Properties of Period 3 Elements & their Oxides (AQA A Level Chemistry): Flashcards

True.

Direct combustion of sulfur gives SO₂ (g). SO₃ is not produced by burning sulfur directly — it is made industrially from SO₂ in the Contact process: 2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g), using a V₂O₅ catalyst.

All three are ionic oxides with giant ionic lattice structures. They are formed by reactions between metals (Na, Mg, Al) and the non-metal oxygen.

SiO₂ has a giant covalent structure (like diamond). Millions of strong covalent bonds must be broken to melt it, so it has a very high melting point.

False.

Direct combustion of sulfur in oxygen gives SO₂ (oxidation state +4), not SO₃ (+6). SO₃ is only produced via the Contact process. Na, Mg, Al, Si and P do react in oxygen to give oxides in their highest oxidation states.

A giant ionic lattice is a three-dimensional structure in which cations and anions are held together by strong electrostatic attractions in a regular repeating arrangement. It gives ionic compounds high melting points and boiling points.

Ionic oxides (Na₂O, MgO, Al₂O₃) have high melting points due to strong electrostatic attraction in giant ionic lattices. SiO₂ is also high (giant covalent). Simple molecular oxides (P₄O₁₀, SO₂, SO₃) have very low melting points due to weak intermolecular forces.

Mg²⁺ has a greater charge and a smaller ionic radius than Na⁺, giving it a higher charge density.

The electrostatic attraction between Mg²⁺ and O^2- is therefore stronger, resulting in a higher lattice energy and melting point.

True.

Both are simple molecular with weak intermolecular forces, but SO₃ molecules are slightly larger than SO₂ molecules, increasing the intermolecular attractions and raising the melting point slightly.

SiO₂ has a giant covalent structure (macromolecule). Millions of strong covalent bonds must be broken simultaneously to melt it, requiring a very large amount of energy.

False.

SO₂ is a gas at room temperature (bp −10 °C), but SO₃ is a liquid (bp ~45 °C). Both have simple molecular structures with weak intermolecular forces, giving low boiling points compared with the ionic or giant covalent oxides.

A weak acid is an acid that partially dissociates in aqueous solution, establishing an equilibrium between the undissociated acid and its ions.

Na₂O (s) + H₂O (l) → 2NaOH (aq)

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

P₄O₁₀ (s) + 6H₂O (l) → 4H₃PO₄ (aq)

False.

Neither Al₂O₃ nor SiO₂ reacts with water. Al₂O₃ is held too strongly in its ionic lattice and SiO₂ has millions of strong covalent bonds, making both insoluble in water.

An equilibrium constant (K_c) is the ratio of the product of the molar concentrations of the products to the product of the molar concentrations of the reactants at equilibrium, with each concentration raised to the power of its stoichiometric coefficient.

MgO (s) + 2HCl (aq) → MgCl₂ (aq) + H₂O (l)

Al₂O₃ (s) + 2NaOH (aq) + 3H₂O (l) → 2NaAl(OH)₄ (aq)

SO₂ (g) + 2NaOH (aq) → Na₂SO₃ (aq) + H₂O (l)

Na₂O is far more soluble in water than MgO, so it produces a much higher [OH⁻] in solution. The pH of NaOH (aq) formed is around 14, whereas Mg(OH)₂ solution only reaches around pH 10 due to its partial solubility.

True.

Metal oxides on the left (Na₂O, MgO) are basic; Al₂O₃ in the middle is amphoteric; non-metal oxides on the right (SiO₂, P₄O₁₀, SO₂, SO₃) are acidic.

H₂SO₄ (aq) → H⁺ (aq) + HSO₄^- (aq)

A one-way arrow is used because H₂SO₄ is a strong acid and fully dissociates in the first step. The second step (HSO₄^- ⇌ H⁺ + SO₄^2-) uses an equilibrium arrow as HSO₄^- is a weak acid.