Thermal decomposition and solubility
Thermal decomposition and solubility
- Thermal decomposition breaks a compound apart by heating.
- Its ease changes in a clear trend down Group 2.
- So do the solubilities of the hydroxides and sulfates.
Decomposing carbonates and nitrates
- carbonates → oxide + carbon dioxide: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$.
- nitrates → oxide + brown $\text{NO}_2$ + oxygen: $2\text{Ca(NO}_3)_2 \rightarrow 2\text{CaO} + 4\text{NO}_2 + \text{O}_2$.
Practice
Heating a Group 2 carbonate produces:
Thermal decomposition: CaCO₃ → CaO + CO₂.
Practice
Heating a Group 2 nitrate produces:
e.g. 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂ (brown NO₂ gas is given off).
Thermal stability rises down the group
- Both carbonates and nitrates get harder to decompose down the group.
- A larger metal ion pulls less on (polarises less) the carbonate/nitrate ion, so it is harder to break apart.
- So MgCO₃ decomposes most easily; BaCO₃ is the hardest.
Practice
Thermal stability of the carbonates increases down Group 2 because:
A big cation distorts the carbonate ion less, so more energy (a higher temperature) is needed to decompose it.
Solubility trends
| Compound | Down the group |
|---|---|
| hydroxides | solubility increases (Mg(OH)₂ almost insoluble; Ba(OH)₂ soluble) |
| sulfates | solubility decreases (MgSO₄ soluble; BaSO₄ insoluble) |
Practice
Down Group 2:
Hydroxides become more soluble (Ba(OH)₂ soluble); sulfates become less soluble (BaSO₄ insoluble).
You've got it
Key idea
- carbonates → oxide + CO₂; nitrates → oxide + NO₂ + O₂ (on heating)
- thermal stability increases down the group (larger ion polarises the anion less)
- down the group: hydroxides more soluble, sulfates less soluble (BaSO₄ insoluble)