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

3.2.2.1 Group 2, the alkaline earth metals

Trends down Group 2 (Mg → Ba): atomic radius and first ionisation energy

Atomic radius increases (an extra occupied shell each time). First ionisation energy decreases: the outer electron is further out and better shielded, outweighing the greater nuclear charge.

Trend in melting point down Group 2

Generally decreases (from Ca to Ba): the 2+ ions get larger, so the attraction between ions and delocalised electrons weakens. Magnesium is lower than calcium and does not fit the trend.

Magnesium with cold water vs with steam

Cold water: Mg + 2H₂O → Mg(OH)₂ + H₂ (very slow). Steam: Mg + H₂O → MgO + H₂ (bright white flame, white powder).

Trend in reactivity of Group 2 metals with water

More vigorous down the group, because the outer electrons are lost more easily. For example Ca + 2H₂O → Ca(OH)₂ + H₂.

Extraction of titanium using magnesium

TiCl₄ + 2Mg → Ti + 2MgCl₂ Ti is reduced (+4 → 0); Mg is the reducing agent (0 → +2).

Solubility trends of Group 2 hydroxides and sulfates

Hydroxides become more soluble down the group (Mg(OH)₂ is sparingly soluble). Sulfates become less soluble (BaSO₄ is insoluble).

Uses of Mg(OH)₂, Ca(OH)₂ and BaSO₄

Mg(OH)₂: antacid (milk of magnesia). Ca(OH)₂: raising the pH of acidic soil. BaSO₄: barium meal for X-rays; it is insoluble, so toxic Ba²⁺ is not absorbed.

How are CaO and CaCO₃ used to remove SO₂ from flue gases?

They are bases that react with the acidic gas: CaO + SO₂ → CaSO₃ CaCO₃ + SO₂ → CaSO₃ + CO₂

Test for sulfate ions, and why the solution is acidified

Add acidified BaCl₂(aq): white precipitate of BaSO₄. The acid removes carbonate ions, which would also give a white precipitate (BaCO₃). Do not use H₂SO₄: it adds sulfate.