Thermal Decomposition

Thermal decomposition is one of the most frequently examined reaction types in 0620. Understanding which compounds decompose and what they produce is essential for Paper 2 and Paper 4.

Definition

Thermal decomposition: the breakdown of a compound into two or more simpler substances by the action of heat.

Key features:

• Heat must be supplied continuously (reaction stops when heating stops)
• It is endothermic
• It is not a combustion reaction (no reaction with oxygen from the air)
• It produces simpler compounds or elements

Thermal decomposition of metal carbonates

General equation

metal carbonate -> metal oxide + carbon dioxide

MCO3 -> MO + CO2

Specific examples

Carbonate	Equation	Colour change
Calcium carbonate	CaCO3 -> CaO + CO2	White stays white
Copper(II) carbonate	CuCO3 -> CuO + CO2	Green to black
Zinc carbonate	ZnCO3 -> ZnO + CO2	White to yellow (hot), white on cooling
Magnesium carbonate	MgCO3 -> MgO + CO2	White stays white
Iron(II) carbonate	FeCO3 -> FeO + CO2	Green/brown to black

Stability and the reactivity series

The more reactive the metal, the more thermally stable its carbonate:

• Group I carbonates (Na2CO3, K2CO3): do not decompose with a Bunsen burner (too stable)
• Group II carbonates (CaCO3, MgCO3): decompose with strong, sustained heating
• Transition metal carbonates (CuCO3, ZnCO3): decompose easily

This trend links to position in the reactivity series.

Testing the gas

CO2 produced can be tested with limewater (turns milky/cloudy):

Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)

Thermal decomposition of metal hydroxides

metal hydroxide -> metal oxide + water

Hydroxide	Equation
Copper(II) hydroxide	Cu(OH)2 -> CuO + H2O
Zinc hydroxide	Zn(OH)2 -> ZnO + H2O
Iron(II) hydroxide	Fe(OH)2 -> FeO + H2O

Group I hydroxides (NaOH, KOH) do not decompose on heating.

Thermal decomposition of metal nitrates

The products depend on the metal’s reactivity:

Group I nitrates -> nitrite + oxygen

2NaNO3 -> 2NaNO2 + O2

Other metal nitrates -> metal oxide + NO2 + oxygen

2Cu(NO3)2 -> 2CuO + 4NO2 + O2

2Pb(NO3)2 -> 2PbO + 4NO2 + O2

Observations: brown fumes of NO2 gas. Test for O2: relights a glowing splint.

Silver and gold nitrates -> metal + NO2 + oxygen (metal is produced because it is very unreactive)

2AgNO3 -> 2Ag + 2NO2 + O2

Hydrated compounds

CuSO4.5H2O -> CuSO4 + 5H2O

Blue crystals turn to white powder. This is a reversible reaction: adding water to anhydrous CuSO4 turns it blue again and releases heat.

See water of crystallisation.

The limestone cycle

The thermal decomposition of calcium carbonate is the first step of the limestone cycle:

1. CaCO3 -> CaO + CO2 (thermal decomposition)
2. CaO + H2O -> Ca(OH)2
3. Ca(OH)2 + CO2 -> CaCO3 + H2O

How to recognise in exams

Key words: “heated strongly”, “decompose”, “break down on heating”. Look for questions about:

• Writing decomposition equations
• Predicting products
• Comparing stability of carbonates
• Colour changes observed
• Testing the gases produced

Copper(II) carbonate is heated. Describe the observations and write the equation. [3 marks]

• The green solid turns black [1]
• Effervescence / a gas is given off [1]
• CuCO3 -> CuO + CO2 [1]

Explain why sodium carbonate does not decompose when heated with a Bunsen burner, but copper(II) carbonate does. [2 marks]

• Sodium is more reactive than copper / sodium is higher in the reactivity series [1]
• The more reactive the metal, the more thermally stable its carbonate, so sodium carbonate requires much higher temperatures to decompose [1]