A new table of the thermodynamic quantities of ionic hydration: values and some applications (enthalpy–entropy compensation and Born radii)

Abstract

Absolute single-ion thermodynamic quantities of hydration at 298.15 K are derivable from the conventional enthalpies and entropies if the values of S°(Haq+) and ΔhydH°(H+) are known. Here we suggest S°(Haq+) = −5.5 J K−1 mol-1 based on the thermodynamics of the dissociation of water. This assignment, in turn, corresponds to ΔhydH°(H+) = −1078 kJ mol-1 according to a self-consistent analysis of Krestov. Using these values, as a main result, the anions are more strongly hydrated than usually thought, in line with recent calculations. Only the group 1, 2, and 15 nobel gas ions are dealt with. For each series, the conventional enthalpies and entropies are linearly related to one another. From these linear free energy relationships (LFERs) a relationship between S°(Haq+) and ΔhydH°(H+) is derived. Further, a connection is detected between the Born radii rB, calculated from the free energies of hydration, and the distances d, corresponding to the upper limits of the experimental first peak position of the ionoxygen radial distribution curves, upon implication, in the case of a cation, the covalent radius rcov of oxygen, and in the case of an anion, the water radius rwater, Finally, from the differences between the enthalpies and free energies of hydration the temperature derivatives of the Born radii are determined.