Properties of Period 3 elements and their oxides and chlorides (A Level) flashcards

3.2.4.1 Properties of Period 3 elements and their oxides and chlorides

Basic oxide

Neutralises an acid, giving a salt plus water (Na₂O, MgO).

Acidic oxide

Neutralises a base, giving a salt plus water (SiO₂, P₄O₁₀, SO₂, SO₃).

Amphoteric (oxide)

Reacts with acids and with bases. Al₂O₃.

Equation for aluminium reacting with chlorine, and why the product is a dimer

2Al + 3Cl₂ → Al₂Cl₆ Al in AlCl₃ has only six outer electrons, so a Cl lone pair from another unit forms a co-ordinate bond; two such bonds make the dimer.

pH of the solutions formed by Na₂O, MgO, P₄O₁₀ and SO₂ with water

Na₂O: 14 (NaOH). MgO: about 9 (Mg(OH)₂ sparingly soluble). P₄O₁₀: about 0 (H₃PO₄). SO₂: about 3 (H₂SO₃, a weak acid).

Why do Al₂O₃ and SiO₂ give pH 7 with water?

They are insoluble (giant structures). Al₂O₃ is still amphoteric and SiO₂ is still acidic: they react with bases (and Al₂O₃ with acids).

Ionic vs covalent Period 3 chlorides with water

NaCl and MgCl₂ dissolve (pH 7 and about 6). SiCl₄ and PCl₅ are hydrolysed, giving steamy fumes of HCl and pH about 1: SiCl₄ + 2H₂O → SiO₂ + 4HCl PCl₅ + 4H₂O → H₃PO₄ + 5HCl

Why is aluminium chloride solution acidic (about pH 3)?

The small, highly charged Al³⁺ ion polarises the O–H bonds of its water ligands: [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅(OH)]²⁺ + H⁺

Why does MgO have a much higher melting point than P₄O₁₀ or SO₂?

MgO is giant ionic, so melting overcomes strong ionic attraction (2+ and 2− ions). P₄O₁₀ and SO₂ are simple molecular: only weak van der Waals forces are overcome.