The thermal decomposition reactions of copper(II) maleate and of copper(II) fumarate

Abstract

Kinetic studies of the thermal decomposition reactions of copper(II) maleate and of copper(II) fumarate have been undertaken. The mechanistic interpretations of these observations are supported by complementary analytical measurements and microscopic observations for the reactants, the products and salt partially decomposed to appropriate known extents of reaction. Analytical evidence showed that both reactants decompose with stepwise reduction of the cation and reaction was completed through two distinct, consecutive rate processes. During these first reactions copper(II) was reduced to copper(I) by approximately the half-way stage. The second reactions, in which copper(I) reduction to the metal was completed, was, for both salts, the significantly slower process. The first reaction of copper(II) maleate was accompanied by anion isomerization to form predominantly copper(I) fumarate before the onset of the second reaction. Copper(II) fumarate, containing the more stable anion, underwent no such transformation. Electron microscopic observations gave strong evidence that the first reaction in the decomposition of copper(II) maleate was accompanied by at least partial melt formation. Enhanced reactivity within the fused salt explains the sigmoid-shaped product yields vs. time curve. The first reaction of copper(II) fumarate was predominantly deceleratory. The second reactions of both salts obeyed the first-order equation and reactivities are compared quantitatively with the decompositions of the copper(I) salts synthesized within the reaction vessel by heating the acid with excess copper(I) oxide. Mechanisms of the decompositions of both salts [and of copper(II) malonate] are discussed with reference to the kinetic observations and interpretations are complemented by a consideration of the microscopic and the analytical evidence.