Complex equilibria in unsaturated vapor over AlBr₃

Abstract

Unsaturated AlBr 3 vapor pressure was measured over the temperature and pressure ranges 560–845 K and 54–145 torr by the static method using a quartz diaphragm pressure gauge with increased sensitivity (the confidence interval of pressure, including thermal drift of zero pressure gauge point, was 0.3 torr, and that of temperature, 0.3 K). Two equilibrium models were considered, one including AlBr₃ and Al₂Br₆ and the other, AlBr₃, Al₂Br₆, and Al₃Br₉. The molecular constants of all vapor constituents were determined using density functional theory at the B3LYP/6-31G(d,p) level. The thermodynamic functions of all bromides were calculated in the rigid rotator-harmonic oscillator approximation. The enthalpies of independent equilibria for each model were determined by minimizing the residual sum of the squares of pressure discrepancies. According to the first model, 0.5Al₂Br₆ = AlBr₃, ΔH ^o(298.15) = 13629.1 ± 9 cal/mol. According to the second model, 0.5Al₂Br₆ = AlBr₃, ΔH ^o(298.15) = 13638.8 ± 8 cal/mol, and 1.5Al₂Br₆ = Al₃Br₉, ΔH ^o(298.15) = −8528 ± 800 cal/mol. The second model, for which the variance of pressure differs insignificantly from the experimental variance of pressure, should be given preference.