Group 7 trends and halogen reactionsEdexcel International A Level Chemistry: Revision notes
Section 1
Physical properties and trends
The halogens exist as diatomic molecules, X₂, held together in the solid and liquid by London forces.
- Cl₂: gas, pale yellow-green
- Br₂: liquid, red-brown
- I₂: solid, grey-black (purple vapour)
Down the group the molecules have more electrons, so the London forces are stronger. More energy is needed to overcome them, so the melting and boiling temperatures increase. Covalent bonds are not broken on melting or boiling.
Never say the covalent bond is stronger down the group to explain boiling temperature. The intermolecular forces change.
Section 2
Colours in water and in a non-polar solvent
In aqueous solution chlorine is pale green, bromine is orange and iodine is brown.
In a non-polar organic solvent such as hexane the colours are clearer and the halogens are more soluble:
- chlorine: very pale green or colourless
- bromine: orange (or red)
- iodine: purple (violet)
The hexane forms an upper layer, and shaking the tube transfers the halogen into it.
In a displacement test, add hexane and shake; the colour of the upper layer tells you which halogen has formed.
Section 3
Electronegativity and reactivity trends
Electronegativity and oxidising ability decrease down the group.
Down the group the atomic radius and the shielding increase, so the nucleus attracts the incoming electron (or the bonding pair) less strongly. Chlorine is the strongest oxidising agent here and iodine the weakest.
In displacement reactions a more reactive halogen oxidises the halide ions of a less reactive one:
- Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ (orange)
- Cl₂ + 2I⁻ → 2Cl⁻ + I₂ (brown)
- Br₂ + 2I⁻ → 2Br⁻ + I₂
No reaction occurs when a less reactive halogen is added to the halide of a more reactive one, e.g. I₂ with Br⁻.
Section 4
Reactions with metals
Halogens oxidise Group 1 and Group 2 metals to ionic halides. The halogen is reduced from 0 to −1.
- 2Na + Cl₂ → 2NaCl
- Mg + Br₂ → MgBr₂
- 2K + I₂ → 2KI
The reactions become less vigorous down the group, because the oxidising power of the halogens falls. Each metal atom loses electrons (Na to +1, Mg to +2), so it is oxidised.
Section 5
Disproportionation of chlorine
Disproportionation is a reaction in which the same element is both oxidised and reduced.
- With water: Cl₂ + H₂O ⇌ HClO + HCl. Chlorine goes from 0 to +1 and −1. The chlorate(I) kills bacteria, so chlorine is used in water treatment.
- With cold dilute sodium hydroxide: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. The mixture is bleach.
- With hot concentrated sodium hydroxide: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O. Chlorine goes from 0 to −1 and +5.
Bromine and iodine react in analogous ways, e.g. Br₂ + 2NaOH → NaBr + NaBrO + H₂O.
State both oxidation numbers and the direction of change for disproportionation. The answer needs 'oxidised and reduced'.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Group 7 trends and halogen reactions
- The melting and boiling temperatures of three halogens are: chlorine, Cl₂, melting 172 K and boiling 239 K; bromine, Br₂, melting 266 K and boiling 332 K; iodine, I₂, melting 387 K and boiling 457 K. Room temperature is taken as 298 K.Explain why the boiling temperature increases from chlorine to iodine.2 marks
- A student adds a few drops of aqueous halogen solutions to separate aqueous solutions of potassium halides. She then adds a few cm³ of hexane to each tube, shakes it and leaves the layers to separate.Write the ionic equation for the reaction of aqueous bromine with aqueous iodide ions, and identify the oxidising agent.2 marks
- A swimming pool is treated with chlorine to kill bacteria. A manufacturer makes bleach by passing chlorine into cold dilute aqueous sodium hydroxide, forming sodium chloride and sodium chlorate(I), NaClO. When the sodium hydroxide is hot and concentrated, sodium chlorate(V), NaClO₃, forms instead of NaClO.Write the equation for the reaction of chlorine with water. Use oxidation numbers to show that this is disproportionation, and state why it is useful in treating water.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).