Thermodynamic Properties of Nitrogen Trifluoride from 12°K to Its Boiling Point. The Entropy from Molecular and Spectroscopic Data

Abstract

The heat capacity of nitrogen trifluoride has been measured from 12° to 144°K. The heat of the transition at 56.62°K was found to be 361.8 cal/mole. The heat of fusion at the triple point temperature, 66.37°K, was found to be 95.1 cal/mole. The heat of vaporization at the normal boiling point, 144.15°K, was found to be 2769 cal/mole. The vapor pressure has been measured, and the data are closely represented by the equation log10Pmm=−673.5828/T+1.869858 log10T−0.00783355T+4.64615.The entropy of nitrogen trifluoride gas has been calculated from the calorimetric data and the third law of thermodynamics to be 54.50 cal/deg/mole at 144.15°K. This value agrees very well with the value 54.61 calculated from spectroscopic data and structural parameters which include a pyramidal model for the molecule.