9701 Chemistry · Topic 11 · AS Level
Group 17 Halogens Cheat Sheet — A Level Chemistry 9701
Group 17 questions almost always turn on one idea: oxidising power falls down the group while reducing power of the halides rises. This sheet lays out the colours and states you are expected to recall, the displacement reactions and their observations, the silver nitrate and concentrated sulfuric acid halide tests, and the disproportionation of chlorine in cold and hot alkali.
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What’s on this cheat sheet
Group 17 — The Halogens
01 · Appearance and volatility
| Halogen | State | Colour |
|---|---|---|
| Cl₂ | gas | pale green |
| Br₂ | liquid | red‑brown |
| I₂ | solid | grey‑black, purple vapour |
All are diatomic. Boiling point rises down the group: more electrons per molecule means stronger London forces, so volatility falls.
02 · Oxidising power
Decreases down the group. The atom gets larger and more shielded, so it attracts an electron less strongly and is reduced less readily.
Electronegativity and electron affinity both fall down the group, and E° values follow: F₂ +2.87, Cl₂ +1.36, Br₂ +1.07, I₂ +0.54 V.
03 · Displacement reactions
A halogen displaces the halide of any halogen below it.
| Added to | Cl⁻ | Br⁻ | I⁻ |
|---|---|---|---|
| Cl₂ | — | orange | brown |
| Br₂ | none | — | brown |
| I₂ | none | none | — |
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Shake with an organic solvent to confirm: Br₂ orange, I₂ violet in the hydrocarbon layer.
04 · Halide ions as reducing agents
Reducing power increases down the group — the larger ion loses its outer electron more easily. This is the reverse of the halogens’ oxidising trend.
Shown by the reaction with concentrated H₂SO₄, where the halide reduces the sulfur to progressively lower oxidation states.
05 · Halides with concentrated H₂SO₄
| Halide | Products | Seen |
|---|---|---|
| Cl⁻ | HCl only | steamy fumes |
| Br⁻ | HBr + Br₂ + SO₂ | red‑brown fumes |
| I⁻ | HI + I₂ + H₂S | purple vapour, bad‑egg smell |
Sulfur is reduced from +6 to +4 (SO₂) by Br⁻ and as far as −2 (H₂S) by I⁻, proving iodide is the strongest reducing agent.
06 · Testing for halide ions
Acidify with dilute HNO₃ (removes carbonate), then add AgNO₃(aq):
| Ion | Precipitate | In NH₃(aq) |
|---|---|---|
| Cl⁻ | white | dissolves in dilute |
| Br⁻ | cream | dissolves in concentrated |
| I⁻ | pale yellow | insoluble |
Ag⁺ + X⁻ → AgX(s). Solubility in ammonia falls down the group, which is what separates the three.
07 · Chlorine with water
Chlorine disproportionates: 0 → −1 and +1.
HClO is the bactericide used to treat water; the risk of chlorine’s toxicity is judged against the far larger benefit of killing waterborne pathogens.
08 · Chlorine with alkali
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
0 → −1 and +1 · the bleach reaction
Hot concentrated NaOH
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
0 → −1 and +5
09 · Hydrogen halides
All are colourless gases that fume in moist air and dissolve to give acidic solutions.
Thermal stability decreases down the group — the H–X bond gets longer and weaker, so HI decomposes to iodine and hydrogen on gentle heating while HCl does not.
10 · Worked example — identify the halide
Cream + concentrated NH₃ → Br⁻
Red‑brown fumes = Br₂, choking gas = SO₂
2Br⁻ + 2H₂SO₄ → Br₂ + SO₂ + 2H₂O + SO₄²⁻
11 · Uses
Chlorine — water treatment, bleach, PVC and solvents. Bromine — flame retardants and silver bromide in photography. Iodine — antiseptic and dietary supplement.
Chlorofluorocarbons were once used as refrigerants and propellants but are now banned because their chlorine radicals catalyse ozone depletion.
12 · Equations to know
Cl₂ + 2I⁻ → 2Cl⁻ + I₂
Br₂ + 2I⁻ → 2Br⁻ + I₂
Ag⁺ + Cl⁻ → AgCl(s)
AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻
Cl₂ + H₂O ⇌ HCl + HClO
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
13 · Worked example — displacement
The aqueous layer turns brown as iodine forms; the hexane layer above turns violet because iodine is more soluble in the non‑polar solvent.
Cl₂ + 2I⁻ → 2Cl⁻ + I₂
Chlorine is the stronger oxidising agent, so it takes electrons from iodide.
Trends in one line
Down Group 17: radius ↑, electronegativity ↓, boiling point ↑, oxidising power of X₂ ↓, reducing power of X⁻ ↑, stability of HX ↓.
Two ideas do all the explaining: increasing atomic size with more shielding, and increasing numbers of electrons for London forces.
Marks lost here
— Reversing the two trends: X₂ gets less oxidising down the group while X⁻ gets more reducing.
— Acidifying the halide test with HCl, which adds chloride of its own.
— Missing the change in oxidation numbers when asked to prove disproportionation.
— Explaining boiling points by “stronger covalent bonds”; it is London forces between molecules.
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Group 17 Halogens — Frequently Asked Questions
Why does oxidising power decrease down Group 17?
Atomic radius increases and shielding increases down the group, so the nucleus attracts an incoming electron less strongly. The halogen is therefore less able to gain an electron and is a weaker oxidising agent.
What are the silver nitrate test results for halides?
Chloride gives a white precipitate that dissolves in dilute ammonia; bromide gives a cream precipitate that dissolves only in concentrated ammonia; iodide gives a pale yellow precipitate insoluble in concentrated ammonia.
What happens when chlorine reacts with cold and with hot alkali?
In cold dilute alkali chlorine disproportionates to chloride (−1) and chlorate(I) (+1). In hot concentrated alkali it disproportionates to chloride (−1) and chlorate(V) (+5). In both cases chlorine starts at 0 and ends in two different oxidation states.
