Vapor Pressures of Zinc Chloride and Zinc Bromide and Their Gaseous Dimerization

Abstract

The absolute vapor pressures of zinc chloride and zinc bromide have been measured by a quasistatic method. In addition, transpiration experiments have been made to determine the vapor density of the gas. Mass spectrometric observations of the gas over the liquid salts have indicated that the gas consists mainly of zinc halide monomer with some dimer also present. The individual partial pressures of the zinc halide monomers and dimers calculated from the results of the pressure and density measurements can be represented by the equation logp(mm)=(−A/T)−BlogT+C.Constants A, B, and C are: for zinc chloride monomer, 8415.2, 5.034, 26.420, respectively; for dimer, 4698.8, 0, 5.891; for zinc bromide monomer 7784.5, 5.034, 26.112; and for dimer 5612.5, 0, 7.915. The various thermodynamic functions have also been calculated and are discussed. The zinc halides are unique among salts that have polymeric gaseous species in that the heat of vaporization of the monomer is greater than the heat of vaporization of the dimer whereas the monomer is the predominant species in the temperature range investigated.