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

Chapter 9Neptunium group 17 (halogen)compounds and complexes9.1 Neptunium halide compounds9.1.1 IntroductionThe experimental thermodynamic data for the neptunium halides are far from complete,consisting essentially of some measurements of enthalpies of solution and somevapour pressure determinations. The only experimental data for low temperature heatcapacities, and thus standard entropies, are those for NpF 6 (cr). This means that many,indeed most, of the data given in this section are interpolated (or extrapolated) fromthose of the corresponding thorium, uranium and plutonium compounds. In practice,this means that the trivalent compounds are related more to the U and Pu systems,the quadrivalent compounds to Th and U systems (where the data are often fairly welldefined). The comparisons for the few compounds of higher valency rely heavily on thedata for the corresponding uranium compounds, since there are often no Pu analogues.9.1.2 Neptunium fluoride compounds9.1.2.1 NpF(g) and NpF 2 (g)These species were observed in the mass-spectrometric study of NpF 4 by Kleinschmidtet al. [92KLE/LAU] but no thermodynamic data (other than appearance potentials)were reported. However their stabilities can be estimated fairly reliably from the datafor the corresponding Th, U and Pu compounds.9.1.2.1.a Enthalpies of formationThe enthalpy of formation of ThF(g), UF(g) and PuF(g) are close to a linear functionof atomic number, suggesting that the enthalpy of formation of NpF(g) is close to themean of those for UF(g) (−(52±30) kJ·mol −1 [92GRE/FUG]) and PuF(g) (−(112.6±10.0) kJ·mol −1 , see Section 18.1.2.1). The selected value is f Hm ◦ (NpF, g, 298.15 K) = −(82 ± 30) kJ·mol−1The situation for NpF 2 (g) is less clear-cut, since the enthalpies of formation ofThF 2 (g) [85HIL/GUR] andPuF 2 (g) (see Section 18.1.2.1) are very similar, while131