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This is an unstable acid which readily breaks down to form carbon dioxide gas and water:
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H2CO3(aq) |
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The salts of carbonic acid are the carbonates. If an acid is added to a carbonate, a reaction will occur if the acid added is stronger than carbonic acid. In this case it donates protons to the carbonate ion forming carbonic acid. This promptly breaks down and effervescence of carbon dioxide gas is seen. However, if the acid added is less acidic than carbonic acid, very little proton transfer occurs and no effervescence is seen. |
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We can check the relative strength of the acids by looking up the acid dissociation constants, Ka, in the data book.
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Acid |
dissociation constant/mol dm-3 |
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Ethanoic acid |
1.7 × 10-5 |
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Carbonic acid |
4.5 × 10-7 |
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Phenol |
1.28 × 10-10 |
Consequently, whilst ethanoic acid will react with a carbonate, phenol does not. This is a popular examination question.
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Na2CO3(aq) + 2CH3COOH(aq) ⇒ 2CH3COONa(aq) + H2O(l) + CO2(g) |
H2O(l) +
CO2(g)