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

356 18. Plutonium group 17 (halogen) compounds and complexes18.1.3.1.d Fusion dataThe melting point is taken to be (1041 ± 2) K from Bjorklund et al. [59BJO/REA],corrected to the 1990 Temperature scale. The enthalpy of fusion is calculated from thevaporisation data for the solid and liquid by Phipps et al. [50PHI/SEA], corrected toT fus = 1041 K (see Section 18.1.3.2 for further details). fus Hm ◦ (PuCl 3, cr, 1041 K) = (55.0 ± 5.0) kJ·mol −1The heat capacity of the liquid has been estimated by Rand and Fuger[2000RAN/FUG],basedonthatestimatedforUF 3 (l) by Glushko et al.[82GLU/GUR]and comparison with lanthanide chlorides [63DWO/BRE, 71DWO/BRE], to beC p,m (PuCl 3 , l) = (145 ± 15) J·K −1·mol −1 .18.1.3.2 PuCl 3 (g)18.1.3.2.a Enthalpy of formationThe Knudsen effusion measurements of vapour pressure in tantalum from 879 to1224 K by Phipps et al.[50PHI/SEA], which have very little scatter, have been correctedto the 1990 Temperature scale and the 1986 set of Fundamental constants. Theseauthors interpreted their data to give a melting point of 1008 K, appreciably lowerthan the value of (1041 ± 2) K directly measured by Bjorklund et al. [59BJO/REA](see Section 18.1.3.1.d). However, if the two relevant points are moved to refer tosublimation of the solid rather than vaporisation of the liquid, the least square fit forthe sublimation pressures becomeslog 10 (p/bar) = 9.3677 − 15472.6/Tfrom 879 to 1034 K.Third law treatment of the data for the sublimation using the thermal functions forPuCl 3 (g) estimated below givesand hence, sub Hm ◦ (298.15 K) = (312.2 ± 0.5) kJ·mol−1 f H ◦ m (PuCl 3, g, 298.15 K) = −(647.4 ± 2.0) kJ·mol −1The least-square fit for the vapour pressure of the liquid from 1051 to 1136 K,subject to the constraint that the vapour pressures of the solid and liquid be equal at themelting point, defines the enthalpy of fusion given above.18.1.3.2.b Standard entropy and heat capacityThe thermal functions of PuCl 3 (g) were calculated assuming the molecular parametersgiven by Rand and Fuger [2000RAN/FUG], using the rigid-rotator, harmonic oscillatorapproximation. These molecular parameters giveSm ◦ (PuCl 3, g, 298.15 K) =−1(368.62 ± 10.00) J·K−1·molCp,m ◦ (PuCl 3, g, 298.15 K) =−1(78.47 ± 5.00) J·K−1·mol