Thermodynamics of macroscopic and microscopic proton ionization from protonated 4-aminobenzoic acid in aqueous solution from 298.15 to 393.15 K

Macroscopic and microscopic thermodynamic quantities (equilibrium constants and enthalpy, entropy, and heat capacity changes) for proton ionization fr om protonated 4-aminobenzoic acid In aqueous (H2O and D2O) solutions at tem peratures from 298.15 to 393.15 K have been determined by a combination of NMR, potentiometric, and calorimetric methods. There are mathematical relat ionships between the thermodynamic quantities associated with the macroscop ic and microscopic reactions. The reactions occurring during the proton ion ization are much better understood by examining the thermodynamic quantitie s for the microscopic rather than for the macroscopic reactions. The change s of the Delta H degrees and Delta S degrees values for the microscopic rea ctions with temperature give insight into the interactions that occur durin g proton ionization since the trends of these quantities for a given ioniza tion with temperature vary depending on whether the other functional group is protonated. There is a direct relationship between the temperature varia tions of the Delta H degrees and Delta S degrees values and the Delta C(p)d egrees values. Therefore, a knowledge of the Delta C(p)degrees values, whic h can only be obtained by making measurements at more than one temperature, is helpful in gaining an understanding of the interactions of the reactant s and products with the solvent and with each other. Large negative or posi tive Delta C(p)degrees values indicate that extent of interaction between t he solvent molecules with the reactants and products is very different. Equ ations typically used to describe the temperature dependence of pK, Delta H degrees, Delta S degrees, and Delta C(p)degrees for simple ionic reactions in aqueous solutions are used to correlate these values for the microscopi c reactions. The trends of the macroscopic constants with temperature are n ot those typically found in simple ionic reactions.