Thermodynamics of the Dimerization and Trimerization of Gaseous Tungsten Trioxide and Molybdenum Trioxide

Abstract

A mass‐spectrometric investigation of gaseous tungsten oxides has yielded the thermodynamic values ΔH°1800=−127 kcal/mole and ΔS°1800=−40±1 eu for 2 WO3(g)=W2O5(g), and ΔH°1800=−240 kcal/mole and ΔS°1800=−77±3 eu for 3 WO3(g)=W3O9(g). The experiments were conducted at reduced activity of WO3 to increase the relative proportion of monomer in the gas phase and permit simultaneous measurement of partial pressures of WO3, W2O6, and W3O9. The activity was reduced by dissolution of tungsten oxide in a molten silicate. Use of similar techniques in studies of gaseous molybdenum oxides yielded ΔH°1800=−112 kcal/mole for 2 MoO3(g)=Mo2O6(g) and ΔH°1800=−209 kcal/mole for 3 MoO3(g)=Mo3O9(g).