Heat content and thermal electromotive force of the molten system, AgCl + CuCl

Abstract

Measurements have been made of the heat content and thermal e.m.f. of AgCl, CuCl and of mixtures of these compounds.

Heat content measurements were made in the temperature range 350 to 950 K by drop calorimetry. Results for the pure compounds agree well with the recent measurements of Thompson and Flengas. The integral heats of mixing calculated from the data are small (∼–0.5 kJ mol^–1 at 750 K and 0.5 mole fraction). The variation of the specific heat of the liquid mixtures with mole fraction is anomalous, and may indicate complex formation in the melts.

Measurements of the initial thermal e.m.f. (θ) were made with reversible electrodes. A recent extension of the Wagner–Pitzer approximation was used to calculate the heterogeneous contribution to θ. The heat of transport term, ∑_i(t_iQ^*_i/z_i) has been calculated from the homogeneous contribution, θ_hom, for each composition studied. It varies from about 9.9 kJ mol^–1 for pure AgCl to 14.6 kJ mol^–1 for pure CuCl. The heat of transport term was also estimated from the temperature dependence of θ_hom and the results are consistent with the values calculated directly. A simple model is presented which accounts qualitatively for the variation of ∑_i(t_iQ^*_i/z_i/z_i) with composition. The data of Sinistri, Pezzati and co-workers on other binary halide systems are reconsidered in terms of this model.