Xenon Tetrafluoride: Heat Capacity and Thermodynamic Functions from 5 to 350°K. Reconciliation of the Entropies from Molecular and Thermal Data

The heat capacity of a very pure sample of xenon tetrafluoride, XeF₄, was determined by adiabatic calorimetry between 5 and 350°K. No irregular thermal behavior was observed, except in the region near 210°K, where slightly enhanced heat capacity values, ascribed to a small amount of impurity, were observed. The values of the thermodynamic quantities $C_p{\mathrm{°,\; S°,\; (H°\; -\; H\; °}}_0\mathrm{)/T,}$ and ${\mathrm{(G°\; -\; H\; °}}_0\mathrm{)/T}$ at 298.15°K are $\text{118.39 ± 0.12, 167.00 ± 0.17, 77.24 ± 0.08,}$ and $\text{−89.76± 0.09\hspace{0.17em}}\mathrm{J}\mathrm{\hspace{0.17em}°}{\mathrm{K}}^{\text{−1}}·{\mathrm{mole}}^{\text{−1}},$ respectively. The standard entropy of the gas at 298.15°K was calculated from the thermal data to be $\text{323.2± 2.0\hspace{0.17em}}\mathrm{J}\mathrm{\hspace{0.17em}°}{\mathrm{K}}^{\text{−1}}·{\mathrm{mole}}^{\text{−1}},$ in excellent agreement with the value $\text{323.98 ± 0.4\hspace{0.17em}}\mathrm{J}\mathrm{\hspace{0.17em}°}{\mathrm{K}}^{\text{−1}}·{\mathrm{mole}}^{\text{−1}}$ calculated from electron‐diffraction data and the frequency assignment of Tsao, Cobb, and Claassen. The standard entropy of formation of solid XeF₄ at 298.15°K was found to be $\text{−407.95 ± 0.17\hspace{0.17em}}\mathrm{J}\mathrm{\hspace{0.17em}°}{\mathrm{K}}^{\text{−1}}·{\mathrm{mole}}^{\text{−1}},$ and the standard Gibbs energy of formation at 298.15°K is $\text{−145.48 ± 0.88\hspace{0.17em}}{\mathrm{kJ\; mole}}^{\text{−1}}.$ The corresponding quantities for gaseous XeF₄ at 298.15°K are $\text{−250.97 ± 0.40\hspace{0.17em}}\mathrm{J}\mathrm{\hspace{0.17em}°}{\mathrm{K}}^{\text{−1}}·{\mathrm{mole}}^{\text{−1}}$ and $\text{−131.36 ± 0.91\hspace{0.17em}}{\mathrm{kJ\; mole}}^{\text{−1}}.$ The standard enthalpies of formation of the solid and of the gas at 0°K are $\text{−266.29 ± 0.88\hspace{0.17em}}{\mathrm{kJ\; mole}}^{\text{−1}}$ and $\text{−201.39 ± 0.90\hspace{0.17em}}{\mathrm{kJ\; mole}}^{\text{−1}},$ respectively. The equilibrium constant for the formation of gaseous XeF₄ from the elements at various temperatures is tabulated and compared with the experimental measurements by Weinstock, Weaver, and Knop.