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

9.1 Neptunium halide compounds 1519.1.4 Neptunium bromide compounds9.1.4.1 NpBr 3 (cr)NpBr 3 (cr) exists in two crystal modifications, the hexagonal structure isomorphouswith UBr 3 (cr) (space group P6 3 /m) and the orthorhombic form (TbCl 3 -type, spacegroup Cmcm) in which PuBr 3 (cr) crystallises.9.1.4.1.a Enthalpy of formationHurtgen [78HUR] has measured the enthalpy of solution of the hexagonal formof NpBr 3 (cr) to be −(161.4 ± 2.0) kJ·mol −1 in oxygen-free 10 −3 M HCland −(158.8 ± 0.9) kJ·mol −1 in oxygen-free 0.1 M HCl. The partial molarenthalpy of formation of HBr in HCl solutions is assumed to be the same asin HBr solutions of the same molality, which have been recalculated from theenthalpy of dilution data given by Parker [65PAR], and are −(121.33 ± 0.15)and −(120.71 ± 0.15) [2000RAN/FUG]. These and the currently assessed valuefor f Hm ◦ (Np3+ , aq, 298.15 K) = −(527.2 ± 2.1) kJ·mol −1 assumed to be thesame as in 10 −3 and 0.1 M HCl give the values of −(729.8 ± 2.9) kJ·mol −1 and−(730.5 ± 2.9) kJ·mol −1 for f Hm ◦ (NpBr 3, cr, hex, 298.15 K) from these solutions.The selected value is the weighted mean f H ◦ m (NpBr 3, cr, hex, 298.15 K) = −(730.2 ± 2.9) kJ·mol −1The use of the infinite dilution value for f H ◦ m (Np3+ , aq, 298.15 K) in 0.001and 0.1 M HCl solutions in these calculations has been justified by Fuger et al.[83FUG/PAR] on the grounds that the enthalpies of formation of all the actinidetrivalent ions vary by less than 0.5 kJ·mol −1 between 0.1 M HCl solutions and theinfinitely dilute solution. We accept this reasoning, but have increased the uncertaintyon the value derived from the measurement in 0.1 M HCl.9.1.4.1.bStandard entropy and heat capacityThere are no low-temperature measurements of the heat capacity of NpBr 3 (cr),and the standard entropy is derived from three reasonably consistent estimates by[2000RAN/FUG]. From these three estimates of 206.1, 194.9, 195.6 J·K −1·mol−1 ,the selected value is biased towards the lower estimates [2000RAN/FUG]Sm ◦ (NpBr −13, cr, hex, 298.15 K) = (196 ± 8) J·K−1·mol9.1.4.1.cHigh temperature heat capacityLacking experimental data, the heat capacity of NpBr 3 (cr) is assumed to be similar toUBr 3 (cr) and those estimated for PuBr 3 (cr) by this review. The values suggested forNpBr 3 (cr) are:Cp,m ◦ (NpBr 3, cr, hex, T) = (101.23 + 20.68 × 10 −3 T − 3.2 × 10 5 T −2 −1) J·K−1·mol