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A kinetic study of some homogeneous reactions of molecular hydrogen with metal ions in aqueous solution Webster, Arthur Hubert

Abstract

The kinetics of the silver ion catalysed reduction of dichromate by molecular hydrogen in aqueous solution were investigated over a wide range of conditions between 30 and 120°C. The rate was found to be independent of dichromate at sufficiently high concentrations of this oxidizing agent. The observed kinetics were consistent with the two simultaneous reaction paths, (1) H₂ + 2Ag⁺ → intermediates → products, giving third order kinetics; and, (2) H₂ + Ag+ ⇄ AgH + H⁺ AgH + Ag⁺ → intermediates → products. The mechanism (1) predominated at low temperatures while the mechanism (2) accounted for most of the reaction at higher temperatures. Mechanism (2) was supported by experiments in which deuterium, added as D₂O to the aqueous phase appeared as HD in the gas phase during the course of the reaction. As the dichromate concentration approached zero (at 110°C) the reaction assumed a high order in the Ag⁺ concentration; this appears to reflect the aggregation of silver ions into incipient nuclei of metal particles. The precipitation by hydrogen of silver metal from solutions of silver perchlorate, acetate and ethylene diamine was examined. The basic complexing agents accelerated the reaction, probably by stabilizing the protons released in the first step of reaction (2). The magnitude of the effect follows the order acetate, ethylene diamine, H₂O. The reduction of permanganate by molecular hydrogen was studied over a wide pH range. The initial step, H₂ + MnO₄⁻ → MnO₄³⁻ + 2 H⁺ was proposed to account for the kinetics in both acidic and basic solutions. Silver perchlorate was found to act as a homogeneous catalyst for this reduction. The mechanism of the catalysed reaction may be, Ag⁺ + MnO₄⁻ ⇄ AgMnO₄ (fast) AgMnO₄ + H₂ → intermediates → products

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