Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

Abstract

AbstractThe kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl–, NH+4, and N2O. The rate law is given by −[d[NH2Cl]/dt] = k+[NH3OH+][NH2Cl] + k0[NH2OH][NH2Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k+= (1.03 ± 0.06) ×103 L · mol−1 · s−1 and k0=91 ± 15 L · mol−1 · s−1. Isotopic studies demonstrate that both nitrogen atoms in the N2O come from the NH2OH/NH3OH+. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH‡ = 36 ± 3 kJ · mol–1 and Δ S‡ = −66 ± 9 J · K−1 · mol−1, and Δ H‡ = 12 ± 2 kJ · mol−1 and Δ S‡ = −168 ± 6 J · K−1 · mol−1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006