Thermodynamics of the actinide elements. Part IV. Heats and free energies of formation of the tetrachlorides, tetrabromides, and tetraiodides of thorium, uranium, and neptunium

Abstract

Heats of solution measured at 298·15 ± 0·05 K for ThCl₄(s), ThBr₄(s), Thl₄(s), UBr₄(s), UL₄(s), and NpBr₄(s) in 1 M-HCl are –241·8± 0·7, –283·5 ± 1·3, –322·0 ± 0·6, –264·6 ± 0·5, –289·9 ± 1·3, and –258·8 ± 0·5 kJ mol^–1 respectively, and in 6 M-HCl, –188·3 ± 0·4, –233·9 ± 0·8, –276·0 ± 1·2, –205·7 ± 0·9, –240·2 ± 1·3, and –199·2 ± 0·4 kJ mol_–1 respectively. Combination of these values with other thermodynamic data yields the following standard heats of formation: ThCl₄(s), –1184·4 ± 1·0 kJ mol^–1; ThBr₄(s), –963·4 ± 1·4 kJ mol^–1; Thl₄(s), –662·0 ± 1·8 kJ mol^–1; Ubr₄(s), –802·5 ± 3·4 kJ mol^–1; Ul₄(s), –511·5 ± 4·0 kJ mol^–1; and NpBr₄(s), –773·2 ± 1·7 kJ mol^–1. By use of suitable entropy and heat capacity data, the thermodynamic functions associated with the formation of these compounds, and of UCl₄(s) and NpCl₄(s), have been calculated as a function of temperature from 298 K to the m.p. of the halide.