Thermodynamics of the actinoid elements. Part V. Enthalpies and gibbs energies of formation of some protactinium-(IV) and -(V) halides

Abstract

The following enthalpies of solution (kJ mol^–1) have been measured: protactinium metal in oxygen-free 12M-HCl–0.05M-HF to yield Pa^IV, –666 ± 13, and in 12M-HCl–0.05M-HF containing oxygen to yield Pa^V, –817 ±15; PaCl₄(s) in oxygen-free 12M-HCl–0.05M-HF and 12.05M-HCl, –160.9 ± 2.6 and –105.9 ± 2.5, respectively; and PaCl₅(s) and PaBr₅(s) in 12M-HCl –0.05M-HF. –203.2 ± 3.0, and –242.0 ± 5.0, respectively. Combination of these values with previously reported enthalpies of solution for other protactinium compounds and with other thermodynamic data yields the following enthalpies of formation (in kJ mol^–1) at 298 K: Pa^IV in 1 and 6M-HCl, –620 ± 13 and –608 ± 1 3, respectively; Pa^V in 12M-HCl–0.05M-HF. –672 ±15; PaF₄(s), –1 918 ± 30; PaCl₄(s), –1 045 ± 13; PaBr₄(s), –827 ± 13; PaCl₅(s), –1 143 ± 15; PaBr₅(s), –865 ± 16; and Cs₂[PaCl₆], –2 011 ± 13. By use of suitably interpolated entropy and heat-capacity data, thermodynamic functions associated with formation of the halides have been calculated as a function of temperature.