Thermal stability and thermodynamics of copper(II) chloride dihydrate

Four tensimetric methods were used to measure the equilibrium water vapour pressure for the reaction 1/2CuCl2 · 2H2O(s) = 1/2CuCl2(s)  +  H2O(g) in the temperature range (301.40 to 404.65) K. Formation of the metastable hydrate super disperse phase [O.G. Polyachenok, E.N. Dudkina, N.V. Branovitskaya, L.D. Polyachenok, Thermochim. Acta 467 (2008) 44–53] was taken into account at studying this process. A linear equation was derived for the temperature dependence of the water vapour pressure: ln(p/p∘) = 17.237 − 6980.4 · (K/T). Standard enthalpy and entropy of this process were calculated at the mean temperature 358 K: ΔrH∘(358) = 58038 ± 73 J, ΔrS∘(358) = 143.32 ± 0.21 J · K−1. These data being processed together with the results of [R. Perret, Bull. Soc. Chim. France 2 (1966) 755–758] enabled to obtain the change of heat capacity ΔrCp∘=-10±2J·K-1·mol-1, and hence a more precise trinomial equation was deduced: ln(p/p°) = 25.515 – 7409.4 · (K/T) – 1.203 · ln(T  /K). Standard thermodynamic characteristics of copper(II) chloride and its dihydrate were obtained: ΔfHm∘(298)CuCl2(s)=-219.1±1.8kJ·mol-1; Sm∘(298)CuCl2·2H2O(s)=195.3±1.4J·K-1·mol-1; Cp,m∘(298)CuCl2·2H2O(s)=159±4J·K-1·mol-1.