Mechanisms and Kinetics of the Decomposition of Trioxalatocobaltate(III) in Aqueous Sulfuric Acid

Abstract

Abstract The kinetics of the thermal decomposition of the trioxalatocobaltate(III) complex were studied in a sulfuric acid solution in the absence and in the presence of oxygen from 25 to 40°C. The decomposition of trioxalatocobaltate(III) was described by the equation: -d[Co(C2O4)33−]/dt=(k1+k2[H+])[Co(C2O4)33−]. The values of k1 and k2 were 6×10−6 sec−1 and 1.8×10−5 M−1 sec−1 respectively at 25°C. The values of the activation enthalpy change, ΔH1\eweq and ΔH2\eweq, were 29.7 and 30.0 kcal mol−1 respectively. The values of the activation entropy change, ΔS1\eweq and ΔS2\eweq, were 17.4 and 20.9 eu respectively at 40°C. Although hydrogen peroxide was formed in the presence of oxygen, and although a white precipitate of the polymerized products of acrylonitrile was found in the presence of acrylonitrile, neither oxygen nor acrylonitrile affected the kinetics of the decomposition of trioxalatocobaltate(III). The decomposition of peroxodisulfate was induced by the decompossition of trioxalatocobaltate(III). However, the rate of the decomposition of trioxalatocobaltate(III) was not affected by peroxodisulfate. On the other hand, both copper(II) and iron(III) ions were accelerators of the decompsition of trioxalatocobaltate(III). An anion radical of CO2− was predicted as an intermediate. The reaction mechanisms will be presented to account for these facts.