Heat Capacity and Thermodynamics of Solid and Liquid Pyridine-3-carboxylic acid (Nicotinic Acid) Over the Temperature Range 296 K to 531 K

Abstract

The molar heat capacity of pyridine-3-carboxylic acid (nicotinic acid) for T   = (296 to 531) K was investigated by differential scanning calorimetry (DSC) and Calvet-drop microcalorimetry. The measurements extended up to the liquid range and also covered the interval where a reversible and fast solid-solid (cr II → cr I) phase transition occurs. The molar enthalpies and entropies of that phase transition and of fusion were obtained as T_trs = (455.0 ± 0.2) K, ∆_trsH^o_m = (0.90 ± 0.10) kJ/mol, ∆_trsS^o_m = (1.98 ± 0.22) J/(K mol), T_fus = (509.91 ± 0.04) K, ∆_fusH^o_m = (28.2 ± 0.1) kJ/mol, and ∆_trsS^o_m = (55.30 ± 0.16) J/(K mol). By combining these experimental results with the previously reported ∆_subH_m(NA,cr II) at T =   366.5 K, the corresponding entropy in the gaseous state calculated at the B3LYP/6-31+G(d,p) level of theory, and ∆_fH^o_m (NA, cr II) at T =   298.15 K, it was possible to estimate the standard molar Gibbs energy of formation functions necessary for the construction of the ∆_fG^o_m vs. T diagram illustrating the enantiotropic nature of this system.

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