Kinetics of reduction of hexachlororuthenate(IV) in hydrochloric acid solutions

Abstract

The reduction of [RuCl₆]_2– by Br^– in 6.15 mol dm^–3 HCl is first order in the concentrations of the reactants. At 298.1 K the second-order rate constant is (7.24 ± 0.25)× 10^–3 dm³ mol^–1 s^–1 and ΔH^‡= 59.2 ± 6.3 kJ mol^–1 and ΔS^‡=–86.9 J K^–1 mol^–1. The products of the reaction are [RuCl₆]^3– and bromine. The reaction proceeds to the equilibrium [RuCl⁶]^2–+ Br^–⇌[RuCl₆]^3–+½ Br₂ whose constant at 298 K and in 6.15 mo dm^–3 HCl is 1.06 ± 0.03 dm mol^–½. In concentrated hydrochloric acid [RuCl₆]^2– slowly and spontaneously reduces. The initial rate is proportional to the 0.6 power of the initial ruthenium concentration. The reduction process is complicated and appears to be catalysed by an unknown impurity in the concentrated acid.