Thermodynamics of Some Reactions Involving Aqueous Carboxylic Acids at Elevated Temperatures

Polyfunctional carboxylic acids are important aqueous species for researchers with interests as disparate as power plant chemistry and the origin of life. Although thermodynamic data for many of these species are available in the SUPCRT databases, values for some important species are not readily available. We have applied the Sedlbauer-O'Connell-Wood (SOCW) functional group-contribution model (Sedlbauer et al. 2000 Chem. Geol. 163: 43-63; Yezdimer et al., 2000 Chem. Geol. 164: 259-280) to the prediction of values of Δ_fGº for a number of polyfunctional carboxylic acids for which other data have not yet been published. Among the most significant input parameters are Δ_fHº (T_r,P_r) and Sº(T_r,P_r), the enthalpy of formation and entropy of the compound of interest as an aqueous species at some specific reference temperature and pressure. Although many of these values of interest have been published (Alberty, 2006 Biochemical Thermodynamics: Wiley) values for many compounds of interest have not yet been measured. It is still possible to make inferences by considering the sign and magnitude of ΔH and ΔC_P for similar reactions. Reductions of double bonds are typically exothermic and have small but positive changes in heat capacity. Reactions involving the reduction of CO₂ to form carbonyl groups are endothermic with positive changes in heat capacity. The reactions become less endothermic as compound chain length increases. Thus reactions such as acetate + CO₂ + H₂ = pyruvate + H₂O are considerably more endergonic at low temperature that at high temperature. Reactions in which CO₂ is reduced to form carboxyl groups have little change in enthalpy at low temperature, but negative changes in heat capacity. The negative ΔC_P values exceed those for the dissociation of water. The equilibrium constants for these reactions are largest at temperatures ranging from ~75-125ºC and are more endergonic at lower and higher temperatures.