Vapor pressures and standard molar enthalpies, entropies and Gibbs energies of sublimation of three 4-substituted acetanilide derivatives

Abstract

The vapor pressures of three 4-substituted acetanilide derivatives (namely, 4-ethoxyacetanilide, 4-hydroxyacetanilide and 4-bromoacetanilide) were measured above their liquids using the mass loss rates determined by thermogravimetry under both isothermal and nonisothermal diffusion-controlled conditions. The coefficient used to convert mass loss rates in vapor pressures showed a slight increase with increasing temperature. Therefore, the small deviations observed from the mean value enable the use of a single calibration value to be considered reasonable. From the temperature dependence of the vapor pressure estimated using a calibration constant for each technique the molar enthalpies of vaporization Δ l g H m ° ( T ) were determined, respectively, at the middle 〈 T〉 of the respective temperature intervals. The melting temperatures and the molar enthalpies of fusion of these compounds were measured by differential scanning calorimetry. Finally, the standard molar enthalpies of sublimation Δ cr g H m ° ( 298.15 K ) were obtained at the reference temperature of 298.15 K using the standard molar enthalpies of vaporization Δ l g H m ° ( T ) , the molar enthalpies of fusion, the heat capacity differences between gas and liquid and the heat capacity differences between gas and solid, Δ l g C p,m ° and Δ cr g C p,m ° , respectively, both estimated by applying a group additivity procedure. Therefore, the averages of the standard ( p° = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation at 298.15 K, have been derived: Compound Δ cr g H m ° ( 298.15 K ) (kJ mol −1) Δ cr g S m ° ( 298.15 K ) (J mol −1 K −1) Δ cr g G m ° ( 298.15 K ) (kJ mol −1) 4-Ethoxyacetanilide 120 ± 3 232 ± 9 52 ± 5 4-Hydroxyacetanilide 138 ± 3 245 ± 9 65 ± 5 4-Bromoacetanilide 110 ± 4 204 ± 10 49 ± 6