Reactions of Acids with Carbonates and Hydrogencarbonates

1Why this matters

Drop a little baking powder into lemon juice and it fizzes. Take an indigestion tablet and the burning feeling fades. Drip acid onto a rock and, if it fizzes, a geologist knows it is limestone. All three are the same reaction: an acid meeting a carbonate or a hydrogencarbonate and releasing carbon dioxide.

2Acid + carbonate → salt + water + carbon dioxide

• CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) (marble chips or limestone fizz and dissolve)
• Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
• CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g) (green powder gives a blue solution)

Ionic equation for any carbonate: CO₃²⁻ + 2H⁺ → H₂O + CO₂. (For an insoluble carbonate, the whole solid is written: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g).)

3Acid + hydrogencarbonate → salt + water + carbon dioxide

A hydrogencarbonate (also called a bicarbonate) contains the HCO₃⁻ ion and reacts in the same way, but needs only one hydrogen ion:

• NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
• 2NaHCO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)

Ionic equation: HCO₃⁻ + H⁺ → H₂O + CO₂. Hydrogencarbonates also react with alkalis, because the HCO₃⁻ ion can give up its hydrogen ion: NaHCO₃ + NaOH → Na₂CO₃ + H₂O.

4The test for carbon dioxide

Bubble the gas through limewater (calcium hydroxide solution). It turns milky, because insoluble calcium carbonate forms:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

If you keep bubbling carbon dioxide through for a long time, the milkiness disappears again, because soluble calcium hydrogencarbonate forms: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).

5Why sulfuric acid is not used with marble

With dilute sulfuric acid, marble chips fizz for a moment and then stop, although plenty of acid and marble are left. The calcium sulfate formed is only slightly soluble, so it coats the chips and keeps the acid away. That is why carbon dioxide is prepared in the laboratory from marble chips and dilute hydrochloric acid; calcium chloride is soluble.

6Everyday examples

• Baking: baking powder contains sodium hydrogencarbonate and a solid acid. When water is added in the dough, they react, and bubbles of carbon dioxide make cakes and mandazi rise.
• Indigestion remedies: tablets of calcium carbonate or sodium hydrogencarbonate neutralise excess hydrochloric acid in the stomach. The carbon dioxide produced may make you burp.
• Testing rocks: a drop of dilute acid fizzes on limestone and marble (calcium carbonate) but not on granite.
• Acid rain slowly dissolves limestone and marble buildings and statues by the same reaction.

Worked example: carbon dioxide from limestone

10 g of calcium carbonate reacts completely with excess dilute hydrochloric acid. Calculate the mass and the volume at STP of carbon dioxide produced. (Ca = 40, C = 12, O = 16; molar volume at STP = 22.4 dm³)

1. M(CaCO₃) = 40 + 12 + 48 = 100 g/mol, so 10 g is 0.10 mol.
2. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂: 1 mol of CaCO₃ gives 1 mol of CO₂, so 0.10 mol of CO₂ forms.
3. Mass = 0.10 × 44 = 4.4 g. Volume = 0.10 × 22.4 = 2.24 dm³.

Practice