Thermodynamics of the actinide elements. Part III. Heats of formation of the dicaesium actinide hexachloro-complexes

The heats of solution in aqueous hydrochloric acid of the various dicaesium actinide(IV) hexachlorides have been measured at 25·00 ± 0·05°C. For the thorium, protactinium, uranium, neptunium, and plutonium compounds, the values are, in 1M-HCl, –31·37 ± 0·12, –23·94 ± 0·31, –19·91 ± 0·04, –20·16 ± 0·07, and –13·67 ± 0·19 kcal mol–1, respectively, and in 6M-HCl, –20·97 ± 0·06, –12·74 ± 0·24, –8·11 ± 0·05, –7·48 ± 0·10, and –0·18 ± 0·03 kcal mol–1, respectively. The heat of solution of the plutonium compound in 1M-HClO4 is –17·74 ± 0·05 kcal mol–1. Combination of these values with other thermodynamic data yields the following heats of formation at 298°K: Cs2ThCl6(c), –508·7 ± 0·5 kcal mol–1; Cs2UCl6(c)–476·5 ± 1·1 kcal mol–1; Cs2NpCl6(c), –468·5 ± 0·4 kcal mol–1, and Cs2PuCl6(c), –466·9 ± 1·0 kcal mol–1.