Thermodynamic Properties of Some Methylphosphonyl Dihalides From 15 to 335 °K

Measurements of the heat capacity of methylphosphonyl difluoride (CH(3)POF(2)), methyl phosphonyl dichloride (CH(3)POCl(2)), and methylphosphonyl chlorofluoride (CH(3)POClF) were made from about 15 to 335 °K by means of an adiabatic calorimeter. These highly reactive and toxic substances were purified in a completely closed glass apparatus by combining slow crystallization and fractional melting procedures. The purities determined by the freezing-curve method are shown to be generally in agreement with those values obtained by the calorimetric method. From the results of the heat measurements, the triple-point temperature, heat of fusion, and their corresponding estimated uncertainties were found to be, respectively, 236.34±0.05 °K and 11,878±12 J/mole for CH(3)POF(2), 306.14± 0.02 °K and 18,076±15 J/mole for CH(3)POCl(2), and 250.70± 0.20 °K and 11,853±30 J/mole for CH(3)POClF. Triple-point temperatures obtained by the freezing-curve method are in agreement with the above values. A table of smoothed values of heat capacity, enthalpy, enthalpy function, entropy, Gibbs free energy, and Gibbs free energy function from 0 to 335 °K was obtained from the data. The entropy and its corresponding estimated uncertainty for CH3POF2, CH3POCl2, and CH3POClF in their respective condensed phase at 298.15 °K and saturation pressure was found to be 208.3± 0.3, 164.8± 0.3, and 216.4± 0.4 J/deg mole, respectively. The entropies in the gaseous state at 298.15 °K and 1 atm pressure were found to be 312.7±3, 339.7±3, and 335.0±3 J/deg mole, respectively.