Observation of an order change during the course of a reaction in a kinetic study of the reduction of trans-dicyanodibromoaurate(III) by octacyanotungstate(IV) in an acidic medium

The reduction of dibromodicyanoaurate(III) ion by octacyanotungstate(IV) ions was studied under near stoichiometric conditions in an acidic medium. An order change occurred during the course of the reaction. The reaction was first order in Au(CN)₂(Br)₂^– and second order in W(CN)₈^4– for the first half-life, but after ca. 60% conversion became first order in both reactants. The order change is ascribed to two different Au^II intermediates that form during the reaction course. A third order rate constant of k₃ = 7.80 × 10⁵ M⁻² min⁻¹ and a second order rate constant of k₂ = 108 M⁻¹ min⁻¹ was found at [H⁺] = 1.633 × 10^–3 M, an ionic strength of I = 0.11 M (NaBr) and at 25.0 ± 0.1 °C for the reaction. The reaction rate decreases with increasing [H⁺] in the reaction mixture at low pH and an experimental pK_a of 1.80 ± 0.03 was found at 25.0 ± 0.1 °C for the deprotonation of HW(CN)₈^3–. The reaction rate is independent of [Br⁻] and decrease with addition of W(CN)₈^3– in the reaction mixture. Activation parameters have been obtained by a least-squares fit of the temperature data directly to the Eyring equation.