1. Physical Trends Down Group 7
The halogens exist as diatomic covalent non-metal molecules (\(\text{X}_2\)):
- Fluorine (\(\text{F}_2\)): Pale yellow gas. Most reactive and most electronegative element.
- Chlorine (\(\text{Cl}_2\)): Pale green gas with a pungent, choking odour.
- Bromine (\(\text{Br}_2\)): Dense red-brown liquid giving off dense orange-brown vapour.
- Iodine (\(\text{I}_2\)): Shiny grey-black crystalline solid that sublimes on gentle warming to form a purple vapour.
Boiling points increase down the group: As molecular size increases, the electron count rises (\(18 \rightarrow 34 \rightarrow 70 \rightarrow 106\)). Larger electron clouds produce stronger London dispersion forces between molecules, requiring more thermal energy to separate them.
2. Oxidising Ability and Displacement Reactions
An electron acceptor. Halogen atoms gain electrons to form 1- halide ions: \\[ \text{X}_2 + 2e^- \rightarrow 2\text{X}^- \\]
Oxidising ability decreases down Group 7: \(\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2\). As atomic radius and electron shielding increase, incoming electrons experience weaker electrostatic attraction to the halogen nucleus.
Displacement Reactions
A more reactive halogen will displace a less reactive halide ion from aqueous solution:
- \(\text{Cl}_2(aq) + 2\text{Br}^-(aq) \rightarrow 2\text{Cl}^-(aq) + \text{Br}_2(aq)\) (Solution turns yellow-orange).
- \(\text{Cl}_2(aq) + 2\text{I}^-(aq) \rightarrow 2\text{Cl}^-(aq) + \text{I}_2(aq)\) (Solution turns brown).
- \(\text{Br}_2(aq) + 2\text{I}^-(aq) \rightarrow 2\text{Br}^-(aq) + \text{I}_2(aq)\) (Solution turns brown).
3. Reducing Ability of Halide Ions with Concentrated Sulfuric Acid
Reducing ability increases down Group 7: \(\text{F}^- < \text{Cl}^- < \text{Br}^- < \text{I}^-\). Iodide ions have the largest ionic radius and greatest shielding, so their outer electrons are most easily lost.
| Solid Halide | Reaction Type | Observations | Products Formed |
|---|---|---|---|
| NaCl | Acid-base only (no redox) | Steamy fumes turning moist blue litmus paper red. | \(\text{HCl}(g) + \text{NaHSO}_4(s)\) |
| NaBr | Acid-base AND Redox | Steamy fumes (\(\text{HBr}\)), red-brown vapour (\(\text{Br}_2\)), choking colourless gas (\(\text{SO}_2\)). | \(\text{HBr}, \text{Br}_2, \text{SO}_2\) (Sulfur reduced from +6 to +4) |
| NaI | Strong Redox | Steamy fumes (\(\text{HI}\)), black solid / purple vapour (\(\text{I}_2\)), choking gas (\(\text{SO}_2\)), yellow solid sulfur (\(\text{S}\)), rotten egg smell gas (\(\text{H}_2\text{S}\)). | \(\text{HI}, \text{I}_2, \text{SO}_2, \text{S}, \text{H}_2\text{S}\) (Sulfur reduced from +6 down to -2) |
4. Qualitative Testing with Acidified Silver Nitrate
To identify unknown halide ions in solution:
- Add dilute nitric acid (\(\text{HNO}_3\)) to eliminate carbonate (\(\text{CO}_3^{2-}\)) or hydroxide (\(\text{OH}^-\)) ions that would produce false precipitates. Never use hydrochloric acid, as it introduces chloride ions!
- Add aqueous silver nitrate (\(\text{AgNO}_3\)): \[ \text{Ag}^+(aq) + \text{X}^-(aq) \rightarrow \text{AgX}(s) \]
- Confirm the precipitate by adding dilute and concentrated aqueous ammonia.
5. Disproportionation Reactions of Chlorine
A redox reaction in which the same element is simultaneously oxidised and reduced.
Household Bleach Production
Chlorine reacts with cold dilute aqueous sodium hydroxide:
\[ \text{Cl}_2(g) + 2\text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{NaClO}(aq) + \text{H}_2\text{O}(l) \]
Chlorine is simultaneously reduced to -1 in \(\text{NaCl}\) and oxidised to +1 in sodium chlorate(I), \(\text{NaClO}\) (active ingredient in bleach).
Water Purification
Chlorine disinfects drinking water by killing bacteria:
\[ \text{Cl}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{HCl}(aq) + \text{HClO}(aq) \]
Chloric(I) acid (\(\text{HClO}\)) acts as a powerful oxidising disinfectant. In bright sunlight, chloric(I) acid decomposes:
\[ 2\text{Cl}_2(aq) + 2\text{H}_2\text{O}(l) \rightarrow 4\text{HCl}(aq) + \text{O}_2(g) \]
6. Practice Questions
Procedure & Observations:
- Add dilute nitric acid (\(\text{HNO}_3\)), followed by aqueous silver nitrate (\(\text{AgNO}_3\)). (1 mark)
- A cream precipitate of silver bromide (\(\text{AgBr}\)) forms. (1 mark)
- Add dilute ammonia solution: the precipitate does not dissolve. (1 mark)
- Add concentrated ammonia solution: the precipitate dissolves to produce a colourless solution. (1 mark)