The Lattice Energies of the Ammonium Halides and the Proton Affinity of Ammonia

The lattice energies of NH₄F and of the CsCl‐type lattices of the other ammonium halides are computed by the method of Born and Mayer, assuming ionic crystals with a van der Waals potential. The calculations appear to be quantitatively satisfactory for the chloride, bromide and iodide. The method fails, however, for the asymmetrical fluoride lattice, and another method is employed. The theoretically calculated lattice energies in kilocalories are, NH₄Cl, 161.6; NH₄Br, 154.0; NH₄I, 145.5. From these lattice energies by means of a thermal cycle the energy change at 0°K of the reaction ${\mathrm{NH}}_{3_{\mathrm{gas}}}{\mathrm{+H}}_{\mathrm{gas}}^{+}{\mathrm{\to NH}}_{4\mathrm{gas}}^{+},$ which may be called the proton affinity of ammonia, is found to be —206.4 kcal. With this proton affinity and the same cyclic process the lattice energy of NH₄F is calculated to be 191.4 kcal.