Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi, 710069, People’s Republic of China
Two crystal samples, sodium 5-methylisophthalic acid monohydrate (C9H6O4Na2·H2O, s) and sodium isophthalic acid hemihydrate (C8H4O4Na2·1/2H2O, s), were prepared from water solution. Low-temperature heat capacities of the solid samples for sodium 5-methylisophthalic acid monohydrate (C9H6O4Na2·H2O, s) and sodium isophthalic acid hemihydrate (C8H4O4Na2·1/2H2O, s) were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 379 K. The experimental values of the molar heat capacities in the measured temperature region were fitted to a polynomial equation on molar heat capacities (Cp,m) with the reduced temperatures (X), [X = f(T)], by a least-squares method. Thermodynamic functions of the compounds (C9H6O4Na2·H2O, s) and (C8H4O4Na2·1/2H2O, s) were calculated based on the fitted polynomial equation. The constant-volume energies of combustion of the compounds at T = 298.15 K were measured by a precise rotating-bomb combustion calorimeter to be ΔcU(C9H6O4Na2·H2O, s) = −15428.49 ± 4.86 J g−1 and ΔcU(C8H4O4Na2·1/2H2O, s) = −13484.25 ± 5.56 J g−1. The standard molar enthalpies of formation of the compounds were calculated to be ΔfHmθ (C9H6O4Na2·H2O, s) = −1458.740 ± 1.668 kJ mol−1 and ΔfHmθ(C8H4O4Na2·1/2H2O, s) = −2078.392 ± 1.605 kJ mol−1 in accordance with Hess’ law. The standard molar enthalpies of solution of the compounds, ΔsolHmθ(C9H6O4Na2·H2O, s) and ΔsolHmθ(C8H4O4Na2·1/2H2O, s), have been determined as being −11.917 ± 0.055 and −29.078 ± 0.069 kJ mol−1 by an RD496-2000 type microcalorimeter. In addition, the standard molar enthalpies of hydrated anion of the compounds were determined as being ΔfHmθ(C9H6O42−, aq) = −704.227 ± 1.674 kJ mol−1 and ΔfHmθ(C8H4O4Na22−, aq) = −1483.955 ± 1.612 kJ mol−1, from the standard molar enthalpies of solution and other auxiliary thermodynamic data through a thermochemical cycle.