A KINETIC AND MECHANISTIC STUDY OF THE THERMAL-DECOMPOSITION OF COPPER(II) OXALATE

A kinetic and mechanistic study of the thermal decomposition of copper(II) oxalate has been completed. The reaction stoichiometry was well represented as CuC2O4 . 0.25H2O --> 1.92CO2 + 0.06CO + 0.25H2O + [Cu + 0.01C]. Reactant crystallites were below the minimum size required for resolution of surface textural detail by scanning electron microscopy. It was shown that the crystallites formed adherent aggregates and there was significant particle re-texturing during the latter half of the reaction. Analytical measurements confirmed that decomposition proceeded with stepwise cation reduction (Cu2+ --> Cu+ --> Cu0). Fractional reaction (alpha)-time curves were sigmoid in shape. The acceleratory stage in the reaction obeyed the exponential law in the range 0.04 < alpha < 0.3 and the activation energy was 140 +/- 7 kJ mol-1 (515-550 K). The first-order reaction was obeyed in the range 0.24 < alpha < 0.91 and the activation energy was somewhat larger at 180 +/- 7 kJ mol-1 (515-550 K). The reaction was significantly reduced in crushed mixtures of the salt with either Cu2O or CuCl. It is concluded that reaction proceeded with intracrystalline but not comprehensive melting; perhaps local and temporary fusion within the particles took place. The decomposition of copper(II) oxalate, dominant when alpha < 0.5, proceeded somewhat more rapidly than the subsequent (alpha > 0.5) decomposition of copper(I) oxalate. These conclusions are supported by comparative kinetic studies of the thermal decompositions of equimolar mixtures of oxalic acid with either Cu2O or CuO. There was some evidence that the rate-limiting step was electron transfer from anion to cation.