3.5 - Thermal Decomposition of Carbonates

Thermal decomposition of metal carbonates

• Thermal decomposition is when a substance breaks down on heating.
• When a metal carbonate is heated, it decomposes into a metal oxide and carbon dioxide:
  • metal carbonate → metal oxide + carbon dioxide

Decomposing copper(II) carbonate

• Copper(II) carbonate is a green powder that decomposes easily on heating to form black copper(II) oxide and carbon dioxide gas.
• The visible colour change is from green to black.
• copper(II) carbonate → copper(II) oxide + carbon dioxide
• CuCO₃(s) → CuO(s) + CO₂(g)

How readily carbonates decompose

• How readily a carbonate decomposes depends on how reactive its metal is: the more reactive the metal, the more stable its carbonate, and the more heat is needed to decompose it.
• Carbonates of magnesium, copper, zinc, calcium, and lithium all break down on heating:
  • Copper(II) carbonate decomposes with gentle heating.
  • Calcium carbonate must be heated strongly for several minutes before it breaks down.
• Other Group 1 carbonates, such as sodium and potassium carbonate, are far more stable and need temperatures higher than a Bunsen burner can reach to decompose.