2.3.2Inorganic Chemistry
Group 7(17) Redox Reactions
5 min read 5 parts Lesson 10 of 27
Section 1 of 5Halogen Displacement
Chlorine and bromide
- Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq); the solution turns orange.
Chlorine and iodide
- Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq); the solution turns brown, and a black solid may form.
Bromine and iodide
- Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq); the solution turns from orange to brown.
No reaction
- Iodine does not displace bromide or chloride, as it is a weaker oxidising agent.
| Added to | Hexane layer colour | Halogen present |
|---|---|---|
| Cl₂ | Pale green or colourless | Chlorine |
| Br₂ | Orange | Bromine |
| I₂ | Violet | Iodine |
Section 2 of 5Concentrated Sulfuric Acid
1
Solid sodium chloride reacts to give steamy fumes of hydrogen chloride: NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g).
2
This is not a redox reaction, because chloride ions are too weak a reducing agent to reduce the sulfuric acid.
3
Sodium bromide first gives HBr, but bromide is strong enough to reduce H₂SO₄ to SO₂ and itself form orange-brown bromine.
4
Sodium iodide reduces the acid much further, to SO₂, solid sulfur and hydrogen sulfide, and produces purple iodine vapour.
5
A rotten egg smell indicates H₂S, whereas a yellow solid is sulfur.
Section 3 of 5Equations and Oxidation States
Bromide
- 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O; bromine is oxidised from −1 to 0 and sulfur reduced from +6 to +4.
Iodide
- 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O; sulfur is reduced from +6 to −2, and iodine is oxidised from −1 to 0.
Product trend
- Iodide reduces sulfur to −2, bromide only to +4 and chloride not at all, showing increasing reducing power.
Reason
- The larger the ion, the further the outer electron is from the nucleus and the more easily it is lost.
Section 4 of 5Interpreting Observations
Interpreting Observations
Aqueous colours
- In water chlorine is pale green, bromine yellow-orange and iodine brown, but non-polar solvents give clearer colours.
Solvent test
- Shaking with hexane extracts the halogen, giving a distinct upper layer coloured orange for bromine and violet for iodine.
Oxidation numbers
- The halogen that is reduced gains electrons and goes from 0 to −1, and the halide displaced rises from −1 to 0.
Section 5 of 5Exam Checklist & Tips
2.3.2 – Key Points for the Exam
- Displacement reaction A reaction in which a more reactive element takes the place of a less reactive one in a compound.
- Oxidising agent A species that takes electrons from another species.
- Reducing agent A species that gives electrons to another species.
- Oxidation state A number showing the charge an atom would have if its bonds were fully ionic.
Common Mistakes
- Saying iodide ions reduce sulfuric acid less than chloride ions do.
- Describing sulfuric acid as the reducing agent when it is the oxidising agent.
- Forgetting to give the observations, such as steamy fumes, orange-brown vapour or purple vapour.
- Confusing the colours of halogens in water with those in hexane.