Enthalpies of Mixing in the Binary Liquid Systems Alk(Cl–Br), Alk(Cl–I), and Alk(Br–I)

The enthalpies of mixing in the A(X–Y) binary liquid alkali halide mixtures have been measured by high-temperature reaction calorimetry (A denotes Li–Cs; X, Y denote Cl, Br, I). All the considered systems have positive enthalpies of mixing with maximum values which range from about +4cal/mole for Cs(Cl–Br) to about +117 cal/mole for Na(Cl–I). The molar enthalpies of mixing can be represented by the semi-empirical relations, Δ HM≅ N1N2Aδ122.In this expression, N1 and N2 are the mole fractions of the two components; δ12=(d1−d2)/d1d2, where d1 and d2 are the interionic distances characteristic of the two salts; A is a constant which for Li as a common cation is about +125 kcal·Å2/mole; for Na, K, Rb, Cs, A ≈ +250 kcal·Å2/mole. An excellent correlation is found between the positive enthalpies of mixing and calculated values derived from estimates of the London-van der Waals' interaction between second-nearest-neighbor anions. However, the observed values are about 5 times larger than the calculated ones. It is possible that this may be due to use of too low values for the characteristic energy in the London expression.