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

The total vapor pressures of lead di- and tetrafluoride were measured by a torsion-effusion method. The vapor above PbF2(s) is prevalently PbF2(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 PbF2, Delta H-sub(degrees)(298 K) = (225 +/- 5) kJ.mol(-1), was selected. Lead tetrafluoride decomposes according the reaction: PbF4(s) -> PbF2(s) + F-2(g) (a). In the range (596 to 684) K, the temperature dependence of the fluorine pressure above PbF4(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 PbF4(s) -> 1/2 PbF2(s) + F(g) (b) was also studied. The second-law enthalpies at 650 K, Delta H degrees(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 PbF4(s), S degrees(650 K) = (272 +/- 15) J.K-1.mol(-1) was also evaluated.