Thermal Decomposition of Nitrates & Carbonates (CIE AS Chemistry)

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)  XO (s) + CO₂ (g)

X = Group 2 element

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

MgCO₃ (s) 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)    XO (s)  +   2NO₂ (g)  +   ½O₂ (g)

OR

 2X(NO₃)₂ (s)   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) 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