AQA A-Level Chemistry Paper 1, 2023: Question 3

✅ Correct Equation

P₄ + 5 O₂ → P₄O₁₀

Also allowed: 4 P + 5 O₂ → P₄O₁₀

💡 Key Knowledge

White phosphorus exists as tetrahedral P₄ molecules. When heated with an excess of oxygen, it burns vigorously to form phosphorus(V) oxide, P₄O₁₀ (oxidation state of P is +5).

🧠 Exam Technique

Look out for the keyword "excess". If oxygen were limited, phosphorus(III) oxide (P₄O₆) would form. "Excess" signifies complete oxidation to P₄O₁₀.

❌ Common Errors

• Writing P₂O₅ as the molecular formula (this is only the empirical formula, though P₄O₁₀ is the molecular species required).
• Unbalanced equations such as forgetting the factor of 5 in front of O₂.

✅ Correct Equation

SO₂ + H₂O → H₂SO₃

Acceptable ionic forms:

• SO₂ + H₂O → 2 H⁺ + SO₃²⁻
• SO₂ + H₂O → H⁺ + HSO₃⁻

💡 Key Knowledge

Sulfur dioxide (oxidation state +4) reacts reversibly with water to produce sulfuric(IV) acid (sulfurous acid, H₂SO₃), which is a weak acid (pH ≈ 2–3).

❌ Common Errors

• Writing H₂SO₄: Sulfur dioxide forms H₂SO₃, not H₂SO₄. Sulfur trioxide (SO₃) is needed to form H₂SO₄.
• Confusing oxidation states: SO₂ has sulfur in +4, so it cannot yield sulfuric(VI) acid (+6) without an oxidizing agent.

✅ Correct Structure

When SO₃ dissolves in water, it forms sulfuric acid: SO₃ + H₂O → H₂SO₄ . The anion formed is the sulfate ion, SO₄²⁻ (or hydrogensulfate, HSO₄⁻).

Hydrogensulfate (HSO₄⁻) is also allowed:

💡 Key Knowledge

A displayed formula must show every bond explicitly as a line:

• The central sulfur atom forms two double bonds to oxygen ( S=O ) and two single bonds to negatively charged oxygen atoms ( S—O⁻ ).
• Sulfur expands its octet to accommodate 12 valence electrons.

🧠 Exam Technique

• Remember to include the negative charges ( ⁻ ) clearly on the single-bonded oxygens.
• Read the prompt carefully: it asks for the anion (negatively charged ion), not the neutral H₂SO₄ molecule!

❌ Common Errors

• Drawing neutral sulfuric acid H₂SO₄ instead of the anion.
• Drawing the sulfite ion SO₃²⁻ from SO₂ instead of SO₃.
• Omitting the negative charges on oxygen atoms.

✅ Correct Answers

Equation:
Mg + H₂O → MgO + H₂

Observation (any one):

• White solid / white powder / white ash
• Bright white light / white flame

💡 Steam vs Cold Water

• With steam (g): Vigorous reaction yielding magnesium oxide (solid) and hydrogen gas:
  Mg(s) + H₂O(g) → MgO(s) + H₂(g)
• With cold water (l): Very slow reaction yielding magnesium hydroxide:
  Mg(s) + 2 H₂O(l) → Mg(OH)₂(aq/s) + H₂(g)

❌ Common Examiner Traps

• Forming Mg(OH)₂: Writing Mg + 2 H₂O → Mg(OH)₂ + H₂ is completely wrong for steam. Mg(OH)₂ decomposes thermally at high temperatures to MgO.
• Rejecting "White precipitate": The product is formed on a solid ribbon in a tube of gas/steam, NOT in an aqueous solution. Precipitates only form in solution.
• Rejecting "Effervescence / bubbling": Hydrogen gas burns or escapes rapidly, but bubbles are only seen in liquid water, not steam.

✅ Correct Equation

3 MgO + 2 H₃PO₄ → Mg₃(PO₄)₂ + 3 H₂O

Or ionic:
3 MgO + 2 H₃PO₄ → 3 Mg²⁺ + 2 PO₄³⁻ + 3 H₂O

📐 Balancing Strategy

1. Identify ion charges:
   Magnesium ion: Mg²⁺
   Phosphate ion: PO₄³⁻
2. Deduce salt formula:
   To balance charges (+6 / -6): Mg₃(PO₄)₂
3. Balance elements:
   Need 3 Mg → 3 MgO
   Need 2 PO₄ → 2 H₃PO₄
   Water produced: 3 O atoms from MgO + 6 H atoms from acid → 3 H₂O

💡 Key Knowledge

Metal Oxide + Acid → Salt + Water

MgO contains basic oxide ions ( O²⁻ ) which act as Brønsted-Lowry bases, accepting protons from phosphoric acid ( H₃PO₄ ) to form neutral water.

❌ Common Errors

• Writing incorrect formulae for magnesium phosphate, such as MgPO₄ or Mg(PO₄)₂ .
• Forgetting that phosphoric acid is triprotic ( H₃PO₄ ), leading to unbalanced hydrogen and oxygen counts.