THE THERMODYNAMICS OF MIXED-CRYSTALS OF (AMMONIUM-CHLORIDE PLUS AMMONIUM BROMIDE) .4. THE EXCESS GIBBS FREE-ENERGY, EXCESS ENTHALPY, AND EXCESS ENTROPY AT THE TEMPERATURE T=298.15 K AND AT T=0

Measurements have been made of the activity coefficients at the temper ature T = 298.15 K of ammonium chloride and ammonium bromide, both in aqueous solutions of the separate salts and in five solutions containi ng both salts. The results have been extrapolated to give these activi ty coefficients in saturated solutions, and the values so obtained hav e been combined with published results for (ammonium chloride + ammoni um bromide + water) to estimate G(m)(E), the excess molar Gibbs free e nergy increase on the formation of solid solutions from the two solid components. Calorimetric determinations have also been made of H-m(E), the molar enthalpy of formation, for three solid solutions. The resul ts for G(m)(E) and H-m(E) show that at T = 298.15 K the excess molar e ntropy S-m(E) is positive. The values of these three excess molar func tions for {xNH(4)Cl + (1 - x)NH4Br}(s.s.) at x = 0.5 and T = 298.15 K are 934 J . mol(-1), 1116 J . mol(-1), and +0.61 J . K-1. mol(-1), res pectively. Entropy values, determined calorimetrically, are available for the pure salts and for three solid solutions, and when these are c ombined with the results for S-m(E) at T = 298.15 K, it is found that, for all three solutions, S-m(E) at T = 0 is negative. Thus, for the s olid solution of composition NH4Br0.55Cl0.45, S-m(E) at T = 298.15 K i s +0.58 J . K-1. mol(-1) while at T = 0 it is -1.34 J . K-1. mol(-1). It is difficult to assess the probable error associated with these val ues of S-m(E)(T-->0), and it may amount to +/-0.8 J . K-1. mol(-1). Ne vertheless, the results suggest that there is some local positional or der of the anions in the solid solutions, but that, at T = 0, there is little if any residual orientational disorder of the ammonium ions. ( C) 1995 Academic Press Limited