Solubility equilibrium - Review question help!

Question

The solubility of ZnCO₃ in water is quite low. What could you add to increase the solubility? Explain how your method works in detail.

Answer 1

I agree with the answer provided by @James M. However, I'd like to add remarks trying to explain why raising the temperature or adding acid would work to increase the solubility of ZnCO3.

Assuming the dissolution process for ZnCO₃ is endothermic (absorbs heat from the surroundings), then one can look at heat as a reactant. Heat + ZnCO₃(s) ---H₂O---> Zn²⁺(aq) + CO₃^2-(aq). And according to Le Chatelier, adding a reactant (heat) will cause the equilibrium to shift to the right (more soluble).

Because ZnCO₃ is the salt of a weak acid (H₂CO₃), adding acid (H⁺) will also increase the solubility of ZnCO₃. Again, this is in accord with Le Chatelier's principle, as explained below:

ZnCO₃(s) ==> Zn²⁺(aq) + CO₃^2-(aq)

Adding H⁺ will react with the CO₃^2-(aq) to produce the weak acid H₂CO₃. Since it is weak, it will not readily dissociate back to H⁺ and CO₃^2-, thus essentially "removing CO₃^2- from solution. Le Chatelier predicts that removing a product from the reaction will cause the equilibrium to shift to the product side (more soluble).

Hope this helps to explain why increasing temperature and adding acid will increase the solubility of ZnCO₃.

Answer 2

Increasing the temperature will increase the solubility, also if an acid is added to the solution it will increase the solubility of ZnCO₃.