Heat capacities and thermodynamic functions of p-chlorobenzoic acid

Abstract

The heat capacities of p-chlorobenzoic acid were measured in the temperature range from 80 to 580 K by means of an automatic adiabatic calorimeter equipped with a small sample cell of internal volume of 7.4 cm 3 . The construction and procedures of the calorimetric system were described in detail. The performance of this calorimetric apparatus was evaluated by heat capacity measurements on α-Al 2O 3. The deviations of experimental heat capacities from the corresponding smoothed values lie within ±0.28 per cent, while the inaccuracy is within ±0.40 per cent, compared with the recommended reference data in the whole experimental temperature range. A fusion transition at T=512.280 K was found from the C p – T curve of p-chlorobenzoic acid. The enthalpy and entropy of the phase transition, as well as the thermodynamic functions {G (T)−G (298.15)}, {H (T)−H (298.15)} and { S ( T) − S 298.15}, were derived from the heat capacity data. The mass fraction purity of p-chlorobenzoic acid sample used in the present calorimetric study was determined to be 0.99935 by fraction melting approach.