For most ionic solids (for example: NaCl), water solubility increases with temperature. How can we explain (on the basis of structural or thermodynamic considerations) why the opposite is true of calcite CaCO3, whose solubility in water decreases with increasing temperature?
I could imagine that this is somehow correlated to the autoprotolysis of water: whether you have CO32- or HCO3- in your solution, depends strongly on the pH and since the autoprotolysis constant of water changes massively by three orders of magnitude from 0° to 100°C,
https://en.wikipedia.org/wiki/Self-ionization_of_water#Dependence_on_temperature,_pressure_and_ionic_strength
the carbonate protonation equilibrium should also be strongly affected by the temperature.
Cl-, on the other hand, is barely affected by the pH of its dissolving water, so there's not much of a reason why it wouldn't act "normal".
Please note that this answer is to be classified as an "educated guess", I didn't calculate the coupled equilibria.
Thank you, Jürgen, for this clear suggestion!
Regards,
Jean-Claude
The presence of dissolved CO2 plays a key role on CaCO3 solubility, by decreasing the pH, which in turn enhances CaCO3 dissolution through HCO3-/CO2- equilibrium shift. CO2 solubility decreases as the temperature increases, so this effect is less noticeable at higher temperatures, and more significant at high pressures. These aspects are analysed in this work: doi:10.1016/j.fluid.2012.03.020 .
Best regards
- Badges
- Science topic
- Similar topics
- Chemistry, Organic
- Organic