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

摘要

The heat capacity of iodine pentafluoride has been measured by an adiabatic method between 5 and 350deg;K. The triple point of the compound is 282.553thinsp;plusmn;thinsp;0.01deg;K and the enthalpy of fusion isDgr;Hfusionthinsp;equals;thinsp;11222thinsp;plusmn;thinsp;11J moleminus;1. The vapor pressure measured between 283 and 378deg;K is represented by the equation:log10PmmHgthinsp;equals;thinsp;minus;thinsp;3090.14thinsp;sol;thinsp;Tthinsp;minus;thinsp;6.96834log10Tthinsp;plus;thinsp;29.02167. A determination of the vapor density at 330 and 372deg;K resulted in the values minus;thinsp;2813 and minus;thinsp;1232 cm3moleminus;1, respectively, for the second virial coefficientBto be used with the equation of statePVthinsp;equals;thinsp;RTthinsp;plus;thinsp;BP. The enthalpy of vaporization at 330deg;K was calculated to beDgr;Hvaplpar;330deg;Krpar;thinsp;equals;thinsp;39320thinsp;plusmn;thinsp;200J moleminus;1. At 298.15deg;K the standard state values of the thermodynamic functions obtained from the heat capacity data are:CPdeg;thinsp;equals;thinsp;174.7thinsp;plusmn;thinsp;0.17Jdeg;Kminus;1middot;moleminus;1, Sdeg;thinsp;equals;thinsp;224.85thinsp;plusmn;thinsp;0.22Jdeg;Kminus;1middot;moleminus;1, lpar;Hdeg;thinsp;minus;thinsp;Hdeg;0rpar;thinsp;equals;thinsp;37086thinsp;plusmn;thinsp;37J moleminus;1, andlpar;Gdeg;thinsp;minus;thinsp;Hdeg;0rpar;thinsp;sol;thinsp;Tthinsp;equals;thinsp;minus;thinsp;100.47thinsp;plusmn;thinsp;0.1Jdeg;Kminus;1middot;moleminus;1. The standard entropy of iodine pentafluoride vapor at 330deg;K calculated from calorimetric data is 347.53thinsp;plusmn;thinsp;0.84 J deg;Kminus;1middot;moleminus;1. The fundamental frequencyngr;9was observed in the infrared spectrum of the gas at 189.0thinsp;plusmn;thinsp;0.7 cmminus;1. To bring the standard entropy calculated from molecular data into agreement with the calorimetric value, the unobserved vibrational frequencyngr;5must be 181thinsp;plusmn;thinsp;20 cmminus;1. A revised calculation of the standard enthalpy, entropy, and Gibbs energy of formation of IF5from the elements at 298.15deg;K yielded the valuesDgr;Hfdeg;thinsp;equals;thinsp;minus;thinsp;881.99thinsp;plusmn;thinsp;1.34kJ moleminus;1, Dgr;Sfdeg;thinsp;equals;thinsp;minus;thinsp;340.0thinsp;plusmn;thinsp;0.6Jdeg;Kminus;1middot;moleminus;1, andDgr;Gfdeg;thinsp;equals;thinsp;minus;thinsp;780.62thinsp;plusmn;thinsp;1.35kJ moleminus;1.