Flashcards · 7405 · AS Topic 14

Group 2, the alkaline earth metals (AS Level) flashcards

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Trends down Group 2 (Mg to Ba): atomic radius and first ionisation energy: Atomic radius increases (one more shell each time). First ionisation energy decreases: the outer electron is further away and more shielded.
Why does the melting point generally decrease from Ca to Ba?: Each ion is 2+, but the ions get larger, so the attraction between the ions and the delocalised electrons is weaker.
Magnesium with cold water and with steam: Cold water: very slow, Mg + 2H₂O → Mg(OH)₂ + H₂. Steam: burns with a bright white light, Mg + H₂O → MgO + H₂.
Solubility trends of Group 2 hydroxides and sulfates: Hydroxides: solubility increases down the group (Mg(OH)₂ sparingly soluble). Sulfates: solubility decreases down the group (BaSO₄ insoluble).
Test for sulfate ions, and why the acid is added: Add dilute HCl (or HNO₃), then BaCl₂(aq): a white precipitate of BaSO₄. The acid removes carbonate (and sulfite) ions, which would also give a white precipitate. Never use H₂SO₄.
Role of magnesium in titanium extraction: TiCl₄ + 2Mg → Ti + 2MgCl₂. Magnesium is oxidised, so it is the reducing agent.
Uses of Mg(OH)₂, Ca(OH)₂, CaO/CaCO₃ and BaSO₄: Mg(OH)₂: antacid. Ca(OH)₂: raises the pH of acidic soil. CaO or CaCO₃: removes SO₂ from flue gases. BaSO₄: barium meal for X-rays (insoluble, so toxic Ba²⁺ is not absorbed).