Halogen displacement reactions (Edexcel GCSE Chemistry): Revision Notes
📚 Revision Notes
Halogen displacement reactions
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A more reactive halogen will displace a less reactive halogen from a compound. These are examples of redox reactions, where oxidation and reduction happen simultaneously.
Redox Reactions in Halogen Displacement
- Halogens (e.g., chlorine, bromine, iodine) gain electrons when they react, which means they are reduced.
- Halide ions (ions of halogens, such as chloride or bromide) lose electrons, meaning they are oxidised.
Ionic Equations for Halogen Displacement
The general pattern for halogen displacement reactions can be represented as:
Or:
This shows that the more reactive halogen replaces the less reactive halogen in the compound.
Aqueous Solutions Displacement Reactions
Chlorine (Cl₂) is more reactive than bromine (Br₂). If you add chlorine water to a solution of potassium bromide (KBr), the chlorine will displace the bromine:
- Chlorine is reduced to form chloride ions (Cl⁻), producing potassium chloride (KCl).
- Bromide ions (Br⁻) are oxidised to form bromine (Br₂), which turns the solution orange.
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Reaction Example:
You can use displacement reactions to observe the reactivity trend of the halogens.
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Method:
- Add aqueous halogens (chlorine water, bromine water, or iodine solution) to separate solutions of potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI).
- Observe the colour change:
- A colour change indicates that a reaction has taken place, meaning the halogen in the salt has been displaced by a more reactive halogen.
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Reactivity Trend:
- Chlorine (the most reactive) displaces both bromine and iodine from their salts.
- Bromine displaces iodine, but cannot displace chlorine.
- Iodine cannot displace either chlorine or bromine.
- Based on this trend, you can predict that astatine, the least reactive halogen, cannot displace any other halogens.