Thermodynamics of Vaporization of Lithium Oxide, Boric Oxide, and Lithium Metaborate

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Abstract
The vapor pressures of lithium oxide, boric oxide, and lithium metaborate have been measured by the torsion—effusion method. For lithium oxide, the magnitude of the observed pressure indicates vaporization largely to lithium oxide molecules. The boric oxide measurements yield an entropy for B2O3(g) that is consistent with calculated entropies based on recent matrix-isolation spectral data, and, therefore, support the assigned molecular constants. Lithium metaborate, the one-to-one mixture of the two oxides, vaporizes congruently to mixed-oxide molecules. Third-law treatment of the vapor pressure data gives heats of sublimation at 298°K of 103.6±3, 103.1±2, and 86.3±3 kcal/mole for Li2O(g), B2O3(g), and LiBO2(g), respectively. When combined with data for the condensed phases, the results yield ΔH298=—75 kcal for the reaction Li2O(g)+B2O3(g)=2LiBO2(g).

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