When the enthalpy of dissolution is positive, should solubility increase/decrease w/ increase in temperature?

Question

For whatever the answer is, can you also explain why that's the answer?

Answer 1

It depends on the sign of the entropy term. Remember that:

dG=dH-TdS

So as you increase the temperature, you decrease the relative importance of the enthalpy term.

Another way to see this is to write the equilibrium constant expression for the dissolution:

Ksp=exp(-dG/RT)
dG=dH-TdS
so Ksp=exp(-dH/RT)*exp(dS/R)

When you take the limit as T-->infinity, the enthalpy term goes to 1 no matter what the sign of dH, and the solubility is dominated by the entropy term.

Answer 2

The entropy change during dissolving is always positive.
If the enthalpy change is also positive, then increasing the temperature will cause the system to try to take in heat (Le Chatelier's principle) and more solute will dissolve.