What type of double displacement reactions yield carbonic acid as a product?
The reactions between an acid and a metal carbonate or a metal hydrogen carbonate are double displacement reactions that produce carbonic acid.
In a double displacement reaction, the cations change partners:
A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻
To form H₂CO₃ as a product, the equation must be of the form:
2H⁺B⁻ + (C⁺)₂CO₃²⁻ → H₂CO₃ + 2C⁺B⁻
H⁺B⁻ + C⁺HCO₃⁻ → H₂CO₃ + C⁺B⁻
Typical examples are
2HCl + Na₂CO₃ → H₂CO₃ + 2NaCl
H₂SO₄ + CaCO₃ → H₂CO₃ + CaSO₄
HCl + NaHCO₃ → H₂CO₃ + NaCl
H₂SO₄ + Ca(HCO₃)₂ → H₂CO₃ + CaSO₄
The carbonic acid is unstable. It rapidly decomposes into water and carbon dioxide.
H₂CO₃ → H₂O + CO₂
Many baking powders are of a weak acid and a hydrogen carbonate salt such as NaHCO₃. They work by forming carbonic acid in a double displacement reaction. The bubbles of carbon dioxide expand in the moist batter and leaven the mixture.