Thermal Decomposition of Group 2 Nitrates & Carbonates (Cambridge International A-Level Chemistry)

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)$\overset{\mathrm{HEAT}}{\rightarrow}$XO (s) + CO₂(g)

X = Group 2 element

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

MgCO₃(s)$\overset{\mathrm{HEAT}}{\rightarrow}$MgO (s) + CO₂(g)

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)$\overset{\mathrm{HEAT}}{\rightarrow}$XO (s) + 2NO₂(g) + ½O₂(g)

OR

2X(NO₃)₂ (s)$\overset{\mathrm{HEAT}}{\rightarrow}$2XO (s) + 4NO₂(g) + O₂(g)

X = Group 2 element

• Nitrogen dioxide gas is observed as brown fumes and is toxic

  • This is often seen as brown fumes when heating Group 2 nitrates in test tube reactions

• An example of this reaction is:

2Ca(NO₃)₂(s)$\overset{\mathrm{HEAT}}{\rightarrow}$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