chemical thermodynamics of neptunium and plutonium - U.S. ...

18.1 Plutonium halide compounds 347the CODATA Key Value [89COX/WAG] −(335.35 ± 0.65) kJ·mol −1 , the differencebeing accommodated in the enthalpies of dilution from the lowest concentration usedby [73JOH/SMI] and infinite dilution.Hence, for the reactionPu(cr,α)+ 1.5F 2 (g) + 0.4H 2 O(l) = PuF 3·0.4H 2 O(cr) r Hm ◦ (298.15 K) = −(1582.93 ± 3.00) kJ·mol−1The enthalpy of dehydration of PuF 3·0.4H 2 O(cr) has been taken to be r H(dehydration) = −(3.8 ± 2.1) kJ·mol(PuF 3 ) −1as discussed by Fuger et al.[83FUG/PAR].Hence, f H ◦ m (PuF 3, cr, 298.15 K) = −(1586.7 ± 3.7) kJ·mol −118.1.2.2.b Standard entropy and heat capacityThe standard entropy and heat capacity at 298.15 K are taken from the low-temperatureheat capacity measurements of Osborne et al.[74OSB/FLO], using a sample containingnearly isotopically pure 242 Pu, to minimise problems from self-heating at very lowtemperatures.Sm ◦ (PuF 3, cr, 298.15 K) =−1(126.11 ± 0.36) J·K−1·molCp,m ◦ (PuF 3, cr, 298.15 K) =−1(92.64 ± 0.28) J·K−1·mol18.1.2.2.c High temperature heat capacityThere are no measured values of the heat capacity above 350 K. Values have beenestimated to be similar to those of UF 3 (cr), assessed by Grenthe et al. [92GRE/FUG],with the constraint that they merge smoothly with the experimental values from 298.15to 350 K from [74OSB/FLO]:Cp,m ◦ (PuF 3, cr, T) = (104.078 + 0.707 × 10 −3 T − 10.355 × 10 5 T −2 −1) J·K−1·molfrom 298.15 K to 1700 K.18.1.2.2.dFusion dataThe melting point is taken to be (1700 ± 3) K, corrected from the experimental valueof (1699 ± 3) K (assumed to be on the IPTS68 Temperature scale) given by Westrumand Wallmann [51WES/WAL]. The enthalpy of fusion ((35.4 ± 5.0) J·K −1·mol −1 )and the heat capacity of the liquid ((130 ± 20) J·K −1·mol −1 ) have been estimated andgiven by Rand and Fuger [2000RAN/FUG].