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

摘要

Molar heat capacities (C_(p,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 C_(p,m) vs. 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, C_(p,m)/J mol~(-1) K~(-1) = 19.086X~4 + 15.951X~3 - 5.2548X~2 + 90.192~X + 176.65, [X = (T-230.50/152.5)]. The thermodynamic functions on the base of the reference temperature of 298.15 K, {ΔH_T-ΔH_(298.15)} and {S_T-S_(298.15)}, were derived. Combustion energy of aspirin (Δ_cU_m) was determined by static bomb combustion calorimeter. Enthalpy of combustion (Δ_cH_m~0) and enthalpy of formation (Δ_fH_m~0) were derived through Δ_cU_m as - (3945.26 ± 2.63) kJ mol~(-1) and -(736.41 ± 1.30) kJ mol~(-1), respectively.