The Kinetics and Stoichiometry of the Reaction between Manganese(III) and Hydrogen Peroxide in Acid Perchlorate Solution

Abstract

The kinetics of the reaction 2Mn(III) + H2O2 → 2Mn(II) + O2 + 2H+ has been studied spectrophotometrically with a stopped-flow apparatus at 25° in 1.00-3.70 M perchloric acid solutions of ionic strength between 2.23 and 4.90 M. The stoichiometry was determined by measuring the difference in the concentration of one of the reactants before and after reaction and by direct analysis of the oxygen produced. For conditions where the initial excess of peroxide was fivefold or less, the ratio of manganese(III) consumed to hydrogen peroxide consumed had the value of 1.97 ± 0.09; i.e., Δ[Mn(III)]/ Δ[H2O2] = 2, within experimental error. Deviations from this stoichiometry were observed at very high excesses of H2O2 by direct oxygen analysis (relative error in an individual determination = ±3%). The reciprocal of the observed second- order rate constant is dependent on [Mn2+]/[Mn(III)]0 and [H2O2]0/[Mn(III)]0. The rate is independent of ionic strength. The proposed mechanism involves the over-all rate-determining steps [formula omitted] where k1 = 7.3 × 104 M-l sec-1 and [formula omitted] (relative error = ±20%). The role in the mechanism of complexed Mn(III) species is discussed. The dependence of rate on [Mn2+] arises predominantly from the reverse of reaction 1′. The Mn3+ and MnOH2+ react at similar rates with H2O2+. Comparison of these results with those of the reactions of Co (III) and Ce(IV) indicates a relationship between reaction rate and the number of available sites in unfilled metal d orbitals. No correlation of rates with oxidation potential could be established. The observation that catalyzed decomposition of H2O2 occurs at high [H2O2]0 indicates that there is a competition between Mn(III) and H2O2 for H2O2+. The apparent rate of the latter reaction obtained from kinetic analysis is too high to be consistent with the measured stoichiometry and suggests the presence of complexed Mn(III) species. © 1968, American Chemical Society. All rights reserved.