The standard molar enthalpies of formation of chromone-3-carboxylic acid, 6-methylchromone-2-carboxylic acid, and 6-methyl-4-chromanone determined by micro-combustion calorimetry

Abstract

The energies of combustion of chromone-3-carboxylic acid (C3CA), 6-methylchromone-2-carboxylic acid (6MCC), and 6-methyl-4-chromanone (6M4C) were determined using an isoperibolic micro-combustion calorimeter. The calorimeter used in the present work has been assembled, calibrated, and tested in our laboratory with the desired results. Prior to the measurement of the energies of combustion, the purities, heat capacities (C p), fusion temperatures (T fus), and enthalpies of melting (Δfus H) for each compound were determined by differential scanning calorimetry. The values of the energies of combustion were used to derive standard molar enthalpies of combustion ( $$ \Delta _{{\text{c}}} H_{{\text{m}}}^{\circ } $$ ) and standard molar enthalpies of formation ( $$ \Delta _{{\text{f}}} H_{{\text{m}}}^{\circ } $$ ) in the crystalline phase at T = 298.15 K. The values found for the $$ \Delta _{{\text{f}}} H_{{\text{m}}}^{\circ } $$ of C3CA, 6MCC, and 6M4C were −(619.5 ± 2.6), −(656.2 ± 2.2), and −(308.9 ± 3.0) kJ mol−1, respectively.