Thermodynamic Properties of Silicon Tetrafluoride from 15°K to Its Triple Point. The Entropy from Molecular and Spectroscopic Data

Abstract

The heat capacity of silicon tetrafluoride has been measured from 15° to 194°K for a sample of 99.97 mole % purity. The solid—liquid—vapor triple-point temperature and pressure were, respectively, 186.35°K and 1679.3 mm. The heat of fusion and heat of vaporization at the triple point were found to be, respectively, 2242 and 3555 cal/mole. The vapor pressure of the solid has been measured from 163° to 186.35°K and the data fitted to the equation log10Pmm=−5.5025–894.93/T+5.9593log10T.Only one vapor-pressure point other than the triple point itself was taken for the liquid. The entropy of silicon tetrafluoride in the ideal gas state at a pressure of one atmosphere and a temperature equal to the triple-point temperature of 186.35°K was found to be 59.85±0.20 cal/(mole °K). This value agrees within the experimental uncertainty with the value of 60.037 cal/(mole °K) calculated theoretically from the molecular parameters and fundamental frequency assignment.