Thermodynamic properties of NpO(g)

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Abstract
The thermodynamic properties of NpO(g) were determined by mass−spectrometric studies of two isomolecular exchange reactions of the type M(g) + M′O(g) = M′ (g) + MO(g) involving the gaseous atoms and monoxides of lanthanum and yttrium. In one series of determinations, both reactions were studied simultaneously. The standard free energy of formation of NpO(g) is given by ΔGf° (NpO,g) = −12890 −13.73T (cal mol−1) over the temperature range 1660−1980 K. An uncertainty of ±1 kcal mol−1 is estimated. The derived thermodynamic properties of NpO(g) suggest the existence of low−lying electronic states in the gaseous molecule. The total volatility of neptunium−bearing species over the univariant system, NpO2(s) + Np(l) + vapor, was measured by the mass−effusion method, and the partial pressures of NpO(g) obtained therefrom were used to confirm the accuracy of the isomolecular results and to derive the standard free energy of formation of NpO2(s): ΔGf° (NpO2,s) ? −256000 + 40.4 T (cal mol−1).