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Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion
Author(s) -
Robinson David M.,
Hoppe Todd J.,
Paslay Timothy J.,
Purser Gordon H.
Publication year - 2006
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.20141
Subject(s) - hydroxylamine , chemistry , protonation , stoichiometry , ionic strength , aqueous solution , kinetics , reaction mechanism , inorganic chemistry , medicinal chemistry , ionic bonding , ion , catalysis , organic chemistry , physics , quantum mechanics
The 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 N 2 O. The rate law is given by −[d[NH 2 Cl]/d t ] = k + [NH 3 OH + ][NH 2 Cl] + k 0 [NH 2 OH][NH 2 Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k + = (1.03 ± 0.06) ×10 3 L · mol −1 · s −1 and k 0 =91 ± 15 L · mol −1 · s −1 . Isotopic studies demonstrate that both nitrogen atoms in the N 2 O come from the NH 2 OH/NH 3 OH + . 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 Δ H ‡ = 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