The vapour composition and pressure over solid potassium chloride: new mass-spectrometric results and effusion masses

Abstract

The composition of the vapour phase over solid potassium chloride was analyzed mass spectrometrically in the temperature range 930 K to 1040 K. A linear least-squares fit of dimer mole fraction x against temperature T is given by: x = {7.45·10−4·(T/K)−0.5458} ±0.02. We combined our mass-spectrometric result with the vapour compositions from three different literature sources to obtain a best fit for x in the temperature range 600 K to 1050 K. The result is x = 8.51·10−7·(T/K)2−8.70·10−4·(T/K)+ 0.23. The apparent vapour pressure was determined by means of effusion masses in the temperature range 840 K to 1040 K. The vapour compositions were combined with our effusion masses and the vapour pressures from seven literature sources to obtain a best value for the third-law molar enthalpies of sublimation of the monomer and of the dimer. The results are ΔgsHom(298.15 K, KCl) = (221.7 ± 0.7) kJ·mol−1, and ΔgsHom(298.15 K, K2Cl2) = (260.2 ± 1.0) kJ·mol−1. With these results we calculated the standard molar enthalpies of formation of the two gaseous species and the reaction enthalpy between monomer and dimer: ΔfHom(298.15 K, KCl, g) = −215.0 kJ·mol−1; ΔfHom(298.15 K, K2Cl2, g) = −613.2 kJ·mol−1; ΔrHom(298.15 K, 2KCl = K2Cl2) = 183.2 kJ·mol−1. A review of the literature on this subject is given from 1936 until now.