Edexcel International A Level Chemistry

Revision Notes

2.4.5 Nitrates & Carbonates

Thermal Stability

  • 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) rightwards arrow with HEAT on top  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) rightwards arrow with HEAT on top XO (s) + CO2 (g)

X = Group 2 element

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

MgCO3 rightwards arrow with HEAT on top 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) rightwards arrow with HEAT on top 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) rightwards arrow with HEAT on top 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  rightwards arrow with HEAT on top MgO + 1 halfO2 + 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)(aq) + CO2 (g) → CaCO3 (s) + H2O (l)

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

C (s) + O2 (g) → CO(g) 

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

2NO(g) + H2O (l) → 2HNO3 (aq) + HNO2 (aq) 

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.