Nitrates & Carbonates (Edexcel International AS Chemistry)

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

Thermal decomposition of carbonates

• In Group 1, lithium carbonate when heated will decompose producing lithium oxide and carbon dioxide

Li₂CO₃ (s)   Li₂O (s)  +  CO₂ (g)

• The rest of the Group 1 carbonates don't decompose at Bunsen temperatures
  • The decomposition temperatures increase as you go down the Group
• 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:

XCO₃ (s)  XO (s) + CO₂ (g)

X = Group 2 element

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

MgCO₃  MgO + CO₂

Thermal decomposition of nitrates

• The only Group 1 nitrate that will decompose to produce nitrogen dioxide (which is a brown toxic gas) and oxygen is lithium nitrate LiNO₃

4LiNO₃ (s)  2Li₂O (s) + 4NO₂ (g) + O₂ (g)

• The rest of the Group don't decompose so completely producing the metal nitrite (NO₂^-) and oxygen, but no nitrogen dioxide

2XNO₃ (s)  2XNO₂ (s) + O₂ (g)

• All the nitrates from sodium to caesium decompose in this same way, the only difference being how hot they have to be to undergo the reaction. 
• Down Group 1, the decomposition gets more difficult, and you have to use higher temperatures.
• The Group 2 nitrates break decomposed when they are heated to form the metal oxide, oxygen gas and nitrogen dioxide gas 
• Since the formed nitrogen dioxide gas is toxic, the decomposition of nitrates is often carried out in a fume cupboard

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

Mg(NO₃)₂   MgO + O₂ + 2NO₂

X = Group 2 element

• Going down Groups 1 and 2, more heat is needed to break down the carbonate and nitrate ions
• The thermal stability of the Group 1 and 2 carbonates and nitrates therefore increases down the group 
  • The smaller positive ions at the top of the groups will polarise the anions more than the larger ions at the bottom of the group
    • The small positive ion attracts the delocalised electrons in the nitrate or carbonate ion towards itself
    • The higher the charge and the smaller the ion the higher the polarising power
  • The more polarised they are, the more likely they are to thermally decompose as the bonds in the carbonate and nitrate ions become weaker 

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)