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

Chapter 18Plutonium group 17 (halogen)compounds and complexes18.1 Plutonium halide compounds18.1.1 IntroductionThere are experimental data for the enthalpies of formation at 298.15 K of some ofthe more important plutonium halides, but those of some technologically importantcompounds, such as PuF 4 and PuF 6 have still to be estimated from non-calorimetricobservations, and thus are relatively uncertain. The only halides for which low temperatureheat capacity measurements (and thus reliable standard entropies) are availableare PuF 3 (cr) and PuF 4 (cr), so that the entropies of all the other halides have had to beestimated. The lack of a precise value for the standard entropy of PuCl 3 is particularlynoteworthy.There are precise determinations of the vapour pressures of PuF 3 ,PuF 6 ,PuCl 3 andPuBr 3 . The derived entropies of sublimation and vaporisation of the first three are inexcellent accord with those predicted from the thermal functions (often estimated), butthose of PuBr 3 are appreciably higher than anticipated, for reasons which are not clear.18.1.2 Plutonium fluoride compounds18.1.2.1 PuF(g) and PuF 2 (g)18.1.2.1.a Enthalpy of formationKent [68KEN] has studied the equilibria involving Pu(g), PuF(g), PuF 2 (g) and PuF 3 (g)evaporating from a tantalum Knudsen cell, containing a mixture of PuF 3 and electrorefinedplutonium metal, attached to a mass-spectrometer. The PuF + 2ion was formedfrom both PuF 2 (g) and PuF 3 (g) present in the vapour, but the fractions arising from thetwo species were estimated by making measurements at different accelerating voltages.The ion current analogues of the equilibrium constants for the two reactionsPuF 3 (g) + 2Pu(g) Å 3PuF(g) (18.1)2PuF 3 (g) + Pu(g) Å 3PuF 2 (g) (18.2)were plotted as a function of reciprocal temperature to give the enthalpiesof these reactions: r H m (18.1, 1414 K) = (97.9 ± 19.2) kJ·mol −1 and345