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Thermal Decomposition of Group 2 Nitrates & Carbonates (CIE AS Chemistry)

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Thermal Decomposition of Group 2 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:

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 (s)rightwards arrow with HEAT on top MgO (s) + CO2 (s)

Thermal decomposition of nitrates

  • Group 2 nitrates also undergo thermal decomposition 
  • Group 2 nitrates decompose to form the metal oxide, nitrogen dioxide gas and oxygen gas
  • The general equation for the decomposition of Group 2 nitrates is:

X(NO3)2 (s) rightwards arrow with HEAT on top   XO (s)  +   2NO2 (g)  +   ½O2 (g)

OR

 2X(NO3)2 (s) rightwards arrow with HEAT on top  2XO (s)  +  4NO2 (g)  +   O2 (g)

X = Group 2 element

  • Nitrogen dioxide gas is observed as brown fumes and is toxic
  • An example of this reaction is:

 2Ca(NO3)2 (s) rightwards arrow with HEAT on top 2CaO (s)  +  4NO2 (g)  +   O2 (g)

Trend in thermal stabilities 

  • Going down Group 2, more heat is needed to break down the carbonate and nitrate ions
  • The thermal stability of the Group 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 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 

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.