Heat Capacity and Other Thermodynamic Functions of Xenon Hexafluoride from 5 to 350°K

Heat‐capacity measurements have been made on a 53‐g sample of XeF₆ between 5 and 350°K. Melting‐point measurements indicated 0.16 mole % liquid‐soluble, solid‐insoluble impurity. Two regions of anomalously high heat capacity and long equilibrium times were observed, near 253.8°K and near 291.8°K, which are believed to arise from structural changes in solid XeF₆. The excess enthalpies associated with these regions are 972 and 1226 J mole⁻¹, respectively. The melting point is 322.63 ± 0.10°K, and the enthalpy of fusion is 5743 J mole⁻¹. The vapor pressure of the liquid is given by ${\log }_{10}{P}_{\mathrm{mmHg}}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}6170.88\hspace{0.17em}/\hspace{0.17em}T\hspace{0.17em}−\hspace{0.17em}23.67815}{\log }_{10}\text{T\hspace{0.17em}+\hspace{0.17em}80.77778}$ from 323 to 350°K. The second virial coefficient was found to be − 955 cm³ mole⁻¹ at 346°K. Densities of the solid were determined at several temperatures by helium displacement. The high entropy of vaporization (136.9 J°K⁻¹·mole⁻¹) at the boiling point (348.72°K), as well as the high and rapidly rising heat capacity of the liquid, suggest that the liquid is associated. Thermodynamic functions of the condensed phases are tabulated at various temperatures. At 298.15°K the values of $C_P{\mathrm{°,\; S°,\; H°\hspace{0.17em}-\hspace{0.17em}H}}_0°$ , and ${\mathrm{(G°\hspace{0.17em}-\hspace{0.17em}H}}_0\mathrm{°)\hspace{0.17em}/\hspace{0.17em}T}$ for solid XeF₆ are 171.59 J°K⁻¹·mole⁻¹, 210.38 J°K⁻¹·mole⁻¹, 30 999 J mole⁻¹, and − 106.41 J°K⁻¹·mole⁻¹, respectively. At 355°K the standard entropy $\mathrm{S°}$ is 255.5 J°K⁻¹·mole⁻¹ for liquid XeF₆ and 402.6 J°K⁻¹·mole⁻¹ for the gas.