Group 2, the alkaline earth metals
Group 2 reactions and trends
Down group 2, atomic radius increases, first ionisation energy decreases, so reactivity increases. The metals react with water: Mg slowly (fast with steam to MgO + H₂), Ca–Ba readily to give hydroxide + H₂, e.g. . Hydroxides become more soluble down the group (Mg(OH)₂ sparingly, Ba(OH)₂ soluble), so solutions become more alkaline. Sulfates become less soluble (BaSO₄ insoluble: used to test for sulfate and in barium meals). Uses: Ca(OH)₂ and CaCO₃ neutralise acid soils; Mg(OH)₂ in antacids.
≈ 45 minGroup 2: thermal stability and solubility
Group 2 carbonates decompose on heating to the oxide and CO₂; nitrates to the oxide, NO₂ and O₂. Thermal stability increases down the group: small Mg²⁺ has high charge density and polarises (distorts) the large carbonate/nitrate ion, weakening its bonds, so MgCO₃ decomposes most easily. Down the group, cations get larger, polarise less, and higher temperatures are needed. Solubility: hydroxides get more soluble down the group; sulfates get less soluble. Solubility depends on the balance between lattice energy and hydration enthalpies: for sulfates, hydration enthalpy of the cation falls faster than lattice energy as the cation grows, so dissolving becomes less favourable.
≈ 40 min