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) 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
- 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