Thermodynamic properties of ZrO (g) and HfO (g); a critical examination of isomolecular oxygen-exchange reactions

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The thermodynamic properties of ZrO(g) and HfO(g) have been determined by mass-spectrometric studies of five isomolecular oxygen-exchange reactions of the type M(g)+M′O(g)=MO(g)+M′(g) involving the gaseous atoms and monoxides of yttrium and thorium. Third law values of the standard free energies of formation over the temperature range 2000–2800 K are adequately (within 2%) expressed by the linear equations ΔG°f(ZrO,g)=13 040−16.04T, ΔG°f(HfO,g)=9930−14.68T. Uncertainties of ±1 kcal mol−1 are estimated. Third law analyses of the data yield the enthalpies of formation at absolute zero, ΔH°0(ZrO,g)=21.4, and ΔH°0(HfO,g) = 18.5 kcal mol−1, and the dissociation energies at absolute zero, D0(ZrO)=180.7, and D0(HfO)=188.9 kcal mol−1, with uncertainties of 1–2 kcal mol−1. The equilibrium constants for the isomolecular reactions in this and in previous studies were obtained from ion currents and relative instrumental sensitivities that were approximated from the mean square radii of the gaseous atoms and the squares of the interatomic distances of the gaseous monoxides; the use of these parameters was shown to improve the closure among several thermodynamic cycles by as much as 2 kcal mol−1