Heterogeneous catalysis in solution. Part VII. Catalysis of the aquation of the bromopenta-amminecobalt(III) ion by mercuric sulphide and silver bromide

Abstract

The catalysis by mercury(II) sulphide and silver(I) bromide, both in powdered form, of the aquation of the [Co(NH₃)₅Br]²⁺ ion was investigated in 0·01 M-perchloric acid solution at 25°. The rate of reaction was found to be surface-controlled and the results conformed fairly well to a model which postulated a Langmuir pre-reaction adsorption equilibrium of the reactant on the catalyst surface. The products of the reaction, [Co(NH₃)₅H₂O]³⁺ and Br^–, exerted slight repressive effects on the catalytic aquation reactions. The adsorption coefficient of [Co(NH₃)₅H₂O]³⁺ on mercury(II) sulphide was found to be much smaller than that for [Co(NH₃)₅Br]²⁺, in agreement with this observation. The heterogeneously catalysed rates were considerably slower than the corresponding homogeneous rates for the second-order reactions between [Co(NH₃)₅Br]²⁺, Ag⁺ and Hg²⁺. Both catalytic reactions, particularly that to silver(I) bromide, were subject to photochemical acceleration.