HCl dissolves lime mortar? how??

Question

Lime mortar is Ca(OH)2, sand, and water. It reacts with CO2 from the air. This is how it becomes hard. What is the equation for this reaction?

ANd then, what is the equation for the reaction between this and hydrochloric acid? How is this dissolved by hydrochloric acid?

Answer 1

Lime is CaO.  (Sometimes referred to as quicklime.)  It is produced from limestone, CaCO3, by heating it and driving off CO2.  The limestone comes from ancient sea shells.

CaCO3(s)  --heat--> CaO(s) + CO2(g)

When CaO is combined with water we get slaked lime, which calcium hydroxide.  It's true that calcium hydroxide is a strong base (and dissociates completely), it's also not very soluble in water.

CaO(s) + H2O(l) -->  Ca(OH)2(s)

The mortar to which you refer, in reacting with CO2, is going back to where the lime originally came from, calcium carbonate.  Carbon dioxide dissolves in water to make an acidic solution.  The acidic solution react with the base, the calcium hydroxide, to produce calcium carbonate.

CO2(g) + H2O(l) <==> H+ + HCO3-

Ca(OH)2(s) + H+  +  HCO3-   --> CaCO3(s) + 2H2O(l)

The catch is that calcium carbonate, the stuff of limestone, is soluble in the acid that is produced when carbon dioxide dissolves in water to make acid rain.  It's also soluble in any other acid, like HCl.

CaCO3(s) + 2H+  -->  Ca2+ + CO2(g) + H2O(l)

CaCO3(s) + 2HCl(aq)  -->  CaCl2(aq) + CO2(g) + H2O(l)

Answer 2

1) Ca(OH)2 + CO2 → CaCO3 + H2O

2)  CaCO3 +2HCl → CaCl2 + CO2 + H2O

It dissolves because the CaCl2 is soluble in water.