Carbonates (WJEC GCSE Chemistry)

• 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

Copper carbonate will decompose into copper oxide and carbon dioxide at 290 °C 

• 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

Trend in stabilities 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
• Metal carbonates that contain a very reactive metal are the most stable and take the most amount of heating to decompose into the metal oxide and carbon dioxide
• Potassium carbonate decomposes at 1200 °C

K₂CO₃ (s) → K₂O (s) + CO₂ (g)

• Metal carbonates that contain a metal with a low reactivity are the least stable and take the least amount of heating to decompose 
• Copper carbonate decomposes at 290 °C

CuCO₃ (s) → CuO (s) + CO₂ (g)

Diagram to show the relationship between the reactivity of the metal and the thermal stability of the metal carbonate

As the reactivity of the metal increases the thermal stability of the metal carbonate also increases