Limits to Exothermic Solution
Most recent answer: 11/21/2017
- Don (age 66)
Fort Collins, CO USA
Great question!. I'm not sure of the answer, but it sounds like you're on the right track.
For low solute concentrations, the enthalpy of going into solution is pretty nearly independent of concentration, and for the exothermic case is negative. Although the entropy change does depend logarithmically on concentration, it should remain positive. So that sounds like the net free energy change should remain negative, and the dissolution should continue to completion. Often (e.g. for isopropyl alcohol and water) that's exactly what happens.
For other solutes (e.g. CaCl2) the solution saturates. That typically happens when the concentration is very high. By that point, the solution isn't really anything like pure water, so there's no particular reason for the enthalpy of going into solution to be close to the low-concentration value. Likewise the entropy increase of solution would be very small simply because the concentration is so large even for an ideal solute, and that is modified further by the solute-solute interactions. It can happen that extra solute ions reduce the entropy of the water molecules by more than the entropy they gain by joining the solution. (I'm speaking loosely here, since the entropy doesn't really break up into separate pieces like that.)
Anyway, sorry for the fuzzy non-chemist answer, but the point is that it's not really a mystery and your hunch is along the right lines.
(published on 11/21/2017)