Trends in Properties of the Halogens
Quick Notes
- Halogens (Group 7): Fluorine (F2), Chlorine (Cl2), Bromine (Br2), Iodine (I2), Astatine (At2).
- Electronegativity decreases down the group due to increased atomic radius and shielding.
- Boiling points increase down the group due to stronger van der Waals forces as molecules get larger.
- Oxidising ability decreases down the group – it becomes harder for the halogen to gain an electron (F is strongest; most readily forms F−).
- Displacement reactions occur where a more reactive halogen displaces a less reactive one from solution.
- Halide ions act as reducing agents (lose an electron and reduce something else)
- Reducing ability increasing down the group.
- Reactions of solid sodium halides with conc. H2SO4 depend on the halide’s reducing power:
- NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
- NaBr(s) + H2SO4(l) → NaHSO4(s) + HBr(g)
- 2HBr(g) + H2SO4(l) → SO2(g) + Br2(g) + 2H2O(l)
- NaI(s) + H2SO4(l) → NaHSO4(s) + HI(g)
- 2HI(g) + H2SO4(l) → SO2(g) + I2(s) + 2H2O(l)
- 6HI(g) + SO2(g) → H2S(g) + 3I2(s) + 2H2O(l)
- Acidified silver nitrate is used to test for halide ions (distinct precipitates); silver halide solubility in ammonia decreases down the group.
- Cl- = white precipitate (AgCl)
- Br- = cream precipitate (AgBr)
- I- = yellow precipitate (AgI)
Full Notes
Trends in Electronegativity
Electronegativity is the ability of an atom to attract a bonding pair of electrons. Electronegativity decreases down the group.
Explanation: Atomic radius increases so bonding electrons are further from the nucleus, and shielding increases, reducing nuclear attraction for bonding electrons.
Trends in Boiling Points
Boiling points increase down the group.
Explanation: Halogens exist as diatomic molecules (X2). Larger molecules have more electrons, giving stronger van der Waals forces; more energy is needed to overcome these.
| Halogen | Relative boiling point | Physical state at 298 K |
|---|---|---|
| F2 | Lowest | Gas (pale yellow) |
| Cl2 | ↑ | Gas (greenish-yellow) |
| Br2 | ↑ | Liquid (red-brown) |
| I2 | Highest | Solid (grey; purple vapour) |
Trend in Oxidising Ability of Halogens
Oxidising ability decreases down the group. Fluorine is the strongest oxidising agent (most electronegative). A more reactive halogen displaces a less reactive halogen from its halide ions in solution.
| Halogen added | In KCl(aq) | In KBr(aq) | In KI(aq) | Typical observation |
|---|---|---|---|---|
| Cl2 | No reaction | Displaces Br2 | Displaces I2 | Orange (Br2) / brown-purple (I2) in organic layer |
| Br2 | No reaction | No reaction | Displaces I2 | Brown-purple colour (I2) |
| I2 | No reaction | No reaction | No reaction | No visible change |
Trend in Reducing Ability of Halide Ions
Reducing ability increases down the group. Iodide ions (I−) are the strongest reducing agents.
Reactions of NaX (sodium halides) with concentrated H2SO4:
Sodium chloride with (concentrated) sulphuric acid:
NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
Sodium bromide with (concentrated) sulphuric acid:
NaBr(s) + H2SO4(l) → NaHSO4(s) + HBr(g)
2HBr(g) + H2SO4(l) → SO2(g) + Br2(g) + 2H2O(l)
Sodium iodide with (concentrated) sulphuric acid:
NaI(s) + H2SO4(l) → NaHSO4(s) + HI(g)
2HI(g) + H2SO4(l) → SO2(g) + I2(s) + 2H2O(l)
6HI(g) + SO2(g) → H2S(g) + 3I2(s) + 2H2O(l)
Explanation: Fluoride and chloride ions are weak reducing agents, so only acid–base reactions occur. Bromide and iodide ions are stronger reducing agents and reduce H2SO4, giving additional products.
| Solid sodium halide | Initial acid–base step | Further redox with H2SO4 | Main products observed |
|---|---|---|---|
| NaCl(s) | NaCl + H2SO4 → NaHSO4 + HCl | None (Cl− too weak a reducing agent) | Steamy fumes of HCl(g) |
| NaBr(s) | NaBr + H2SO4 → NaHSO4 + HBr | 2HBr + H2SO4 → SO2 + Br2 + 2H2O | Brown Br2, choking fumes, SO2 |
| NaI(s) | NaI + H2SO4 → NaHSO4 + HI | 2HI + H2SO4 → SO2 + I2 + 2H2O; 6HI + SO2 → H2S + 3I2 + 2H2O |
Purple I2, rotten-egg smell of H2S, SO2 |
Testing for Halide Ions Using Acidified Silver Nitrate
Silver nitrate solution (AgNO3) identifies halide ions. Halide ions react with Ag+ to form silver halide precipitates. Different solubilities in ammonia help confirm identity (precipitate colours can be similar).
| Halide ion | Observation with acidified AgNO3 |
|---|---|
| Cl− | White precipitate of AgCl |
| Br− | Cream precipitate of AgBr |
| I− | Yellow precipitate of AgI |
| Silver halide | Solubility in NH3(aq) |
|---|---|
| AgCl | Dissolves in dilute ammonia |
| AgBr | Only dissolves in concentrated ammonia |
| AgI | Insoluble in ammonia |
Why is silver nitrate acidified? Add dilute HNO3 first to remove carbonate (CO32−) impurities, which would otherwise give false positives with Ag+.
Summary Table: Trends in Halogens
| Property | Trend down Group 7 | Reason / Note |
|---|---|---|
| Electronegativity | Decreases | Greater radius and shielding reduce nuclear attraction |
| Boiling point | Increases | More electrons → stronger van der Waals forces |
| Oxidising ability of X2 | Decreases | Harder to gain an electron down the group |
| Reducing ability of X− | Increases | I− strongest; reduces H2SO4 to SO2/H2S |
| Displacement reactions | Cl2 displaces Br−, I−; Br2 displaces I−; I2 displaces none | More reactive halogen oxidises the halide of a less reactive halogen |
| AgNO3/NH3 test | AgCl white (dilute NH3 soluble); AgBr cream (conc NH3 soluble); AgI yellow (insoluble) | Solubility in ammonia decreases down the group |