Absolute single-ion thermodynamic quantities of hydration at 298.15 K are derivable from the conventional enthalpies and entropies if the values of and are known. Here we suggest S¡(H aq `) * hyd H¡(H`) S¡(H aq `) \ J K~1 mol~1 based on the thermodynamics of the dissociation of water. This assignment, in turn, [5.5 corresponds to kJ mol~1 according to a self-consistent analysis of Krestov. Using these * hyd H¡(H`) \ [1078 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 and is derived. Further, a connection is detected between S¡(H aq `) * hyd H¡(H`) the Born radii calculated from the free energies of hydration, and the distances d, corresponding to the r B, upper limits of the experimental Ðrst peak position of the ionÈoxygen radial distribution curves, upon implication, in the case of a cation, the covalent radius of oxygen, and in the case of an anion, the water r cov radius r water ,Finally, from the di †erences between the enthalpies and free energies of hydration the temperature derivatives of the Born radii are determined.

This article brings to mind that the textbook acidity constants of the hydrohalic acids (except for HF) are erroneous because of outmoded conjectures about the HX solubilities. Here we invoke theoretical calculations of HX solubilities based on current solvation models. Although we do not claim to have provided the ultimate answer, it is safe to assume that the solubilities of both HCl and HBr are much higher than commonly surmised.