Enthalpies of Mixing in Some Binary Alkaline-Earth Chlorides

The integral enthalpies of mixing in the six binary systems formed by MgCl₂, CaCl₂, SrCl₂, and BaCl₂ have been determined calorimetrically. The results are discussed in terms of the theories of Reiss, Katz, and Kleppa and of Davis and Rice, each suitably modified to take into account the higher charge on the cation. The molar enthalpies of mixing for the systems CaCl₂–SrCl₂, CaCl₂–BaCl₂, and SrCl₂–BaCl₂ can be represented by the following approximate expression which is simply related to the corresponding expression for the binary alkali halides, previously derived by Hersh and Kleppa: $${\mathrm{\Delta H}}^M{\mathrm{=X}}_1{X}_2{\mathrm{(U}}_0^{\mathrm{++}}{\mathrm{-Z}}^2{\text{340δ}}_{12}^2\mathrm{)\hspace{0.17em}}\mathrm{kcal}/\mathrm{mole}.$$ Here X₁ and X₂ are the mole fractions of the two components; δ₁₂=(d₁—d₂)/d₁d₂, where d₁ and d₂ are the sums of the ionic radii of anion and cation in the two salts, while Z=Z₁Z₂, the product of the charges of the anion and cation in the component salts (here Z=2). The term U₀⁺⁺ represents an estimate of the contribution to the enthalpy of mixing arising from the London dispersion interaction between next‐nearest‐neighbor cations. The quoted expression does not hold for the three binary systems involving MgCl₂, which are all much less exothermic and which exhibit considerable energetic asymmetry. In all cases the enthalpy of solution of MCl₂in MgCl₂ is more endothermic than the opposite process. The results for these three systems are interpreted to support the view, originally advanced by Førland, that pure MgCl₂ has a certain tendency to form covalently bonded Mg–Cl–Mg bridges. The breaking of these bridges gives rise to a significant endothermic contribution to the enthalpy of mixing, over and above the contributions arising from Coulombic and dispersion forces.