Thermochemistry of gaseous SiO(OH), SiO(OH)2, and SiO 2

Abstract

The gaseous species SiO(OH) and SiO(OH)2 were detected as products of the reaction of SiO2(1) and H2O(g) near 2000 K, using effusion-beam mass spectrometry. No other silicon-containing hydroxides or oxyhydroxides were observed. From third law analysis of equilibrium data, values of ΔfH298○ of SiO(OH) and SiO(OH) 2 were evaluated as -118 and -213 kcal mol-1, respectively, both ±4 kcal mol-1. Several equilibrium measurements on gaseous SiO2 yielded a ΔfH298○ value of -67 kcal mol-1, which is 6-10 kcal mol -1 less stable than values given in most compilations. It appears that the new thermochemical data for SiO(OH) and SiO(OH)2, plus a literature estimate for Si2O(OH)6, can be used to model the equilibrium behavior of the silica-steam system over a wide range of temperatures and pressures. © 1994 American Institute of Physics.