Physical Properties of the Group 17 Elements
1. Colours and Trend in Volatility of Cl₂, Br₂, I₂
| Element | Colour (at room temperature) | Physical State | Volatility |
|---|---|---|---|
| Cl₂ | Pale green gas | Gas | High (most volatile) |
| Br₂ | Red-brown liquid | Liquid | Moderate |
| I₂ | Shiny grey-black solid (purple vapour) | Solid | Low (least volatile) |
- Trend in volatility: Decreases down the group
→ Due to increasing molecular size and mass → Stronger London dispersion forces → Less volatility
2. Trend in Bond Strength of Halogen Molecules (X–X)
- Bond strength decreases from Cl–Cl to I–I:
- Cl–Cl: Strongest
- Br–Br: Weaker
- I–I: Weakest
- Reason: Down the group, atomic radius increases → Bond length increases → Weaker orbital overlap → Lower bond dissociation energy
3. Volatility and Instantaneous Dipole–Induced Dipole Forces
- Halogens are non-polar molecules → Intermolecular forces are instantaneous dipole–induced dipole (London dispersion forces)
- As molecular size and number of electrons increase down the group:
- Strength of London forces increases
- Boiling point increases
- Volatility decreases
Chemical Properties of the Halogen Elements and Hydrogen Halides
1. Relative Reactivity of Halogens as Oxidising Agents
- Halogens act as oxidising agents: gain electrons to form halide ions
- Cl₂ > Br₂ > I₂ in oxidising strength
- Explanation:
- Cl₂ has highest electron affinity and strongest ability to attract electrons
- Reactivity decreases down the group as the nucleus has less effective attraction for electrons due to increased shielding
Displacement Reactions:
- Cl₂ displaces Br⁻ and I⁻
- Br₂ displaces I⁻
- I₂ displaces neither
2. Reaction with Hydrogen (Formation of Hydrogen Halides)
- General reaction:
H₂ + X₂ → 2HX (where X = Cl, Br, I) - Trend in reactivity:
- Cl₂ reacts explosively with H₂ in sunlight
- Br₂ reacts slowly on heating
- I₂ reacts very slowly and reversibly
- Explanation:
- Reactivity decreases down the group
- Due to decreasing bond strength of X–H
3. Thermal Stability of Hydrogen Halides
- Stability order: HF > HCl > HBr > HI
(More stable to less stable) - Explanation:
- Down the group, H–X bond becomes weaker
- Easier thermal decomposition (breaking) of the bond
Some Reactions of Halide Ions
1. Relative Reactivity as Reducing Agents
- Halide ions act as reducing agents: lose electrons to form halogen
- I⁻ > Br⁻ > Cl⁻
- Down the group, halide ions become better reducing agents
- Due to weaker hold of nucleus on outer electrons
2(a). Reactions with Aqueous Ag⁺ and Aqueous NH₃
- Add AgNO₃ to halide solution → Precipitate forms:
| Halide Ion | Precipitate formed | Colour | Solubility in NH₃ |
|---|---|---|---|
| Cl⁻ | AgCl | White | Soluble (dilute NH₃) |
| Br⁻ | AgBr | Cream | Soluble (conc. NH₃) |
| I⁻ | AgI | Yellow | Insoluble |
Equations:
- Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
2(b). Reaction with Concentrated H₂SO₄
| Halide | Observation | Reaction Type | Products |
|---|---|---|---|
| Cl⁻ | Steamy fumes of HCl | Acid-base only | HCl |
| Br⁻ | Brown fumes of Br₂, SO₂ gas | Redox + Acid-base | HBr, Br₂, SO₂ |
| I⁻ | Violet vapour of I₂, bad smell | Strong redox | HI, I₂, H₂S, SO₂, S, H₂SO₄ |
Sample equation for Br⁻:
- HBr formation:
NaBr + H₂SO₄ → NaHSO₄ + HBr - Oxidation to Br₂:
2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O
Reactions of Chlorine
1. Disproportionation with NaOH
- Cold dilute NaOH:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
(Cl: 0 → -1 in NaCl and +1 in NaClO) - Hot conc. NaOH:
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
(Cl: 0 → -1 in NaCl and +5 in NaClO₃) - Both are disproportionation reactions:
Chlorine is both oxidised and reduced
2. Use in Water Purification
- Cl₂ added to water:
Cl₂ + H₂O ⇌ HCl + HOCl
HOCl ⇌ H⁺ + OCl⁻ - HOCl (hypochlorous acid) and OCl⁻ are active oxidising agents:
- Kill bacteria
- Used in disinfecting water supply
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