Explain why the pH of an aqueous solution formed when a Period 3 oxide dissolves in water changes across the period from sodium to sulfur, referring to the oxides of sodium, magnesium, and sulfur in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Sodium oxide (Na₂O) is an ionic oxide; the O²⁻ ion reacts with water:
Na₂O + H₂O → 2Na⁺ + 2OH⁻
The solution is alkaline (pH > 7).
2. Magnesium oxide (MgO) is also ionic; the same reaction occurs:
MgO + H₂O → Mg²⁺ + 2OH⁻
Again the solution is alkaline.
3. As we move across Period 3 the metal–oxygen bond becomes increasingly covalent; the oxides are no longer purely ionic.
4. Sulfur dioxide (SO₂) and sulphur trioxide (SO₃) are covalent oxides; they react with water to give acidic species:
SO₂ + H₂O → H₂SO₃ → 2H⁺ + SO₃²⁻
SO₃ + H₂O → H₂SO₄ → 2H⁺ + SO₄²⁻
The solutions are acidic (pH < 7).
5. Consequently, the pH of the aqueous solution decreases from above 7 for Na₂O and MgO to below 7 for SO₂/SO₃ as we move from sodium to sulfur across Period 3.
Na₂O + H₂O → 2Na⁺ + 2OH⁻
The solution is alkaline (pH > 7).
2. Magnesium oxide (MgO) is also ionic; the same reaction occurs:
MgO + H₂O → Mg²⁺ + 2OH⁻
Again the solution is alkaline.
3. As we move across Period 3 the metal–oxygen bond becomes increasingly covalent; the oxides are no longer purely ionic.
4. Sulfur dioxide (SO₂) and sulphur trioxide (SO₃) are covalent oxides; they react with water to give acidic species:
SO₂ + H₂O → H₂SO₃ → 2H⁺ + SO₃²⁻
SO₃ + H₂O → H₂SO₄ → 2H⁺ + SO₄²⁻
The solutions are acidic (pH < 7).
5. Consequently, the pH of the aqueous solution decreases from above 7 for Na₂O and MgO to below 7 for SO₂/SO₃ as we move from sodium to sulfur across Period 3.
Examiner tips
- Use the command word ‘explain’ – show cause and effect. Mention ionic vs covalent bonding. Give the balanced reactions for each oxide. State the resulting pH trend clearly.
Common mistakes
- Confusing Na₂O with Na₂O₂ or writing the wrong reaction. Forgetting that SO₂ and SO₃ are acidic oxides. Not linking the change in bonding type to the pH change.
Mark scheme (5 marks)
- Sodium oxide (or magnesium oxide) is a basic/alkaline oxide that reacts with water to produce OH⁻ ions (or an alkaline solution with pH above 7)
- Sodium and magnesium oxides are ionic compounds / contain metal–oxygen ionic bonds, so the oxide ion (O²⁻) reacts with water to form OH⁻
- Across the period the bonding in the oxides changes from ionic to covalent (as the elements become non-metals)
- Sulfur oxide (SO₂ / SO₃) is an acidic oxide that reacts with water to produce H⁺ ions / an acidic solution with pH below 7
- Therefore the pH of the solution decreases (from above 7 to below 7) as you move from sodium to sulfur across Period 3
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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