Halide ions as reducing agents

· Halide ions, X⁻, act as reducing agents because they lose electrons and are oxidised.

· Reducing ability increases down Group 17: Cl⁻ < Br⁻ < I⁻.

· Down the group, ionic radius increases, so the outer electron is further from the nucleus and experiences more shielding.

· Therefore, the outer electron is lost more easily, so the ion is a stronger reducing agent.

· Exam phrase: “The halide ion reduces another species and is itself oxidised.”

The diagram summarises the trend in reducing ability from chloride to iodide ions. It helps students link the trend to increasing ionic radius, shielding and ease of electron loss. Source

Test with aqueous silver ions, Ag⁺(aq)

· Used to identify Cl⁻, Br⁻ and I⁻ in aqueous solution.

· First add dilute nitric acid, HNO₃(aq) to remove ions such as carbonate ions that could form misleading precipitates.

· Then add aqueous silver nitrate, AgNO₃(aq).

· General ionic equation: Ag⁺(aq) + X⁻(aq) → AgX(s).

· Cl⁻ gives white precipitate of AgCl(s).

· Br⁻ gives cream precipitate of AgBr(s).

· I⁻ gives yellow precipitate of AgI(s).

· Exam warning: AgBr and AgI colours can look similar, so use ammonia confirmation.

The image shows the different silver halide precipitates formed with silver nitrate. It is useful for memorising white AgCl, cream AgBr and yellow AgI. Source

Confirmation with aqueous ammonia, NH₃(aq)

· Add dilute ammonia first, then concentrated ammonia if needed.

· AgCl(s): dissolves in dilute NH₃(aq) to give a colourless solution.

· AgBr(s): insoluble in dilute NH₃(aq) but dissolves in concentrated NH₃(aq).

· AgI(s): insoluble in both dilute and concentrated NH₃(aq).

· Required complex formula is not needed for CIE 11.3.

· Quick memory: chloride dissolves easily, bromide dissolves only with concentrated ammonia, iodide does not dissolve.

This diagram shows the full test sequence for halide ions using acidified silver nitrate followed by ammonia. It clearly links precipitate colour and ammonia solubility to the identity of the halide ion. Source

Summary table: silver nitrate and ammonia tests

· Cl⁻: white AgCl precipitate; dissolves in dilute NH₃.

· Br⁻: cream AgBr precipitate; insoluble in dilute NH₃, dissolves in concentrated NH₃.

· I⁻: yellow AgI precipitate; insoluble in dilute and concentrated NH₃.

· Ionic equations:
· Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
· Ag⁺(aq) + Br⁻(aq) → AgBr(s)
· Ag⁺(aq) + I⁻(aq) → AgI(s)

Reaction with concentrated sulfuric acid: key idea

· Concentrated sulfuric acid can act as an acid and an oxidising agent.

· All halide ions first form a hydrogen halide, HX.

· Cl⁻ is too weak a reducing agent to reduce concentrated H₂SO₄.

· Br⁻ and I⁻ are strong enough reducing agents to reduce concentrated H₂SO₄.

· I⁻ is the strongest reducing agent, so it reduces sulfuric acid the furthest.

· Safety: reactions produce toxic gases, so they should be carried out in a fume cupboard.

Chloride ions with concentrated sulfuric acid

· NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g).

· Observation: steamy white fumes of HCl(g).

· No redox reaction occurs.

· Reason: Cl⁻ is not a strong enough reducing agent to reduce concentrated H₂SO₄.

Bromide ions with concentrated sulfuric acid

· First acid-base reaction:
· NaBr(s) + H₂SO₄(l) → NaHSO₄(s) + HBr(g).

· Then redox reaction:
· 2HBr(g) + H₂SO₄(l) → Br₂(g) + SO₂(g) + 2H₂O(l).

· Observations:
· Steamy fumes of HBr.
· Orange-brown fumes of Br₂.
· SO₂ gas formed; it is colourless and has a choking smell.

· Redox explanation:
· Br⁻ is oxidised to Br₂.
· Sulfur in H₂SO₄ is reduced from +6 in H₂SO₄ to +4 in SO₂.

Iodide ions with concentrated sulfuric acid

· First acid-base reaction:
· NaI(s) + H₂SO₄(l) → NaHSO₄(s) + HI(g).

· Then redox reactions:
· 2HI(g) + H₂SO₄(l) → I₂(g) + SO₂(g) + 2H₂O(l).
· 6HI(g) + H₂SO₄(l) → 3I₂(g) + S(s) + 4H₂O(l).
· 8HI(g) + H₂SO₄(l) → 4I₂(g) + H₂S(g) + 4H₂O(l).

· Observations:
· Purple/violet fumes of I₂.
· Dark grey/black solid iodine may form.
· Yellow solid sulfur may form.
· H₂S gas has a bad egg smell.

· Redox explanation:
· I⁻ is oxidised to I₂.
· Sulfur in H₂SO₄ is reduced from +6 to +4 in SO₂, 0 in S, or −2 in H₂S.

Exam comparison: Cl⁻ vs Br⁻ vs I⁻ with H₂SO₄

· Cl⁻: forms HCl only; no reduction of H₂SO₄.

· Br⁻: forms HBr, then reduces H₂SO₄ to SO₂; forms Br₂.

· I⁻: forms HI, then reduces H₂SO₄ to SO₂, S and H₂S; forms I₂.

· Trend: reducing power increases down the group.

· Best exam wording: “Iodide ions are oxidised more readily than bromide ions because the outer electron is less strongly attracted to the nucleus.”

Common exam mistakes

· Do not say halide ions are oxidising agents here; they are reducing agents.

· Do not forget to add dilute nitric acid before silver nitrate.

· Do not confuse precipitate colours: AgCl white, AgBr cream, AgI yellow.

· Do not say AgI dissolves in ammonia; it is insoluble in both dilute and concentrated ammonia.

· Do not omit balanced equations for reactions with concentrated H₂SO₄.