Vapor Composition and Thermodynamic Characteristics of Gaseous Molecules of Alkaline Earth Metal Tungstates

Abstract

Calculated and experimental mass spectra of gaseous tungstates of alkaline earth metals MgWO_4(g), CaWO_4(g), SrWO_4(g), and BaWO_4(g) in the temperature range from 1600 to 2000 K are presented. The partial vapor pressures are determined and equations are derived for the temperature dependences of the partial pressures of gaseous molecules of alkaline earth metal tungstates for liquids in the following form (P, atm): \(\log P\left( {{\text{MgW}}{{{\text{O}}}_{4}}_{{\left( {\text{L}} \right)}}} \right) = - 28\,737{\text{/}}T + 7.95{\text{ for the range from }}1600{\text{ to }}1900{\text{ K}};\) \(\log P\left( {{\text{CaW}}{{{\text{O}}}_{4}}_{{\left( {\text{L}} \right)}}} \right) = - 25\,265{\text{/}}T + 6.913{\text{ for the range from }}1850{\text{ to 20}}00{\text{ K}};\) \(\log P\left( {{\text{SrW}}{{{\text{O}}}_{4}}_{{\left( {\text{L}} \right)}}} \right) = - 25\,052{\text{/}}T + 7.13{\text{ for the range from }}1800{\text{ to 19}}00{\text{ K}};\) \(\log P\left( {{\text{BaW}}{{{\text{O}}}_{4}}_{{\left( {\text{L}} \right)}}} \right) = - 20570{\text{/}}T + 4.58{\text{ for the range from }}1770{\text{ to 19}}00{\text{ K}}{\text{.}}\) On the basis of experimental data on vapor pressure, the enthalpies of evaporation (\(\Delta H_{{s,0}}^{0}\)), formation (\( - \Delta H_{{f,0}}^{0}\)), and atomization (\(\Delta H_{{{\text{at}},0}}^{0}\)) of gaseous tungstates of alkaline earth metals are calculated, which, respectively, amount to (ΔH⁰, kJ/mol) 652, 890, and 2855 for MgWO₄; 644, 974, and 2968 for CaWO₄; 615, 1045, and 3056 for SrWO₄; and 548, 1045, and 3095 for BaWO₄. It is determined that the enthalpy of sublimation of alkaline earth metal tungstates decreases from magnesium to barium.