Thermochemically Consistent Free Energies of Hydration for Di- and Trivalent Metal Ions

Kasper Planeta Kepp*

*Corresponding author for this work

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This paper uses the relationship between the standard half reduction potential, the third ionization potential, and the free energies of hydration (ΔGhyd) of M2+ and M3+ ions to calculate new values of ΔGhyd for M2+ and M3+ ions. The numbers are “thermochemically consistent”, i.e. all numbers agree with the applied thermochemical cycle. This enables the tabulation of many ΔGhyd derived mainly from the data compiled by Marcus, but consistent with ΔGhyd(H+) = 1100 kJ/mol and SHE = 4.44 V. The accuracy of the new values of ΔGhyd(M3+) is by definition similar to the accuracy of the experimental hydration energy of the ΔGhyd(M2+) used for calculation, and vice versa, i.e. the new data have the same accuracy or higher than previously reported. As a result, the literature values for Cr3+ and Au3+, and Pd2+ are substantially revised. The approach also allows the calculation of new ΔGhyd for metal ions such as Mn3+, Ti2+, Ag3+, Ni3+, Cu3+, Au2+, and the theoretically interesting but experimentally inaccessible +2 ions of lanthanides. The new numbers enable a discussion of the previously unreported trend in ΔGhyd(M3+) for the 3d metal ions, which relates to the ligand field stabilization energies and effective nuclear charge as for the M2+ ions. The new tabulated values should be accurate with the applied assumptions to within 10 kJ/mol and may be of value for other thermochemical calculations, for interpretation of the aqueous trend chemistry of the metal ions, and as benchmarks for theoretical chemistry.
Original languageEnglish
JournalJournal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
Issue number37
Pages (from-to)7464-7471
Number of pages8
Publication statusPublished - 2018

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