Flashcards · 9701 · A Level Topic 27

Group 2 (A Level) flashcards

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Why does the thermal stability of Group 2 carbonates and nitrates increase down the group?: The cation gets larger with the same 2+ charge, so its charge density falls. It polarises (distorts) the large anion less, weakening the C–O or N–O bonds less, so a higher temperature is needed to decompose it.
Thermal decomposition equations for Mg(NO₃)₂ and MgCO₃, with observations: 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂: brown gas; oxygen relights a glowing splint. MgCO₃ → MgO + CO₂: CO₂ turns limewater milky.
Energy cycle used to explain the solubility trends of Group 2 hydroxides and sulfates: ΔHsol = −ΔHlatt + ΔHhyd(M²⁺) + ΔHhyd(anions). Down the group both ΔHlatt and ΔHhyd(M²⁺) become less exothermic; the trend depends on which changes more.
Why does the solubility of Group 2 hydroxides increase down the group?: OH⁻ is small, so a larger cation increases the inter-ionic distance by a large fraction: lattice energy falls faster than the cation hydration enthalpy. ΔHsol becomes more exothermic, so solubility increases.
Why does the solubility of Group 2 sulfates decrease down the group?: SO₄²⁻ is large, so the inter-ionic distance changes by only a small fraction and the lattice energy changes little, while the cation hydration enthalpy still falls significantly. ΔHsol becomes less exothermic, so solubility decreases.