Thermodynamic study of Cs3Na(MoO4)2: Determination of the standard enthalpy of formation and standard entropy at 298.15 K

The enthalpy of formation at 298.15 K and low temperature heat capacity of Cs₃Na(MoO₄)₂ have been measured for the first time in this work using solution calorimetry and thermal-relaxation calorimetry in the temperature range T = (1.9–299.6) K, respectively. The solution calorimetry measurements, performed in 2 M HNO₃ solution, have yielded an enthalpy equal to ${\Delta }_r{H}_m$(298.15 K) = (6.79 $\pm$ 1.72) kJ$·$mol⁻¹ for the reaction:$3/2{\mathrm{Cs}}_2{\mathrm{MoO}}_4\left(\mathrm{cr}\right)+1/2{\mathrm{Na}}_2{\mathrm{MoO}}_4\left(\mathrm{cr}\right)={\mathrm{Cs}}_3\mathrm{Na}{\left({\mathrm{MoO}}_4\right)}_2\left(\mathrm{cr}\right)$Combining with the enthalpies of formation of Cs₂MoO₄(cr) and Na₂MoO₄(cr), also determined in this work in 0.1 M CsOH and 0.1 M NaOH solutions, respectively, the standard enthalpy of formation of Cs₃Na(MoO₄)₂ at 298.15 K has been determined as ${\Delta }_f{H}_m^o$(Cs₃Na(MoO₄)₂, cr, 298.15 K) = −(2998.5 $\pm$ 3.0) kJ$·$mol⁻¹. The heat capacity and entropy values of Cs₃Na(MoO₄)₂ at 298.15 K have been derived as $C_{p\text{,}m}^o\left({\mathrm{Cs}}_3\mathrm{Na}{\left({\mathrm{MoO}}_4\right)}_2\text{,}\mathrm{cr}\text{,}298.15\text{K}\right)=(296.3\pm 3.3)$ J$·$K⁻¹$·$mol⁻¹ and $S_m^o\left({\mathrm{Cs}}_3\mathrm{Na}{\left({\mathrm{MoO}}_4\right)}_2\text{,}\mathrm{cr}\text{,}298.15\text{K}\right)=(467.2\pm 6.8)$ J$·$K⁻¹$·$mol⁻¹. Combining the newly determined thermodynamic functions, the Gibbs energy of formation of Cs₃Na(MoO₄)₂ at 298.15 K has been derived as ${\Delta }_f{G}_m^o\left({\mathrm{Cs}}_3\mathrm{Na}{\left({\mathrm{MoO}}_4\right)}_2\text{,}\mathrm{cr}\text{,}298.15K\right)=-(2784.6\pm 3.4)$ kJ$·$mol⁻¹. Finally, the enthalpies, entropies and Gibbs energies of formation of Cs₃Na(MoO₄)₂ from its constituting binary and ternary oxides have been calculated.