AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The trend in oxidising ability of the halogens down the group, including…
Full specification wording
Trends in properties: The trend in oxidising ability of the halogens down the group, including displacement reactions of halide ions in aqueous solution.
A halogen acts as an oxidising agent by gaining electrons: X₂ + 2e⁻ → 2X⁻.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A halogen acts as an oxidising agent by gaining electrons: X₂ + 2e⁻ → 2X⁻. Down the group the atom is larger and better shielded, so the incoming electron is attracted less strongly and the halogen is reduced less readily, which means oxidising power falls from F₂ to I₂. The consequence is that a halogen displaces the ions of any halogen below it. Chlorine added to aqueous sodium bromide gives an orange solution of bromine: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine added to aqueous sodium iodide gives a brown solution, or a black solid, of iodine: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. Bromine displaces iodine but not chlorine, and iodine displaces neither. Shaking with cyclohexane confirms which halogen formed: bromine colours the organic layer orange and iodine colours it violet.
What earns the marks
- Stating that oxidising ability decreases down the group.
- Explaining that by the larger atomic radius and increased shielding, so an electron is gained less readily.
- The ionic equation, for example Cl₂ + 2I⁻ → 2Cl⁻ + I₂.
- The observation of a brown solution or a black solid when iodine is displaced.
- Stating that no reaction occurs when the halogen added is below the halide in the group.
Common mistakes and how to put them right
Describing displaced iodine in aqueous solution as purple, which mark schemes refuse.
Put it right: It is brown in water and only violet in an organic solvent.
Writing a full equation including sodium ions when an ionic equation is asked for.
Leaving the equation unbalanced, as Cl₂ + I⁻ → Cl⁻ + I₂.
Predicting that iodine displaces chloride ions.
Saying the halogen is oxidised in a displacement reaction, when it is reduced.
Exam tips
- Give the colour of the solution, not of the solid, unless the question mentions an organic layer.
- Check that both atoms and charges balance in every displacement equation before moving on.
Try the lesson
The start of the Trends in properties: The trend in oxidising ability of the halogens down the group, including displacement reactions of halide ions in aqueous solution. lesson, made from checked teaching notes for this specification point.
Have a go first
Which halogen is the most powerful oxidising agent?
Listen
A halogen acts as an oxidising agent by gaining electrons: X₂ + 2e⁻ → 2X⁻. Down the group the atom is larger and better shielded, so the incoming electron is attracted less strongly and the halogen is reduced less readily, which means oxidising power falls from F₂ to I₂. The consequence is that a halogen displaces the ions of any halogen below it. Chlorine added to aqueous sodium bromide gives an orange solution of bromine: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine added to aqueous sodium iodide gives a brown solution, or a black solid, of iodine: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. Bromine displaces iodine but not chlorine, and iodine displaces neither. Shaking with cyclohexane confirms which halogen formed: bromine colours the organic layer orange and iodine colours it violet. A strong answer earns marks for these points. stating that oxidising ability decreases down the group. explaining that by the larger atomic radius and increased shielding, so an electron is gained less readily. the ionic equation, for example Cl₂ + 2I⁻ → 2Cl⁻ + I₂. the observation of a brown solution or a black solid when iodine is displaced. Exam tip: Give the colour of the solution, not of the solid, unless the question mentions an organic layer.
Key words
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oxidising ability
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displacement reactions
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ionic equations
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observed colours
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See every definition
- oxidising ability
- A halogen acts as an oxidising agent by gaining electrons, and this power falls from F₂ to I₂ down the group.
- displacement reactions
- A halogen displaces the ions of any halogen below it, so bromine displaces iodine but not chlorine, and iodine displaces neither.
- ionic equations
- Chlorine added to aqueous sodium bromide gives Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, and with aqueous sodium iodide gives Cl₂ + 2I⁻ → 2Cl⁻ + I₂.
- observed colours
- Chlorine added to aqueous sodium bromide gives an orange solution of bromine, and with aqueous sodium iodide gives a brown solution, or a black solid, of iodine.
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