Molecular Association in Alkali Halide Vapors

Abstract

Molecular association equilibria in alkali halide vapors were studied by measuring the temperature dependence of the molecular weights of gaseous NaCl, NaBr, NaI, KCl, KI, RbCl, and CsCl. The molecular weights were determined by measurement of the absolute pressure exerted by a known weight of completely vaporized salt contained in an isothermal fused silica bulb of known volume. The pressure sensing element was a fused silica manometer containing molten gold. The apparatus was used in the temperature range of 1175° to 1430°K and pressures of from 10 to 40 mm were measured with a precision of ±0.05 mm. The temperature dependence of the equilibrium constants for the reaction (MX)₂⇌2MX yielded dissociation energies (evaluated at 1300°K) ranging from 48.0 kcal mole⁻¹ for NaCl to 34.7 for CsCl. The entropies of dissociation were found to fall within a small range, varying from 28.3 eu for NaCl to 25.0 for KI, and a statistical calculation of the entropy changes based on an ionic model was found to agree well with the experimental values. It is shown that these systems may be adequately described with an electrostatic model, although closer attention should be paid to the nature of polarization interactions.