Calorimetric Study of Iodine Pentafluoride: Heat Capacity between 5 and 350°K, Enthalpy of Fusion and Vaporization, Standard Entropy of the Vapor, and Other Thermodynamic Properties

Abstract

The heat capacity of iodine pentafluoride has been measured by an adiabatic method between 5 and 350°K. The triple point of the compound is 282.553 ± 0.01°K and the enthalpy of fusion is ΔHfusion = 11222 ± 11 J mole−1. The vapor pressure measured between 283 and 378°K is represented by the equation: log10PmmHg = − 3090.14 / T − 6.96834 log10T + 29.02167. A determination of the vapor density at 330 and 372°K resulted in the values − 2813 and − 1232 cm3 mole−1, respectively, for the second virial coefficient B to be used with the equation of state PV = RT + BP. The enthalpy of vaporization at 330°K was calculated to be ΔHvap(330°K) = 39320 ± 200 J mole−1. At 298.15°K the standard state values of the thermodynamic functions obtained from the heat capacity data are: CP° = 174.7 ± 0.17 J °K−1·mole−1, S° = 224.85 ± 0.22 J °K−1·mole−1, (H° − H°0) = 37086 ± 37 J mole−1, and (G° − H°0) / T = − 100.47 ± 0.1 J °K−1·mole−1. The standard entropy of iodine pentafluoride vapor at 330°K calculated from calorimetric data is 347.53 ± 0.84 J °K−1·mole−1. The fundamental frequency ν9 was observed in the infrared spectrum of the gas at 189.0 ± 0.7 cm−1. To bring the standard entropy calculated from molecular data into agreement with the calorimetric value, the unobserved vibrational frequency ν5 must be 181 ± 20 cm−1. A revised calculation of the standard enthalpy, entropy, and Gibbs energy of formation of IF5 from the elements at 298.15°K yielded the values ΔHf° = − 881.99 ± 1.34kJ mole−1, ΔSf° = − 340.0 ± 0.6 J °K−1·mole−1, and ΔGf° = − 780.62 ± 1.35kJ mole−1.