Thermodynamics of gas phase species in the Si-O-H system

The transpiration method was used to study the reaction of SiO2 (cristobalite) with water vapor. Measurements were made between T = (1073 and 1728) K with water vapor contents from (0.05 to 0.6) by volume fraction in argon. From the results, the thermodynamic parameters for Si(OH)(4)(g) and SiO(OH)(2)(g) were determined. At T = (1073 to 1373) K, Si(OH)(4) is the primary vapor species and second law measurements lead to Delta(f)H degrees(1200 K) = (-1344.6 +/- 2.7) kJ(.)mol(-1) and S degrees(1200 K) = (544.4 +/- 2.1) J mol(-1) K-1. Third law measurements lead to Delta(f)H degrees(298 K) = (-1351.3 +/- 1.7) kJ(.)mol(-1). These are in very good agreement with previous measurements and previous ab initio calculations. At T = (1673 and 1728) K, there is evidence of a second vapor species, SiO(OH)2(g). By subtracting the extrapolated Si(OH)4(g) pressure, it was possible to do a third law analysis and obtain a Delta(r)H degrees(298 K) for SiO(OH)(2)(g) of (-836 +/- 40) kJ(.)mol(-1). Previous measurements and ab initio calculations are within this range. (c) 2005 Elsevier Ltd. All rights reserved.