11.3 Some reactions of the halide ions
- Syllabus
- 9701–2028–2029
- Topic
- 11.3
- Level
- AS
Learning objectives
11.3.1Reactivity of halide ions as reducing agents• Describe the relative reactivity of halide ions as reducing agents11.3.2Reactions of halide ions• Describe/explain: the reactions of halide ions with:: (a) aq. silver ions followed by aq. ammonia (the formation and formula of the [Ag(NH3)2]+ complex is not required); (b) conc. sulfuric acid, to include balanced chemical equations
Halide reducing power increases down Group 17
A halide ion acts as a reducing agent when it donates an electron and is oxidised: 2X⁻ → X₂ + 2e⁻. Relative reducing power increases F⁻ < Cl⁻ < Br⁻ < I⁻.
| Halide | Ease of oxidation | Evidence with concentrated H₂SO₄ |
|---|---|---|
| F⁻ | hardest | acid-base reaction only; no reduction of sulfuric acid |
| Cl⁻ | very difficult | acid-base reaction only; no reduction of sulfuric acid |
| Br⁻ | easier | Br₂ forms while H₂SO₄ is reduced mainly to SO₂ |
| I⁻ | easiest | I₂ forms while H₂SO₄ can be reduced to SO₂, S and H₂S |
Down the group, ionic radius and shielding increase. The outer electron is farther from the nucleus and less strongly attracted, so it is lost more readily and the ion is a stronger reducing agent.
This is the opposite of halogen oxidising power: X₂ gains electrons, whereas X⁻ loses electrons. Always identify the starting species before choosing the trend.
Silver ions and concentrated sulfuric acid distinguish halides
For an aqueous sample, acidify with dilute nitric acid, add aqueous silver nitrate, then test any precipitate first with dilute aqueous ammonia and, if needed, concentrated aqueous ammonia.
Ag+(aq)+X−(aq)→AgX(s)
| Halide | Silver-ion result | Dilute NH₃ | Concentrated NH₃ |
|---|---|---|---|
| F⁻ | no precipitate because AgF is soluble | — | — |
| Cl⁻ | white AgCl precipitate | dissolves | dissolves |
| Br⁻ | cream AgBr precipitate | does not dissolve | dissolves |
| I⁻ | yellow AgI precipitate | does not dissolve | does not dissolve |
With a solid sodium halide, concentrated sulfuric acid first acts as an acid: NaX(s) + H₂SO₄(l) → NaHSO₄(s) + HX(g). NaF and NaCl stop at this acid-base stage, giving steamy acidic HF or HCl fumes; the sulfur oxidation number remains +6.
| Reducing halide product | Balanced further reaction with concentrated H₂SO₄ | Sulfur product / observation |
|---|---|---|
| HBr | 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O | red-brown Br₂ and colourless choking SO₂; S: +6 → +4 |
| HI | 2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O | purple I₂ vapour / dark iodine and SO₂ |
| HI | 6HI + H₂SO₄ → 3I₂ + S + 4H₂O | yellow sulfur; S: +6 → 0 |
| HI | 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O | H₂S with characteristic rotten-egg odour; S: +6 → −2 |
Br⁻ and I⁻ are oxidised from −1 to 0 in Br₂ or I₂. I⁻ is the stronger reducing agent, so it can reduce sulfuric acid through more oxidation-number steps than Br⁻.
Nitric acid is used before Ag⁺ because chloride-containing acids would create AgCl. The syllabus does not require the formula or formation equation of the soluble silver-ammonia complex, so identify dissolution by observation only. Toxic gases must be handled in a fume cupboard and never smelled directly.