Low-temperature heat capacity and standard molar enthalpy of formation of crystalline (S)–(+)–Ibuprofen (C₁₃H₁₈O₂)(s)

Abstract

Low-temperature heat capacities of the compound,(S)–(+)–Ibuprofen (C₁₃H₁₈O₂), have been measured by a precision automated adiabatic calorimeter in the temperature range 78 – 348 K. The experimental molar heat capacities in the temperature range 78 – 345 K have been fitted to a polynomial equation. The smoothed heat capacities and thermodynamic functions, (HT - H₂₉₈․₁₅ĸ) and (ST - S₂₉₈․₁₅ĸ), of the compound have been calculated by the heat-capacity polynomial equation. The constant-volume energy of combustion of the compound, has been found to be ΔUc (C₁₃H₁₈O₂, s) = – (7136.94 ± 4.74) kJmol⁻¹ by means of a home-made oxygen-bomb combustion calorimeter at T = 298.15 ± 0.001 K. The standard molar enthalpy of combustion of the sample has been calculated to be ΔcHmº (C₁₃H₁₈O₂, s) = – (7145.62 ± 4.74) kJmol⁻¹, from the constant-volume combustion energy. The standard molar enthalpy of formation of the compound has been derived, ΔքHmº (C₁₃H₁₈O₂, s) = – (542.48 ± 5.05) kJmol⁻¹, from the combination of the data of standard molar enthalpy of combustion of the compound with other auxiliary thermodynamic quantities through a Hess thermochemical cycle.