Vapour pressures and activities in the reciprocal fused salt systems ZnCl₂ + Na₂SO₄ and ZnSO₄ + NaCl

Abstract

Zinc chloride vapour pressures, activities and partial molar energies have been determined in the reciprocal fused salt system: Zn,Na/Cl,SO₄ over the range 770-900 K. The transpiration vapour pressure of pure ZnCl₂ was first redetermined for the range 720-900 K and found to follow the equation (in which P° is in mmHg):                Log P°_ZnCl2 = (-8247±17)/T-5.033 log T + 26.213 ± 0.006 Activities of ZnC1, in the reciprocal system differed greatly from ideality, the degree of departure varying with both composition and temperature. In general, at relatively high ZnCl, concentrations, observed activities were greater than ideal and ZnCl₂ partial molar enthalpies were positive; this behaviour is believed to be largely caused by the effect of partial covalent bonding in pure ZnCl₂. At low ZnCl₂ concentrations, activities were much less than ideal and ZnCl₂ partial molar enthalpies were negative; this other effect is believed to be caused by the formation of complex ions within this system.