On p. 130 in Atkins & De Paula Physical Chemistry 10th Ed. so-called absolute ion entropies is introduced. The bearing idea is that H+ should have a proper value of - 21 J/molK instead of the customarily assigned value 0 J/molK.
In the exercise included "brief illustration 3B.4" they convert conventional entropies for Cl- and Mg2+.
For Cl- the entropy should change from 57 J/molK to 36 J/molK and for Mg2+ it should change from - 128 J/molK to - 149 J/molK.
But wouldn't this method of just adjusting all ion entropies have really far-reaching consequenes...?
1. If just all ion entropies all changed this way, then ΔS°sol would change for all salts and hence all ΔG°sol would change accordingly, and hence all Ksp values would have to be recalculated. The latter is in itself unsatisfactory since, in principle, it should be readily measurable for some sparingly soluble salts (with, for example, ion sensitive electrodes).
For example, for NaOH ΔG°sol would be decreased from - 40 kJ/mol to just - 27 kJ/mol using the Atkins method of absolute entropies. For NaCl it would shift from -9 kJ/mol to + 4 kJ/mol (and hence NaCl should be insoluble!).
2. To keep Ksp and ΔG°sol stable, all S° for all solid salts would have to change or...
3. ... all H°sol are 'recalculated' but that would be very counter-intuitive since ΔH°sol are readily measurable as the heat released or absorbed when a salt is dissolved (just measure at different molalities and extrapolate to zero molality).
I thought such a modification of customary ion entropies (with H+ having 0 J/molK) to 'absolute' ion entropies (with H+ having assigned -21 J/molK) should adjust all other values in a different way: All monovalent cations should decrease by 21 J/molK while all monovalent anions would increase by 21 J/molK (and appropriate adjustments for divalent ions of course). For example Cl- should, this way, have 78 J/mol. Then all ΔS°sol would be kept the same, and thus all the three Ksp, ΔG°sol and ΔH°sol would be kept stable.
There must be a flaw somewhere, but I can't figure out where it is...