Thermodynamic Properties of Phosphoryl Fluoride from 12 to 240°K. The Zero-Point Entropy of the Solid

Abstract

The heat capacity of phosphoryl fluoride has been measured from 12 to 240°K for a sample with a purity greater than 99.9 mole %. The solid–liquid–vapor triple-point temperature and pressure were, respectively, 233.45°K and 774.06 torr. The heat of fusion was 3086 cal/mole. The heat of vaporization was determined as 5150 cal/mole at a temperature of 236.00°K and a pressure of 880.25 torr. The vapor pressure data were fitted by the equation lnP(torr) = − 487.49218 + 14 223.578 / T + 79.446599 lnT for the solid and by lnP(torr) = 8.05651 − 1206.41 / T for a short temperature range in the liquid phase. An entropy discrepancy, Sspec − Scalc, of 1.33 cal/mole·°K at 236.00°K was found between the spectroscopic entropy of 64.777 cal/mole·°K and the calorimetric value of 63.45 cal/mole·°K. A tentative model to explain the discrepancy has been proposed in which it is assumed that the degree of degeneracy of the several lowest eigenstates of the molecular motion are reduced when the molecule in the solid is subjected to the field of neighboring molecules of a certain symmetry.