Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

The total vapor pressures of lead di- and tetrafluoride were measured by torsion-effusion method. The vapor above PbF2(s) is prevalently PbF2(g). The temperature dependence of the total 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 second- and third-law, the sublimation enthalpy of PbF2, subH (298 K) = (225 } 5) kJ Emol-1, was selected. The lead tetrafluoride decomposes according the reaction: PbF4(s) PbF2(s) + F2(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 the reaction (a), the reaction: . PbF4(s) . PbF2(s)+ F(g) (b) was also studied. The second-law enthalpies at 650 K, subHdeg (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 the enthalpies. The entropy of PbF4(s), Sdeg (650 K) = (272 +- 15) J*K-1 *mol-1, was also evaluated.