KINETICS AND STOICHIOMETRY OF REACTION BETWEEN BETWEEN MANGANESE(3) AND HYDRAZOIC ACID IN ACID PERCHLORATE SOLUTION

The stoichiometry of the reaction Mn(III) + NH3 → Mn(II) + 3/2N2 + H+ has been studied by standard chemical analysis under conditions of excess Mn(III) and by direct measurement of the nitrogen evolved for large excesses of hydrazoic acid. The kinetics of the reaction has been followed spectrophotometrically with a stopped-flow apparatus at 25° in 1.00-2.87 M HCIO4 and ionic strength of 3.8 M. A pink color produced upon mixing the reactants has been assigned to the formation of a monoazidomanganese(III) complex. The formation constant K = [MnN32] [H+]/[Mn3+] [HN3] has been determined from initial optical densities which give the value K - 89 ± 18; evaluation of the rate data leads to a value of about 70 (±20%). The spectrum of the complex has been determined from 440 to 570 nm. The observed rate is second order in [Mn(III)] and is consistent with the mechanism [formula omitted] Steps III and IV provide the predominant path to nitrogen production. The chief features of this mechanism are the reaction of two monoazidomanganese(III) complexes via a bridged intermediate and the formation of N3+, The values of the kinetic parameters calculated from the mechanism with the additional assumption that k1 and k-1 limited by water exchange in MnN32+ and Mn2+, respectively, are 106 ≤ k1 ≤ 107 M-1 sec-1, k-1 ≤ 3 × 108 M-1 sec-1, k3 ≫ 6k2, and k4 ≈ 0.15 k-3K’, where K’ is the formation constant for MnN3+. © 1969, American Chemical Society. All rights reserved.