Thermodynamics of the actinide elements. Part I. Heats of formation of crystalline neptunium tetrachloride, neptunium oxide dichloride, and neptunium trioxide monohydrate, and a new determination of the heat of formation of the NpIII and NpIV ions in M-hydrochloric acid

Abstract

The following heats of solution have been measured: neptunium metal in oxygen-free 1M-HCl, to yield Np^III, –126·2 ± 0·2 kcal. mole^–1; neptunium metal in 1M-HCl–0·005M-Na₂SiF₆ to yield Np^IV, –166·0 ± 0·2 kcal. mole^–1; NpCl_4(c) in 1M-HCl and 1M-HCl–0·005M-Na₂SiF₆, –53·87 ± 0·13 and –54·40 ± 0·10 kcal. mole^–1, respectively; NpOCl_2(c) in 1M-HCl, –33·64 ± 0·12 kcal. mole^–1; NpO₃,H₂O_(c) in 1M-HCl and 1M-HClO₄, –12·70 ± 0·10 and –12·85 ± 0·10 kcal. mole^–1, respectively.

On combination with other thermodynamic data, these results yield the following heats of formation at 298°K. Np^III in 1M-HCl, –126·2 ± 0·2; Np^IV in 1M-HCl, –132·3 ± 0·3; NpCl_4(c), –236·0 ± 0·3; NpOCl_2(c), –245·8 ± 0·3 and NpO₃,H₂O_(c), –332·4, all in kcal. mole^–1.