Limestone (CaCO3) reacts with hydrogen ions in water. These are always present in water, since water undergoes
H2O(l) = H+(aq) + OH-(aq)
The more acidic the water is, the more limestone will react, and erode. The erosion of marble sculpture and inscriptions
exposed to the elements is one of the consequences of acid rain.
The equilibrium solubility of limestone (and marble) exposed to the atmosphere is dominated by the following reactions:
CaCO3(s) + H3O+(aq) = Ca2+(aq) + HCO3-(aq)
|Eroded marble headstones at Blean, |
Kent County, England. Fred Senese, ©1992
HCO3- + H3O+(aq) = CO2(aq) + 2 H2O(l)
H3O+(aq) + CO32-(aq) = HCO3-(aq) + H2O(l)
These reactions are responsible for the erosion of the marble stones in the photo to the right. The main cast of characters is
calcium ion, Ca2+(aq)
bicarbonate ion, HCO3-(aq)
carbonate ion, CO32-(aq)
hydrated CO2, CO2(aq)
Carbonic acid, H2CO3(aq) makes a cameo appearance. Only about one percent of the total dissolved CO2 present is actually in the form of carbonic acid, so the formation and acid dissociation equilibria are worth mentioning, but are frequently overemphasized:
CO2(aq) + H2O(l) = H2CO3(aq)
H2CO3(aq) + H2O(l) = H3O+(aq) + HCO3-(aq)
There are a couple of bit players as well. Calcium forms
a carbonato complex CaCO30(aq).
Ca2+(aq) + CO32-(aq) = CaCO30(aq)
The log of the equilibrium constant for this reaction has been determined to be 3.22 +/- 0.14 (see Reference 1); it is important for very concentrated calcium and carbonate solutions.
There is also a calcium hydrogencarbanato complex CaHCO3+(aq):
Ca2+(aq) + HCO3-(aq) = CaHCO3+(aq)
with a log K of 1.1 +/- 0.07 (Reference 1) so under certain conditions this, too, can be present in nonegligible quantities.
Last but not least, the hydrated carbon dioxide will diffuse out of solution, adding to atmospheric CO2:
CO2(aq) = CO2(g)
The further adventures of CO2(g) are a story for another time.
- L. N. Plummer, E. Busenberg (1982). The solubilities of calcite, aragonite, and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O.
Geochim. Cosmochim. Acta, 46, 1011-1040.
Author: Fred Senese email@example.com