Redox — GCSE Chemistry practice questions

Question 1

Iron(III) sulfate is used to treat drinking water.

What is the formula of iron(III) sulfate?

Accepted Answer

D: Fe₂(SO₄)₃. Iron(III) ions are Fe³⁺ and sulfate ions are SO₄²⁻. Two Fe³⁺ (+6) balance three SO₄²⁻ (−6), so the formula is Fe₂(SO₄)₃.

Question 2

Nitrite ions, NO₂⁻, are added to some cured meats as a preservative.

What is the oxidation number of nitrogen in NO₂⁻?

Accepted Answer

B: +3. Each oxygen is −2, so the two oxygens total −4. The numbers must add up to the ion's charge, −1, so nitrogen is +3.

Question 3

Four reactions of sulfur dioxide are shown.

1. SO₂ + 2H₂S → 3S + 2H₂O
2. 2SO₂ + O₂ → 2SO₃
3. SO₂ + Cl₂ + 2H₂O → H₂SO₄ + 2HCl
4. SO₂ + CaO → CaSO₃

In which reactions does sulfur dioxide act as a reducing agent?

Accepted Answer

B: 2 and 3. Sulfur in SO₂ is +4. In reactions 2 and 3 it becomes +6 (in SO₃ and H₂SO₄), so sulfur dioxide is oxidised and acts as a reducing agent. In reaction 1 it is reduced to sulfur (0), so it is the oxidising agent; reaction 4 is not redox (sulfur stays +4 in CaSO₃).