Vapor Pressures of Aluminum Tribromide and Aluminum Triiodide and Their Standard Sublimation Enthalpies

Abstract

Total vapor pressures of AlBr 3 and All 3 were measured by a torsion effusion apparatus. Their temperature dependences are expressed by the equations log(p/Pa) = (14.78 ± 0.60) - (4700 ± 200)/(T/K) from (301.0 to 351.0) K and log(p/Pa) = (15.52 ± 0.30) - (5960 ± 150)/(T/K) from (359.5 to 419.5) K for AlBr 3 and AlI 3 , respectively. Practically, AlBr 3 vaporizes in a dimeric form, while AlI 3 vaporizes in monomeric and dimeric forms. Treating the vapor pressures of AlBr 3 by second- and third-law methods, the standard sublimation enthalpy ΔH°(298 K) = (90 ± 4) kJ·mol -1 was derived. For AlI 3 , treating the partial pressures of monomer and dimer gaseous species, deduced from the measured total vapor pressures and the constant of dimer-monomer equilibrium reported in the literature, by second- and third-law methods, the standard enthalpies ΔH°(298 K) = (110 ± 4) kJ·mol -1 and ΔH°(298 K) = (119 ± 4) kJ·mol -1 associated to the sublimation reactions of AlI 3 according to the AlI 3 (s) → AlI 3 (g) and 2AlI 3 (s) → Al 2 I 6 (g), respectively, were selected.