Heat Capacity and Other Thermodynamic Properties of MoF6 between 4° and 350°K

Thermodynamic properties of MoF6 have been measured between 4° and 350°K in order to obtain a third-law value of the entropy for comparison with the entropy calculated from molecular data. The enthalpy change for the solid—solid transition at 263.48°±0.02°K is 1953.3±2.0 cal mole−1, and the enthalpy of fusion at the triple point, 290.73°±0.02°K, is 1034.2±1.0 cal mole−1. The vapor pressure between 291° and 320°K is represented by the equation log10 Pmm=−2047.15/T−4.28004 log10T+20.19354. At 298.15°K the enthalpy of vaporization calculated from the vapor-pressure equation and the measured second virial coefficient, −923 cm3 mole−1, is 6630 cal mole−1. The standard entropy of the gas at 298.15°K obtained from the thermal data is 83.75±0.10 cal deg−1·mole−1. This agrees with the value of 83.77 cal deg−1·mole−1 calculated from Nazarian's electron-diffraction measurement of the Mo–F distance (1.840 Å) and from Weinstock and Goodman's vibrational assignment. Thermodynamic functions are tabulated at selected temperatures. At 298.15°K the values of CP°, S°, (H° − H0°)/T, and (G° − H0°)/T are 40.58±0.04, 62.06±0.06, 34.23±0.03, and −27.83±0.03 cal deg−1·mole−1, respectively, for liquid MoF6 and 28.82, 83.77, 19.25, and −64.51 cal deg−1·mole−1, respectively, for gaseous MoF6.