0 and [HClO 4 ] 0 increase. This observation is rationalized by a mechanism assuming the intermediacy of H 2 I 2 O 3 at concentrations significant compared to those of I -, requiring that the kinetic equations be constrained by the iodide-ion mass balance. It is suggested that H 2 I 2 O 3 is a kinetically and stoichiometrically significant species in the Dushman reaction, while H 2 Br 2 O 3 is not in the analogous bromine system, because of the greater atomic radius, polarizability, and ability to expand its valence shell of iodine as compared to bromine.

The kinetics of the reaction HBrO(2) + HBrO(2) --> HOBr + BrO(3)(-) + H(+) is investigated in aqueous HClO(4) (0.04-0.9 M) and H(2)SO(4) (0.3-0.9 M) media and at temperatures in the range 15-38 degrees C. The reaction is found to be cleanly second order in [HBrO(2)], with the experimental rate constant having the form k(exp) = k + k'[H(+)]. The half-life of the reaction is on the order of a few tenths of a second in the range 0.01 M < [HBrO(2)](0) < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for kare found to be E(double dagger) = 19.0 +/- 0.9 kJ/mol and DeltaS(double dagger) = -132 +/- 3 J/(K mol) in HClO(4), and E(double dagger) = 23.0 +/- 0.5 kJ/mol and DeltaS(double dagger) = -119 +/- 1 J/(K mol) in H(2)SO(4). The activation parameters for k' are found to be E(double dagger) = 25.8 +/- 0.5 kJ/mol and DeltaS(double dagger) = -106 +/- 1 J/(K mol) in HClO(4), and E(double dagger) = 18 +/- 3 kJ/mol and DeltaS(double dagger) = -130 +/- 11 J/(K mol) in H(2)SO(4). The values Delta(f)H(29)(8)(0)[BrO(2)(aq)] = 157 kJ/mol and Delta(f)H(29)(8)(0)[HBrO(2)(aq)] = -33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Delta(f)H(29)(8)(0)[HBrO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOBr(aq)] and Delta(f)H(29)(8)(0)[HBrO(3)(aq)] as Delta(f)H(29)(8)(0)[HClO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOCl(aq)] and Delta(f)H(29)(8)(0)[HClO(3)(aq)]. The estimated value of Delta(f)H(29)(8)(0)[BrO(2)(aq)] agrees well with calculated gas-phase values, but the estimated value of Delta(f)H(29)(8)(0)[HBrO(2)(aq)], as well as the tabulated value of Delta(f)H(29)(8)(0)[HClO(2)(aq)], is in substantial disagreement with calculated gas-phase values. Values of Delta(r)H(0) are estimated for various reactions involving BrO(2) or HBrO(2).