Thermodynamic properties of the methylpyridines. Part 2. Vapor pressures, heat capacities, critical properties, derived thermodynamic functions between the temperatures 250 K and 560 K, and equilibrium isomer distributions for all temperatures >= 250 K

Results are reported for measurements leading to the calculation of standar d thermodynamic properties for temperatures from T = 250 K to T = 560 K for the methylpyridines (Chemical Abstracts registry numbers: 4-methylpyridine , 108-89-4; 2-methylpyridine, 109-06-8; 3-methylpyridine, 108-99-6). The st andard state is defined as the ideal gas at the pressure p = p(O) = 101.325 kPa. Vapor pressures were measured by comparative ebulliometry for the pre ssure range p approximate to 4 kPa to p = 270 kPa. Two-phase (liquid + vapo r) heat capacities were measured for the temperature range approximate to 3 10 (T/K) approximate to 590 with a differential scanning calorimeter (d.s.c .), and saturation heat capacities C-sat were derived. The critical tempera ture T, was determined experimentally by d.s.c., and the critical pressure and critical density were derived. Densities for the liquid phase of 3-meth ylpyridine at saturation pressure were measured with a vibrating-tube densi meter for the temperatures 323 (T/K) < 473. The enthalpy of combustion for 3-methylpyridine was determined to resolve a discrepancy in the literature. Enthalpies of vaporization were calculated for all compounds. Literature v alues for enthalpy increments and entropies for the condensed phases by adi abatic heat-capacity calorimetry and the energies of combustion were combin ed with the present results to derive standard entropies, enthalpy incremen ts, and molar Gibbs free energies of formation for temperatures between T = 250 K and T = 560 K. Comparisons with standard entropies calculated statis tically provide corroboration for the calorimetrically derived values and a re used to demonstrate thermodynamic consistency for properties related thr ough identities and correlations. Isomerization distributions derived with standard molar Gibbs free energies of formation are used to aid interpretat ion of literature fossil fuel characterization and pyridine methylation cat alysis studies. (C) 1999 Academic Press.