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

The vapor pressures in equilibrium over CuF 2 , AgF 2 , and AgF were measured by the torsion-effusion method. CuF 2 and AgF vaporized congruently. The temperature dependences of the vapor pressure of these compounds were found to fit the following equations: CuF 2 (s), log(p/kPa) = (9.78 ± 0.10) - (12690 ± 100)(T/K) (from (881 to 1051) K) and AgF(1), log(p/kPa) = (7.83 ± 0.20) - (10700 ± 300)(T/K) (from (880 to 1074) K). Treating the vapor pressure data by the second- and third-law methods, the standard sublimation enthalpies of these compounds, Δ sub H°(298 K) = (260 ± 4) kJ·mol -1 and (240 ± 10) kJ·mol -1 for CuF 2 and AgF, respectively, were calculated. In the covered temperature range (from (655 to 776) K), AgF 2 decomposes into fluorine and solid or molten AgF so that two temperature dependences of the fluorine pressure above this compound in equilibrium with AgF(s) and AgF(1), log(p/kPa) = (11.39 ± 0.30) -(11060 ± 300)(T/K) (from (655 to 706) K) and log(p/kPa) = (10.27 ± 0.20) - (10220 ± 200)(T/K) (from (710 to 776) K), respectively, were determined. At these temperatures, fluorine is partially decomposed so that two partial molar standard enthalpies of AgF 2 associated to the sublimation reactions, AgF 2 (s) → AgF(s) + F(g) and AgF 2 (s) - AgF(s) + ½F 2 (g), Δ sub H°(298 K) = (195 ± 10) kJ·mol -1 and (125 ± 5) kJ·mol -1 , were calculated.