Thermodynamic characterization of chromium tellurate

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The standard Gibbs energy of formation of chromium tellurate, Cr2TeO6 was determined from the vapour pressure measurement of TeO2(g) over the phase mixture Cr2TeO6(s) + Cr2O3(s) in the temperature range 1,183–1,293 K. A thermogravimetry (TG)-based transpiration technique was used for the vapour pressure measurement. This technique was validated by measuring the vapour pressure of CdCl2(g) over CdCl2(s). The temperature dependence of the vapour pressure of CdCl2(g) could be represented as logp (Pa) (±0.02) = 12.06 − 8616.3/T (K) (734 − 823 K). A ‘third-law’ analysis of the vapour pressure data yielded a mean value of 185.1 ± 0.4 kJ mol−1 for the enthalpy of sublimation of CdCl2(s). The temperature dependence of vapour pressure of TeO2(g) generated by the incongruent vapourisation reaction, $$ {\text{Cr}}_{ 2} {\text{TeO}}_{ 6} (\rm s) \to {\text{Cr}}_{ 2} {\text{O}}_{ 3} (\rm s) + {\text{TeO}}_{ 2} (\rm g) + 1/2\,{\text{O}}_{ 2} (\rm g) $$ could be represented as logp (Pa) (±0.04) = 18.57 – 21,199/T (K) (1,183 – 1,293 K). The temperature dependence of the Gibbs energy of formation of Cr2TeO6 could be expressed as $$ \{ \Updelta G_{\text{f}}^{ \circ } ({\text{Cr}}_{ 2} {\text{TeO}}_{ 6} ,{\text{ s}}){\text{ (kJ}}\,{\text{mol}}^{ - 1} )\pm 4. 0 {\text{\} = }} - 1 6 2 5. 6 { \,+\, 0} . 5 3 3 6\,T({\text{K}}) \, (1{,}183 - 1{,}293\,{\text{K}}). $$ A drop calorimeter was used for measuring the enthalpy increments of Cr2TeO6 in the temperature range 373–973 K. Thermodynamic functions viz., heat capacity, entropy and Gibbs energy functions of Cr2TeO6 were derived from the experimentally measured enthalpy increment values. $$ \Updelta H_{{{\text{f}},298\,{\text{K}}}}^{ \circ } ({\text{Cr}}_{ 2} {\text{TeO}}_{ 6} ) $$ was found to be −1636.9 ± 0.8 kJ mol−1.