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Q & A: limits to exothermic solution

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Most recent answer: 11/21/2017
Q:
Why does an exothermic dissolution ever reach saturation? For an endothermic dissolution, I think of saturation as when the increase in disorder from dissolution becomes too small to overcome the ordering of the surroundings due to the heat that's absorbed.But for an exothermic dissolution, more dissolution should increase the disorder of both the material dissolving and of the surroundings that absorb the heat released. So why would it ever reach saturation?I suppose it has something to do with ordering of the solvent at some point? But what properties do I need to explore to get a handle on this? Something like the partial molar entropy of solution? Just not sure where to look....
- Don (age 66)
Fort Collins, CO USA
A:

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.

Mike W.


(published on 11/21/2017)

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