Thermodynamic studies on Sn–Te–O system

Abstract

The standard Gibbs energy of formation of tin tellurate, SnTe3O8(s) was determined from its vapour pressure measurements over the temperature range 973–1,158 K by employing thermogravimetry-based transpiration method. The temperature dependence of vapour pressure of TeO2 over the mixture SnTe3O8(s) + SnO2(s) generated by the incongruent vapourisation reaction, SnTe3O8(s) → SnO2(s) + 3 TeO2(g) could be represented as: log (p (TeO2, g)/Pa ± 0.03) = 13.943–14,181 (K/T) (973–1,158 K). The standard Gibbs energy of formation of SnTe3O8(s) was also determined by measuring the oxygen potential of SnO2(s)–Te(s)–SnTe3O8(s) phase mixture by the electromotive force method. Enthalpy increments of SnTe3O8(s) were determined by inverse drop calorimetric method in the temperature range 523–973 K. The thermodynamic functions, viz., heat capacity, entropy and free energy functions were derived from the measured values of enthalpy increments. A mean value of −1,642 ± 2.0 kJ mol−1 was obtained for \( \Updelta_{\text{f}} H_{298}^{\circ } \)(SnTe3O8, s) by combining the value of Δf \( G^{\circ } \)(SnTe3O8, s) derived from vapour pressure data and the free energy functions derived from the drop calorimetric data.