Thermodynamic Study of Sublimation of PbF2 and PbF4 from Torsion-Effusion Vapor Pressure Measurements

Abstract

The total vapor pressures of lead di- and tetrafluoride were measured by a torsion-effusion method. The vapor above PbF 2 (s) is prevalently PbF 2 (g). The temperature dependence of the vapor pressure is represented, in the temperature range (793 to 951) K, by the expression: log(p/Pa) = (12.76 ± 0.10) - (10940 ± 200)/ (T/K). Treating the vapor pressures by the second and third laws, the sublimation enthalpy of PbF 2 , Δ sub H°(298 K) = (225 ± 5) kJ·mol -1 , was selected. Lead tetrafluoride decomposes according the reaction: PbF 4 (s) → PbF 2 (s) + F 2 (g) (a). In the range (596 to 684) K, the temperature dependence of the fluorine pressure above PbF 4 (s) at near unit activity is represented by the expression: log(p/Pa) = (11.70 ± 0.20) - (7450 ± 200)/ (T/K). In this temperature range, fluorine is partially dissociated so that, in addition to reaction a, the reaction 1/2 PbF 4 (s) → 1/2 PbF 2 (s) + F(g) (b) was also studied. The second-law enthalpies at 650 K, ΔH°(650 K) = (141 and 152) kJ·mol -1 associated to the reactions a and b, respectively, were obtained with an estimated error of 6 kJ·mol -1 for both enthalpies. The entropy of PbF 4 (s), S°(650 K) = (272 ± 15) J·K -1 ·mol -1 , was also evaluated.