Similarities and Trends in the Properties of the Group 2 Metals, Magnesium to Barium, and Their Compounds
Specification Reference Inorganic Chemistry, Group 2 27.1
Quick Notes
- Thermal stability of Group 2 nitrates and carbonates increases down the group.
- Smaller Group 2 ions (e.g. Mg2+) polarise large nitrate and carbonate anions more, weakening their bonds and making decomposition easier.
- Polarisation decreases down the group as ionic radius increases.
- Solubility of hydroxides increases down the group
(Mg(OH)2 sparingly soluble, Ba(OH)2 highly soluble). - Solubility of sulfates decreases down the group
(MgSO4 soluble, BaSO4 insoluble). - Solubility depends on ΔH°sol, which is a balance between lattice energy and hydration enthalpy.
Full Notes
Thermal Stability of Group 2 Nitrates and Carbonates
Group 2 nitrates (M(NO3)2) and carbonates (MCO3) decompose on heating.
The ease with which they decompose depends on the polarising power of the metal cation.
Thermal stability increases down the group from Mg to Ba.
Smaller cations (like Mg2+) have a higher charge density, which polarises the large nitrate (NO3−) or carbonate (CO32−) anion more.
- This distorts the anion’s electron cloud, weakening its bonds and making decomposition easier.
- As you go down the group, the cations get larger and their polarising ability decreases, meaning the anions remain more stable and require more heat to decompose.
Thermal decomposition reactions:
- Carbonates:
MCO3 (s) → MO (s) + CO2 (g) - Nitrates:
2M(NO3)2 (s) → 2MO (s) + 4NO2 (g) + O2 (g)
Solubility and Enthalpy of Solution (ΔH°sol)
Solubility of Group 2 hydroxides and sulfates shows opposite trends:
Hydroxide solubility increases down the group.
Sulfate solubility decreases down the group.
Explanation:
Solubility depends on the enthalpy change of solution (ΔH°sol), which is influenced by:
- The lattice energy (energy required to break apart the solid).
- The hydration enthalpy (energy released when ions are surrounded by water molecules).
Hydroxides:
- The lattice energy becomes less negative (weaker ionic lattice) down the group.
- The hydration enthalpy decreases too, but not as much as the lattice energy.
- Therefore, ΔH°sol becomes more negative and solubility increases.
Sulfates:
- Lattice energy again becomes less negative.
- But hydration enthalpy decreases more sharply due to the large SO42− ion.
- Therefore, ΔH°sol becomes less negative and solubility decreases.
Summary
- Thermal stability of nitrates and carbonates increases down Group 2 due to decreasing polarisation by larger metal ions.
- Hydroxide solubility increases, sulfate solubility decreases due to changing balances between lattice and hydration enthalpies.
- These trends illustrate the importance of ionic size and energy changes in predicting chemical behaviour.