What happens when acid reacts with limestone?

15 June 2008

Question

What happens in a reaction between acid rain and limestone?

Answer

Limestone is mostly made up of the mineral calcium carbonate (CaCO3). This is not very soluble, so rocks don't dissolve very quickly. But if you add an acid, you add hydrogen ions (H+), which will react with the carbonate to form hydrogen carbonate HCO3- ions, which are very soluble in water, and the limestone will dissolve. Or, if there is more acid, two hydrogen ions will react with a carbonate to form carbonic acid - H2CO3 - which will decompose to form carbon dioxide - CO2 - which eventually bubbles off into the atmosphere, and water H2O.

The acid can come from a variety of sources: sulphur and nitrogen oxides released by burning fossil fuels will form sulphuric, sulphurous, nitrous and nitric acids, and carbon-dioxide itself can dissolve in water to form carbonic acid.

Comments

Does limestone still dissolve if there are two acids

Limestone - calcium carbonate, (CaCO3) - dissolves and at a low rate. Sodium carbonate (Na2CO3) dissolves faster than limestone; it reacts with acids in a chemical reaction, producing carbon dioxide.

Yes it does

Limestone is calcium carbonate (CaCO3). Carbonates react with acids according to the equation: 

CO32- + 2H+ -> H2O + CO2

However, the rate of the reaction will be determined by a number of factors. First, the strength of the acid. A strong acid, like hydrochloric acid, which readily ionises to produces a high concentration of hydrogen ions (H+) will react more rapidly than a weaker acid that ionises more slowly, like acetic acid. Temperature is also important, because the rates of all chemical reactions are proportional to temperature.

Another factor to consider is the salt produced when the acid and the carbonate react. Sulphuric acid is a very strong acid but it won't react very well with calcium carbonate because one product of the reaction is calcium sulphate. This is plaster of paris and is insoluble. As a result, it forms a protective layer around the limestone, preventing further access by the acid and effectively stopping the reaction.

So, all acids in the liquid phase will react with limestone to some extent, but combinations of acids might, or might not, react as well as some strong acids used in isolation.

 

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Very nice
answer

The CaCO3 reacts with water and CO2 to form carbonated calcium which is soluble in water. This is a natural process.

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