Thermal decomposition of copper(II) squarate

Abstract

The thermal decomposition of copper(II) squarate dihydrate, in vacuum between 530 and 670 K, has been shown by kinetic and complementary analytical measurements to proceed (following rapid initial loss of the water of crystallization) through two reactions, involving stepwise reduction of the cation, Cu²⁺→ Cu⁺→ Cu⁰. This pattern of behaviour has been described previously for the decompositions of several copper(II) carboxylates. Isothermal fractional decomposition (α)vs. time curves were deceleratory throughout and were analysed in two parts. The first stage of reaction (α < 0.5) was the decomposition of copper(II) squarate to copper(I) squarate, studied in the temperature interval 530–590 K. The initial part of this rate process was zero-order with an activation energy of 150 ± 15 kJ mol^–1. Subsequently the rate diminished and reaction could be represented approximately by either a contracting-interface model or by one-dimensional advance of a reactant/product interface into flat crystallites of varying thicknesses. The second stage of reaction (α > 0.5) was the decomposition of copper(I) squarate to copper metal produced in the form of small crystallites dispersed on a carbonaceous matrix that was pseudomorphic with the reactant particles. This process, studied in the interval 590–670 K, obeyed the first-order equation approximately, when due allowance was made for overlap of the two regions, and the activation energy was 210 ± 20 kJ mol^–1. Electron microscopic observations of reactant, product and samples of partially decomposed salt did not reveal any recognizable reactant–product interface. It is suggested that Cu₂O is formed as a decomposition intermediate, thus providing an explanation for the formation of product CO₂. The observations are discussed and a reaction mechanism is proposed in the context of previous studies of the decompositions of copper(II) carboxylates and of other metal salts of squaric acid.