Studies of Fused Salts. III. Heats of Mixing in Silver Nitrate-Alkali Nitrate Mixtures

The enthalpies of mixing in binary liquid mixtures of silver nitrate with lithium‐, sodium‐, potassium‐, and rubidium nitrate have been measured. It is found that the molar enthalpy of mixing (ΔH^M) changes in a regular fashion from positive values for lithium‐ and sodium‐ to negative values for potassium‐ and rubidium nitrate. Neglecting second‐order effects, the mixing enthalpies may be represented by the following semiempirical relation, $${\mathrm{\Delta H}}^M{\text{≅−X(1−X)[140\hspace{0.17em}δ}}^2\text{−24(δ−0.01)]\hspace{0.17em}}\mathrm{kcal}/\mathrm{mole}.$$ The parameter δ is (d₁—d₂) / (d₁+d₂), where d₁ and d₂ are the interionic distances characteristic of the two pure components, while X and (1 — X) are their mole fractions. In this expression the δ² term has been taken from the equivalent expression for the binary alkali nitrates, which presumably are essentially ionic. The value of the numerical factor (140 kcal/mole) is of the order of the lattice energy of these salts. The term which is linear in δ takes into account departures from ionicity in silver nitrate, and is of magnitude comparable to the nonionic contribution to the lattice energy of this compound. It is shown by considering available emf data for the silver halide‐alkali halide systems that expressions of this type may be quite useful in predicting the heats of mixing in as yet unexplored systems.