| Summary: | A calorimetric study has been made of proton ionization in dilute aqueous solution from aliphatic carboxylic acids at 10, 25, and 40° (13 acids) and at 25° (3 acids). Values of ΔS° are calculated by combining the resulting ΔH° values with ΔG° values obtained from the literature or determined from potentiometric or calorimetric measurements. The trends in ΔG°, ΔH°, and ΔS° are described by an electrostatic model, and deviations from this model are taken as a measure of nonelectrostatic substituent effects for carboxylic acids. A calorimetric study has also been made of proton ionization in dilute aqueous solution from several primary, secondary and tertiary amines at 10, 25 and 40°. ΔCp° values have been calculated and correlations made between ΔCp° values for proton ionization of several acids and amines and the dipole moments of the unprotonated acids or bases. Linear enthalpy-entropy relationships have been observed for proton ionization as a function of temperature. An equation has been derived to use this linear relationship between ΔH° and ΔS° to calculate ΔG° values at temperatures where only ΔH° values are available.
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