Low-temperature heat capacities and standard molar enthalpy of formation of aspirin

Abstract

Molar heat capacities (Cp,m) of aspirin were precisely measured with a small sample precision automated adiabatic calorimeter over the temperature range from 78 to 383 K. No phase transition was observed in this temperature region. The polynomial function of Cp,mvs. T was established in the light of the low-temperature heat capacity measurements and least square fitting method. The corresponding function is as follows: for 78 K≤T≤383 K, Cp,m/J mol-1 K-1=19.086X4+15.951X3-5.2548X2+90.192X+176.65, [X=(T-230.50/152.5)]. The thermodynamic functions on the base of the reference temperature of 298.15 K, {ΔHT -ΔH298.15} and {ST-S298.15}, were derived. Combustion energy of aspirin (ΔcUm) was determined by static bomb combustion calorimeter. Enthalpy of combustion (ΔcHom) and enthalpy of formation (ΔfHom) were derived through ΔcUm as - (3945.26±2.63) kJ mol-1 and - (736.41±1.30) kJ mol-1, respectively.