# Why does the reaction of dissolution stop at an equilibrium point? [closed]

The formula for Gibbs free energy is $$\Delta G=\Delta H-T\Delta S$$. If Gibbs free energy is negative, the reaction is spontaneous. This also applies to dissolution reactions.

However, we know that every soluble element has a dissolution constant. At a certain equilibrium point, the reaction stops. Which term in the Gibbs free energy indicates that the reaction should stop?

Moreover, according to the Le Chatelier's principle, the dissolution reaction can go in either way if we change some concentrations at the equilibrium point. But only changing concentrations does not seem to affect neither the enthalpy nor the entropy. Why does the value of $$\Delta G$$ vary?

Finally, is there any way to predict the dissolution constant based on $$\Delta H$$ and $$\Delta S$$?

• The entropy of mixing depends on concentrations, which change during the dissolution process. That is the basis of the law of mass action (equilibrium when Q = K) as well.
– Karsten
Jan 21 at 17:04

• The forward reaction, e.g. a salt dissolving in water, $$\ce{NaCl -> Na+ + Cl-}$$. Simplified for illustration.
• The backwards reaction, e.g. a salt precipitating from solution in water, $$\ce{Na+ + Cl- -> NaCl}$$.