1. Thermal Stability and Solubility of Group 2 Compounds
27.1
Thermal decomposition of carbonates and nitrates
Decomposition on heating
MCO3(s) → MO(s) + CO2(g)
2M(NO3)2(s) → 2MO(s) + 4NO2(g) + O2(g)
Nitrates give off brown fumes of NO2 and a gas (O2) that relights a glowing splint. Carbonates give off CO2, which turns limewater milky.
Down the group (Mg → Ba), a higher temperature is needed for decomposition: thermal stability increases.
Explanation
- All Group 2 cations have a 2+ charge, but the ionic radius increases down the group.
- So the charge density of the cation decreases down the group.
- A cation with a high charge density (Mg2+) polarises (distorts) the electron cloud of the large CO32− or NO3− anion more.
- This weakens the C–O (or N–O) bonds in the anion, so less energy is needed to break them and form the oxide.
- Larger cations (Ba2+) polarise the anion less, so more heat is needed.
Practice Question
Explain why magnesium carbonate decomposes at a lower temperature than barium carbonate.
Solubility and enthalpy change of solution
The enthalpy change of solution, ΔH⦵sol, is the enthalpy change when 1 mol of an ionic solid dissolves in enough water to give an infinitely dilute solution. Think of it in two stages: separate the lattice into gaseous ions (the reverse of the lattice energy, endothermic), then hydrate the ions (exothermic).
Energy cycle
ΔH⦵sol = ΔH⦵hyd(cation) + ΔH⦵hyd(anion) − ΔH⦵latt
The more exothermic (or the less endothermic) ΔH⦵sol is, the more soluble the compound tends to be.
Down the group, as the cation gets larger, both the lattice energy and the hydration enthalpy of the cation become less exothermic. Which one falls faster decides the trend.
| Hydroxides, M(OH)2 | Sulfates, MSO4 |
|---|
| Trend down the group | Solubility increases (Mg(OH)2 sparingly soluble, Ba(OH)2 fairly soluble) | Solubility decreases (MgSO4 soluble, BaSO4 insoluble) |
| Lattice energy | OH− is small, so the inter-ionic distance depends strongly on the cation. Lattice energy becomes much less exothermic as the cation grows. | SO42− is large, so the inter-ionic distance hardly changes. Lattice energy changes only a little. |
| Hydration enthalpy of cation | Becomes less exothermic, but by less than the lattice energy. | Becomes less exothermic, by more than the lattice energy. |
| Result for ΔH⦵sol |
Becomes more exothermic → more soluble |