Heterogeneous Reactions Studied by Mass Spectrometry. I. Reaction of B2O3(s) with H2O(g)

Knudsen effusion and mass‐spectrometric techniques were employed in studying the gaseous species in thermodynamic equilibrium with condensed boric oxide and water vapor in the temperature range from 1060° to 1450°K. Water vapor was introduced into a Knudsen cell containing B₂O₃, and the vapor effusing from the cell was analyzed mass spectrometrically. The ions H₂O⁺, HBO₂⁺, H₃BO₃⁺, and (HBO₂)₃⁺ were observed. Of the various boron‐containing species in equilibrium with the system, the one present in largest amounts was found to be HBO₂. ΔH₀° for the reaction $$\frac{1}{2}{\mathrm{H}}_2\mathrm{O}(\mathrm{g}\mathrm{)+}\frac{1}{2}{\mathrm{B}}_2{\mathrm{O}}_3(\mathrm{s}\mathrm{)\to }{\mathrm{HBO}}_2(\mathrm{g})$$ was calculated to be 47.6±2.0 kcal/mole. The data pertaining to the trimer (HBO₂)₃ are rather poor, but, in the temperature range covered, its concentration was less than 1% of that of the monomer HBO₂, and at the highest temperature (1450°K) was approximately equal to the concentration of H₃BO₃. A crude calculation of the heat of formation of the trimer by means of the third law gives ΔH₀° (f) ≈ — 540±10 kcal/mole.