Abstract
The equilibrium between copper(I) oxide and gold + copper alloys (25 to 30 moles per cent Cu) has been studied by low-pressure surface oxidation over the 810 to 900 K temperature range. Values of the standard Gibbs free energy were calculated from published heat capacities and the observed equilibrium oxygen pressures. The values: ΔG f o(Cu 2O, s, 298.15 K) = −(118.2 ± 1.2) kJ mol −1 and ΔH f o(Cu 2O, s, 298.15 K) = −(134.2 ± 1.1) kJ mol −1 were determined by extrapolation. By use of recently selected values for the standard entropies of oxygen and copper the value S o(Cu 2O, s, 298.15 K) = (115.1 ± 0.4) J K −1 mol −1 was obtained. Our results as well as others based upon pressure measurements differ significantly from the results obtained by electrochemical and calorimetric measurements. It is proposed that the deviations are primarily due to surface effects of the very small crystals at the alloy single-crystal surface. Hence, an experimental method is reported whereby standard Gibbs energy changes due to surface phenomena may be studied.
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