Abstract
The stopped-flow apparatus has been used to determine the kinetics of reaction between manganese(III) and the reductants NH3OH+, NH3OCH3+, and HN02 at 25° in acid (0.50-3.70 M) perchlorate media. Nitrate was determined as a product of oxidation of hydroxylamine. Stoichiometric measurements with Mn(III) in excess were used to analyze the kinetic data with excess of substrate in terms of the reactions
6Mn(III) + NH3OH + + 2H20->- 6Mn(II) + NOs- + 8H + 2Mn(III) + 2NH3OCH3-->- 2Mn(II) + N2H3(OCH3)2+ + 3H +
2Mn(III) + HNOi + H20-> 2Mn(II) + 03~ + SH-
AH reactions were second order over-all, and first order in each reactant. The reactions were also independent of initial
concentration of reactants, [Mn!+] [NaClOi], [NaN03], ionic strength, and wavelength. The observed rate constant was dependent on acidity. A mechanism consistent with these results was postulated, in which the rate-determining steps involve the formation of the 20·, · 3, and · 02 radicals for the reactions with 3 -, NH3OCH3+, and HNO¡, respectively. The acidity dependence arises from reaction with Mn3 + and MnOH2+. With the primed rate con- stant designating MnOH2 + reaction, the rate constants for the rate-determining steps are: NH3OH-, ki = (1.4 ± 0.1) X 103 AT-1 sec"1, k,' = (3.1 ± 0.3) X 10= Af-‘ sec"1; NH3OCH3+, W = 6.1 ± 0.4 AT-1 sec"1, h < 0.5 Af_1 sec-1; HNO,, k·, = (2.2 ± 0.2) X 104 M~l sec-1, kj = (4.9 ± 0.4) X 104 Af_1 sec-1. The relative slowness of the reaction with NH3- OCH3+ is ascribed to the lack of hydrogen bonding in this system.