Vapor Pressures and Sublimation Enthalpies of Mercury(I, II) Fluorides by the Torsion-Effusion Method

The vapor pressures in equilibrium over HgF2 and Hg2F2 were measured by the torsion-effusion method. HgF2 vaporized congruently. The temperature dependence of the vapor pressure of this compound was found to fit the following equation: HgF2(s), log(p/kPa) = (9.00 +/- 0.30) - (6750 +/- 200)(K/T) (from (496 to 629) K). Treating the vapor pressure data by the second- and third-law methods, the standard sublimation enthalpy, Delta H-sub degrees(298 K) = (136 +/- 4) kJ.mol(-1), and entropy, Delta S-sub degrees(298 K) = (145 +/- 6) J.K-1.mol(-1), were selected. In the covered temperature range (from (457 to 589) K), Hg2F2(s) decomposes into HgF2(s) and Hg(g) so that the temperature dependence of the mercury pressure above this compound, log(p/kPa) = (7.90 +/- 0.50) - (5400 +/- 300)(K/T), was determined. Also, for this compound, the standard sublimation enthalpy, Delta H-sub degrees(298 K) = (105 +/- 4) kJ.mol(-1), and entropy, Delta S-sub degrees(298 K) = (114 +/- 10) J.K-1.mol(-1), were determined.