Halogen Displacement Reactions

This lesson covers halogen displacement reactions, a key topic in the Edexcel GCSE Chemistry specification (1CH0), Topic 6: Groups in the Periodic Table. You need to understand that a more reactive halogen will displace a less reactive halogen from a solution of its salt, and you must be able to describe the observations and write equations for these reactions.

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The Principle of Halogen Displacement

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its halide salt. This is because the more reactive halogen is better at gaining electrons than the less reactive one — it is a stronger oxidising agent.

The general pattern is:

more reactive halogen + metal halide → metal halide (of more reactive halogen) + less reactive halogen

This is very similar to the displacement reactions of metals, but instead of metals competing to lose electrons, halogens compete to gain electrons.

Exam Tip: Displacement reactions prove the order of reactivity. If chlorine can displace bromide ions from a solution, it proves chlorine is more reactive than bromine. This is the experimental evidence for the reactivity trend.

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The Displacement Experiment

The standard experiment involves adding a halogen solution (chlorine water, bromine water or iodine water) to a solution of a potassium halide (potassium chloride, potassium bromide or potassium iodide) in a test tube.

Colours of Halogen Solutions and Halide Solutions

Substance	Colour in Solution
Chlorine water (Cl₂)	Pale green / almost colourless
Bromine water (Br₂)	Orange / brown
Iodine solution (I₂)	Brown / yellow
Potassium chloride (KCl)	Colourless
Potassium bromide (KBr)	Colourless
Potassium iodide (KI)	Colourless
Chloride ions (Cl⁻) in solution	Colourless
Bromide ions (Br⁻) in solution	Colourless
Free bromine (Br₂) in solution	Orange / brown
Free iodine (I₂) in solution	Brown / yellow

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Results of Displacement Reactions

Chlorine + Potassium Bromide

Word equation: chlorine + potassium bromide → potassium chloride + bromine

Symbol equation: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Observation: The colourless potassium bromide solution turns orange/brown as bromine is displaced.

Why it works: Chlorine is more reactive than bromine — chlorine atoms can gain electrons more easily than bromine atoms because the outer shell is closer to the nucleus with less shielding.

Chlorine + Potassium Iodide

Word equation: chlorine + potassium iodide → potassium chloride + iodine

Symbol equation: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)

Observation: The colourless potassium iodide solution turns brown/yellow as iodine is displaced.

Bromine + Potassium Iodide

Word equation: bromine + potassium iodide → potassium bromide + iodine

Symbol equation: Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

Observation: The colourless potassium iodide solution turns brown/yellow as iodine is displaced. (The initial orange colour of bromine water may also become darker/browner.)

Reactions That Do NOT Occur

• Bromine + potassium chloride — no reaction (bromine is less reactive than chlorine, so cannot displace chloride ions).
• Iodine + potassium chloride — no reaction.
• Iodine + potassium bromide — no reaction (iodine is less reactive than bromine).

In each case, the solution remains unchanged — no colour change is observed.

Exam Tip: If a halogen is added to a solution containing a halide ion of a MORE reactive halogen, no displacement occurs. The solution stays the same colour. Always state "no reaction" and explain why.

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Summary Table of Displacement Results

graph TD
    A["<b>Halogen Displacement Results</b>"]
    A --> B["<b>Cl₂ + KBr</b><br/>✅ Reaction<br/>Orange/brown colour<br/>(Br₂ displaced)"]
    A --> C["<b>Cl₂ + KI</b><br/>✅ Reaction<br/>Brown/yellow colour<br/>(I₂ displaced)"]
    A --> D["<b>Br₂ + KI</b><br/>✅ Reaction<br/>Brown/yellow colour<br/>(I₂ displaced)"]
    A --> E["<b>Br₂ + KCl</b><br/>❌ No reaction"]
    A --> F["<b>I₂ + KCl</b><br/>❌ No reaction"]
    A --> G["<b>I₂ + KBr</b><br/>❌ No reaction"]

    style B fill:#27ae60,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#e74c3c,color:#fff
    style G fill:#e74c3c,color:#fff

The results can also be displayed in a table:

	KCl (aq)	KBr (aq)	KI (aq)
Cl₂ (aq)	— (same element)	Orange/brown (Br₂ formed)	Brown/yellow (I₂ formed)
Br₂ (aq)	No reaction	— (same element)	Brown/yellow (I₂ formed)
I₂ (aq)	No reaction	No reaction	— (same element)

Exam Tip: This table is extremely useful for the exam. Learn it! If you can fill in this table from memory, you can answer any halogen displacement question. Note the pattern: reactions occur above the diagonal (where the added halogen is more reactive).

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Ionic Equations for Displacement (Higher)

For the higher tier, you may need to write ionic equations that show only the species that change.

Chlorine displacing bromide:

Full equation: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Ionic equation: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)

The potassium ions (K⁺) are spectator ions — they do not take part in the reaction.

Chlorine displacing iodide:

Ionic equation: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)

Bromine displacing iodide:

Ionic equation: Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq)

Linking to Redox

In these displacement reactions:

• The halogen (e.g. Cl₂) is reduced — it gains electrons to form halide ions (Cl⁻).
• The halide ion (e.g. Br⁻) is oxidised — it loses electrons to form the halogen molecule (Br₂).

This means halogen displacement reactions are redox reactions.

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Practical Considerations

When carrying out this experiment:

1. Use small volumes (about 2 cm³) of each halide solution in test tubes.
2. Add a few drops of halogen water to each test tube.
3. Observe and record any colour changes.
4. Record "no change" if no reaction occurs — this is still an important result.
5. Chlorine water should be used in a well-ventilated area or fume cupboard as chlorine gas is toxic.

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Why Does Displacement Occur? — Electron Transfer Explanation

Halogen displacement reactions occur because a more reactive halogen has a greater tendency to gain electrons than a less reactive halogen. The more reactive halogen is a stronger oxidising agent.

Consider chlorine displacing bromide ions:

• Chlorine molecules (Cl₂) can gain electrons more easily than bromine molecules (Br₂).
• Each Cl₂ molecule takes electrons from two Br⁻ ions.
• The Br⁻ ions lose their extra electron and become Br₂ molecules.
• The Cl atoms gain an electron each and become Cl⁻ ions.

This is because chlorine's outer shell is closer to the nucleus than bromine's outer shell. The nuclear attraction for an incoming electron is stronger in chlorine, so chlorine gains electrons more readily.

The same reasoning explains why bromine can displace iodine (bromine's outer shell is closer to the nucleus than iodine's), but iodine cannot displace either chlorine or bromine.

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Applying Displacement to Identify Unknown Halides

Halogen displacement reactions can be used as a practical test to identify unknown halide solutions: