What is the reaction of limestone with water?

When limestone (CaCO₃) reacts with the hydronium ion, what are all the products produced?
David, University of Nebraska-Lincoln

Limestone (CaCO₃) reacts with hydrogen ions in water. These are always present in water, since water undergoes autoprotolysis: 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.

Eroded marble headstones at Blean, Kent County, England. Fred Senese, ©1992

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)
HCO₃^- + H₃O⁺(aq) = CO₂(aq) + 2 H₂O(l)
H₃O⁺(aq) + CO₃^2-(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, CO₃^2-(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) + CO₃^2-(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.

References

1. 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 senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/limestone-and-water-reaction.shtml