The kinetics and mechanism of the thermal decomposition of copper(II) malonate

Abstract

A kinetic and mechanistic study of the thermal decomposition of copper(II) malonate has been completed in which the rate measurements have been complemented with microscopic and analytical observations for salt that was partly decomposed to various known extents. Plots of the fractional reaction ( α ) against time for the isothermal reactions were approximately sigmoid but were distorted by a significant diminution in rate at the half-way stage ( α ≈ 0.5). Microscopic observations gave evidence that local fusion occurred soon after the onset of reaction, so that the initial acceleratory behaviour cannot be ascribed to a solid-state nucleation and growth process. From analytical measurements it is concluded that decomposition proceeds to completion by two distinct reactions, involving a stepwise cation reduction: Cu 2+ →Cu + →Cu 0 . The relatively slower rate of the second reaction accounts for the marked diminution in slope of the α -time plots after α = 0.5. The first reaction, copper(II) malonate decomposition, is characterized by a prolonged acceleratory process that obeys the exponential equation (In α = kt ). This is explained by autocatalytic behaviour in which anion breakdown is promoted by acetate, a reaction product, thus (d α /d t ) = kα . Separate experiments confirmed that added copper(II) acetate accelerated the reaction. The decomposition of copper(II) malonate was accompanied by partial fusion, and gas evolution within the viscous melt resulted in the development of an intracrystalline froth-like texture. Such partial melting was, however, localized within the reactant and the sizes and shapes of crystallites did not change markedly during reaction, so that product particles were pseudomorphic with those of the reactant. The final, short deceleratory phase of the first reaction overlapped with the onset of the second reaction.