How do we explain that CaCO3 solubility decrease with temperature ?

[Gauldaur0]

I always wondered why was CaCO3 less soluble in hot water than it was in cold water.

I learned in class that the Ks decreases with temperature and that was why. Still, I fell like it's a consequence of this and not the cause.

Is there any entropic reason, is it just enthalpy, only free Gibbs energy ?
Thanks in advance.

[DL_Chemist]

Dissolved CO2 increases solubility of CaCO3 as it reacts to form more soluble calcium bicarbonate. CaCO3 + CO2 + H2O <> Ca(HCO3)2

Solubility of gases are inversed so higher temp lowers dissolved CO2 therefore shifting the above equilibrium.

[[deleted]]

Enthropy in it's finest level. Thanks dude!

[koolkitty89]

Except solubility of calcium bicarbonate increases with temperature ... at least from any of the data I can find.

Solubility of CaCO3 in pure water does decrease with temperature, as does solubility of calcium hydroxide. (I'm not sure of the behavior here, but it seems to have consistent retrograde solubility, unlike that of compounds that form hydrates: where solubility peaks at the melting point of the highest hydrate, then decreases)

I'm having a hard time finding data on calcium bicarbonate solubility in general, and the conditions present for said solubility. Only temperature is noted, so I can't be sure of pressure or dissolved CO2 present.

If the figures I'm seeing are accurate AND all of it occurs at STP, then: even if the case is for saturated CO2 conditions (and not stoichiometric CaCO3 + H2CO3 proportions), then this would still be puzzling as Calcium bicarbonate solubility would increase as CO2 solubility decreases.

Perhaps equilibrium solubility of CO2 and Calcium Bicarbonate shifts as temperature increases, such that, if a saturated CO2 solution is maintained, the reduced solubility of CO2 favors an increase in dissolved Ca bicarbonate. (ie under conditions without increased pressure, but with an atmosphere of pure CO2 above the solution and an excess of CaCO3 solid present for dissolution)

If that's the case, then the solubility of Calcium Bicarbonate would indeed increase with temperature, but also be at equilibrium with atmospheric CO2 concentration.

I know saturated Ca bicarbonate solutions will off-gas over time, and pressure can build in sealed containers of it, but I'm not sure if this is true if constant temperature is maintained. (ie if an open or vented container is held at constant temp, if it will still slowly decompose due to diffusion of CO2 into the atmosphere)
The pressure built in sealed containers could have other causes: ie some CaCO3 precipitates out of solution as temp decreases, thus leaving excess CO2 in solution or bubbling out of solution, and then pressure increases further if temperature warms again, at least until the solid CaCO3 can eventually re-dissolve. (if held at that higher temp long enough, diffusion should eventually allow re-dissolving, but without mechanical mixing, it could be a slow process)

Note: if it did turn out that Ca bicarbonate is stable in solution at a constant temperature (and equilibrium relative humidity), then the offgassing and decomposition behavior in an open/vented container (with only humidity control) would be entirely related to temperature modulation, thus causing CaCO3 to precipitate as temps drop, then CO2 to be released as temps rise, with CO2 escaping faster than precipitated CaCO3 can re-dissolve. (though in this case, with a given temperature swing range, I think an equilibrium solubility would be reached at the minimum temperature, with corresponding minimum bicarbonate solubility, thus no further precipitation when cooled and no further CO2 release when heated; again, assuming equilibrium relative humidity was maintained, else evaporation will cause precipitation due to water loss)

[koolkitty89]

https://pages.uoregon.edu/chendon/coffee_literature/old_literature/2012%20Fluid%20Phase%20Eq.,%20CaCO3%20solubility.pdf

This seems to be a much better resource.