AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
A more reactive halogen can displace a less reactive halogen from an…
Full specification wording
Group 7: A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
A displacement reaction occurs when a more reactive halogen is added to an aqueous solution containing a less reactive halide salt.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A displacement reaction occurs when a more reactive halogen is added to an aqueous solution containing a less reactive halide salt. The more reactive halogen gains an electron and forms a 1⁻ ion, while the less reactive halide ion loses an electron and becomes a halogen molecule. For example, chlorine displaces bromine from potassium bromide solution because chlorine is above bromine in Group 7. The colour change observed depends on the halogen formed: bromine gives an orange solution and iodine gives a brown solution. This reaction demonstrates the reactivity trend and can be used to place halogens in order of reactivity.
What earns the marks
- A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt.
- The more reactive halogen gains an electron and forms a 1⁻ ion, while the less reactive halide ion is oxidised to form a halogen molecule.
- Chlorine displaces bromine from potassium bromide solution and iodine from potassium iodide solution.
- Bromine displaces iodine from potassium iodide solution but cannot displace chlorine from potassium chloride solution.
- The colour of the solution changes as the halogen formed appears, for example orange for bromine and brown for iodine.
Common mistakes and how to put them right
Writing that a less reactive halogen displaces a more reactive one.
Put it right: The more reactive halogen displaces the less reactive halogen.
Forgetting that the halide must be in aqueous solution.
Put it right: Displacement is described for an aqueous solution of the salt.
Confusing the colour of the halogen with the colour of the halide ion.
Put it right: The colour change is due to the halogen molecule formed, not the halide ion.
Exam tips
- Identify which halogen is more reactive by its position in Group 7 before predicting whether displacement occurs.
- Describe the colour change and name the halogen responsible for it.
- Write a word equation or balanced symbol equation to show the displacement clearly.
Try the lesson
The start of the Group 7: A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. lesson, made from checked teaching notes for this specification point.
Have a go first
Chlorine water is added to potassium iodide solution. Which halogen is displaced?
Listen
A displacement reaction occurs when a more reactive halogen is added to an aqueous solution containing a less reactive halide salt. The more reactive halogen gains an electron and forms a 1⁻ ion, while the less reactive halide ion loses an electron and becomes a halogen molecule. For example, chlorine displaces bromine from potassium bromide solution because chlorine is above bromine in Group 7. The colour change observed depends on the halogen formed: bromine gives an orange solution and iodine gives a brown solution. This reaction demonstrates the reactivity trend and can be used to place halogens in order of reactivity. A strong answer earns marks for these points. A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt. The more reactive halogen gains an electron and forms a 1⁻ ion, while the less reactive halide ion is oxidised to form a halogen molecule. Chlorine displaces bromine from potassium bromide solution and iodine from potassium iodide solution. Bromine displaces iodine from potassium iodide solution but cannot displace chlorine from potassium chloride solution. A common slip: Writing that a less reactive halogen displaces a more reactive one. Instead, the more reactive halogen displaces the less reactive halogen. Exam tip: Identify which halogen is more reactive by its position in Group 7 before predicting whether displacement occurs.
See it
Key words
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Displacement reactions
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Group 7 reactivity
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Halide salts
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Colour changes
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See every definition
- Displacement reactions
- A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt.
- Group 7 reactivity
- The reactivity trend shown by this reaction, which can be used to place halogens in order of reactivity.
- Halide salts
- Salts whose aqueous solutions are used in displacement reactions, where the less reactive halide ion is oxidised to form a halogen molecule.
- Colour changes
- The change observed as the halogen formed appears, for example orange for bromine and brown for iodine.
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