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 from protonated 4-aminobenzoic acid in aqueous (H 2 O and D 2 O) solutions at temperatures from 298.15 to 393.15 K have been determined by a combination of NMR, potentiometric, and calorimetric methods. There are mathematical relationships between the thermodynamic quantities associated with the macroscopic and microscopic reactions. The reactions occurring during the proton ionization are much better understood by examining the thermodynamic quantities for the microscopic rather than for the macroscopic reactions. The changes of the Δ H ° and Δ S ° values for the microscopic reactions with temperature give insight into the interactions that occur during proton ionization since the trends of these quantities for a given ionization with temperature vary depending on whether the other functional group is protonated. There is a direct relationship between the temperature variations of the Δ H ° and Δ S ° values and the Δ C p ° values. Therefore, a knowledge of the Δ C p ° values, which can only be obtained by making measurements at more than one temperature, is helpful in gaining an understanding of the interactions of the reactants and products with the solvent and with each other. Large negative or positive Δ C p ° values indicate that extent of interaction between the solvent molecules with the reactants and products is very different. Equations typically used to describe the temperature dependence of p K, Δ H °, Δ S °, and Δ C p ° for simple ionic reactions in aqueous solutions are used to correlate these values for the microscopic reactions. The trends of the macroscopic constants with temperature are not those typically found in simple ionic reactions.