Thermal Decomposition of Metal CarbonatesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is thermal decomposition?
Thermal decomposition is a chemical reaction in which a single compound breaks down into two or more simpler substances when heated. It is an endothermic reaction, because energy must be supplied (as heat) to break the bonds in the compound.
Section 2
How do metal carbonates decompose?
The carbonates of magnesium, copper, zinc, calcium and lithium all decompose on heating in a similar way, producing a metal oxide and carbon dioxide gas:
metal carbonate → metal oxide + carbon dioxide
Example: copper carbonate (green) decomposes on heating to form copper oxide (black) and carbon dioxide:
CuCO₃ → CuO + CO₂
Copper carbonate is a useful example to describe in exams because the colour change (green to black) is easy to observe: CuCO₃ (green) → CuO (black) + CO₂
Section 3
Do all metal carbonates decompose the same way?
Not all carbonates decompose at the temperature reached by a Bunsen burner:
- The carbonates of calcium, magnesium, copper, zinc and lithium decompose relatively easily when heated with a Bunsen burner
- Most Group 1 metal carbonates (e.g. sodium carbonate, potassium carbonate), other than lithium, are much more thermally stable and do not decompose at Bunsen burner temperatures
- This reflects the general trend that Group 1 compounds are thermally very stable, and lithium (a Group 1 metal that behaves somewhat like Group 2 metals) is the exception
Section 4
How can the carbon dioxide produced be identified?
The carbon dioxide gas released during thermal decomposition can be tested by bubbling it through limewater (calcium hydroxide solution):
- If carbon dioxide is present, the limewater turns from colourless to cloudy (milky) white
- This confirms that the gas released on heating the carbonate is carbon dioxide, supporting the thermal decomposition equation
When describing this experiment, always name the specific observation (cloudy/milky white) — 'changes colour' alone will not gain the mark.
Must Know
- Thermal decomposition = breakdown of a compound into simpler substances on heating; it is endothermic
- Metal carbonate → metal oxide + carbon dioxide
- Carbonates of magnesium, copper, zinc, calcium and lithium decompose at Bunsen burner temperatures
- Most Group 1 carbonates (except lithium) do NOT decompose at Bunsen burner temperatures — they are more thermally stable
- Copper carbonate (green) decomposing to copper oxide (black) is a classic example
- CO₂ gas is confirmed using limewater, which turns cloudy/milky white
That's the notes covered.
Carry on to the next subtopic.