2.4.5 Nitrates & Carbonates

• Thermal decomposition is the breakdown of a compound into two or more different substances using heat

Thermal decomposition of carbonates

• The Group 2 carbonates break down (decompose) when they are heated to form the metal oxide and give off carbon dioxide gas
• The general equation for the decomposition of Group 2 carbonates is:

The thermal decomposition of Group 2 carbonates to form metal oxides and carbon dioxide gas

• Going down the group, more heat is needed to break down the carbonates 

The diagram shows the thermal decomposition of magnesium carbonate to form magnesium oxide and carbon dioxide gas

Thermal decomposition of nitrates

• The Group 2 nitrates break decomposed when they are heated to form the metal oxide, oxygen gas and nitrogen dioxide gas (which is a brown toxic gas)
• Since the formed nitrogen dioxide gas is toxic, the decomposition of nitrates is often carried out in a fume cupboard 

The diagram shows the thermal decomposition of magnesium nitrate to form magnesium oxide, oxygen gas and nitrogen dioxide gas

• The general equation for the decomposition of Group 2 nitrates is: 

• Going down the group, more heat is needed to break down the nitrates
  • The smaller positive ions at the top of Group II will polarise the nitrate ions more than the larger ions at the bottom of the group
  • The more polarised they are, the more likely they are to thermally decompose

Testing for gases

• Carbon dioxide 
  • When bubbled through limewater, carbon dioxide gas will turn the limewater milky 
  • This is a fine white precipitate of calcium carbonate, CaCO₃

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

• Oxygen 
  • If oxygen gas is present, it will relight a glowing splint 

C (s) + O₂ (g) → CO₂ (g) 

• Nitrogen dioxide
  • NO₂ is a toxic brown - orange gas and if dissolved in water it would give an acidic solution

2NO₂ (g) + H₂O (l) → 2HNO₃ (aq) + HNO₂ (aq)