Thermodynamic properties of fluorine compounds. Part 11.—Low-temperature heat capacities of the three tetrafluorobenzenes
The heat capacities of 1,2,3,4-, 1,2,3,5- and 1,2,4,5-tetrafluorobenzene were measured by adiabatic calorimetry at temperatures between 11 and 353 K. Enthalpies of phase transitions were also measured. The entropies of the liquids at 298.15 K were calculated from the measurements as 256.1, 257.3 and 250.4 J K–1 mol–1 respectively. The heat capacity curve for 1,2,3,5-tetrafluorobenzene showed that a transition occurred in the crystals just below the melting temperature. 1,2,3,4-tetrafluorobenzene crystallised first in a metastable form which, on cooling, rapidly transformed to stable crystals. No anomalies were found in the heat capacity curve of 1,2,4,5-tetrafluorobenzene.