Compare halogens as oxidising agents
Rank halogen oxidising strength and use displacement equations to predict reactions with halide ions.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to a halogen molecule when it acts as an oxidising agent?
What you will learn
- Rank F₂, Cl₂, Br₂ and I₂ as oxidising agents
- Predict halogen–halide displacement
- Write balanced ionic equations
Halogens gain electrons and act as oxidising agents
A halogen molecule accepts electrons to form halide ions: X₂ + 2e⁻ → 2X⁻. A stronger oxidising agent is more readily reduced and oxidises another species.
Oxidising strength decreases down the group: F₂ > Cl₂ > Br₂ > I₂. Thus chlorine can oxidise bromide or iodide ions, bromine can oxidise iodide ions, and iodine cannot displace chloride or bromide in the standard aqueous comparison.
Use a displacement rule and an ionic equation
A more reactive halogen X₂ displaces a less reactive halogen Y₂ from Y⁻: X₂ + 2Y⁻ → 2X⁻ + Y₂. Check that the oxidising halogen is the one gaining electrons.
Keep oxidising strength of X₂ separate from reducing strength of X⁻. The direction reverses when you compare halide ions as reducing agents.
Pause and explain
Which is the stronger oxidising agent, bromine or iodine?
Worked example
Predict and balance the reaction of chlorine with aqueous iodide ions.
Show the worked solution
- Chlorine is a stronger oxidising agent than iodine.
- Cl₂ gains electrons and I⁻ loses electrons.
- Two iodide ions are needed for each Cl₂ molecule.
Answer Cl₂ + 2I⁻ → 2Cl⁻ + I₂.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the ionic equation for bromine displacing iodide ions.
Give me a hint
Bromine gains two electrons; two I⁻ ions form one I₂ molecule.
Compare my reasoning
- Br₂ is the stronger oxidising agent than I₂.
- Two I⁻ ions lose two electrons to form I₂.
- The products are 2Br⁻ and I₂.
Br₂ + 2I⁻ → 2Br⁻ + I₂.
Look for these in your work
- Reactants selected
- Coefficients balanced
- Electron-transfer direction correct
Predict whether iodine displaces bromide from aqueous bromide. Explain using oxidising strength.
Give me a hint
The added halogen must be a stronger oxidising agent than the halide's parent halogen.
Compare my reasoning
- I₂ is a weaker oxidising agent than Br₂.
- I₂ cannot oxidise Br⁻ to Br₂ in the standard series.
- No displacement is predicted.
No reaction is predicted: I₂ is a weaker oxidising agent than Br₂ and cannot oxidise Br⁻ to Br₂.
Look for these in your work
- Outcome correct
- Relative order stated
- Reason linked to electron acceptance
A solution becomes brown when chlorine is added to an initially colorless iodide solution. Identify the halogen formed and explain the evidence limit.
Give me a hint
Chlorine oxidises iodide to iodine, which is brown in aqueous solution.
Compare my reasoning
- Cl₂ displaces iodine from I⁻.
- I₂ causes the brown color.
- Color supports iodine formation but a full identification should consider controls and other evidence.
Iodine is formed: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. The brown color is consistent with I₂ but color alone is not unique proof in an uncontrolled mixture.
Look for these in your work
- Species identified
- Equation supplied
- Observation qualified
Common mistakes
- Calling the halide ion the oxidising agent when it loses electrons
- Reversing the displacement trend
- Forgetting the coefficient 2 on halide ions
- Assuming iodine displaces chloride ions
What can you explain now?
What happens to a halogen molecule when it acts as an oxidising agent?
Compare with the explanation
It gains electrons
The halogen is reduced to halide ions.
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Make it stick
How does oxidising strength change down Group 17, and how does that predict displacement?
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