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

Total vapor pressures of AlBr(3) and AlI(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 Delta H degrees(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 Delta H degrees(298 K) = (110 +/- 4) kJ.mol(-1) and Delta H degrees (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.