Thermal Decomposition of Metal Carbonates (Oxford AQA IGCSE Combined Science Double Award)

Thermal Decomposition of Metal Carbonates

• Thermal decomposition is the term used to describe reactions where a substance breaks down due to the action of heat

• One such reaction is the thermal decomposition of metal carbonates

• Carbonates of metals from the lower half of the reactivity series tend to decompose on heating to produce the metal oxide and carbon dioxide gas:

metal carbonate → metal oxide + carbon dioxide

Thermal decomposition of copper carbonate

• The thermal decomposition of copper(II)carbonate occurs readily on heating

• Copper(II) carbonate is a green powder and slowly darkens as black copper(II) oxide is produced

• The carbon dioxide given off can be tested by passing the gas through limewater and looking for it to turn milky

• The equation for the reaction is:

CuCO₃ (s) →  CuO (s) + CO₂  (g) 

copper(II) carbonate → copper(II) oxide + carbon dioxide

Stability of metal carbonates

• A metal carbonate that needs to be heated strongly in order to decompose is described as thermally stable

  • The more strongly it needs heating, the higher the thermal stability

• Lithium carbonate is less thermally stable than other Group 1 metal carbonates

  • This means it requires less heating to decompose 

• Other Group 1 metal carbonates will not decompose at the temperature reached by a Bunsen burner

• For example, potassium carbonate decomposes at 1200 °C

K₂CO₃ (s) → K₂O (s) + CO₂ (g)

• Copper, zinc and calcium carbonate will decompose at the temperature that a Bunsen burner will reach

• Copper carbonate decomposes at 290 °C

CuCO₃ (s) → CuO (s) + CO₂ (g)