AQA · GCSE · Combined Science
Course version: 8464 · 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.
Group 7 halogens gain one electron to form 1⁻ ions, and reactivity decreases down the group as atomic radius increases and shielding rises, so the outer shell is gained less easily.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 7
Study notes
From the checked teaching notes for this specification point.
The idea
Group 7 halogens gain one electron to form 1⁻ ions, and reactivity decreases down the group as atomic radius increases and shielding rises, so the outer shell is gained less easily. A more reactive halogen therefore oxidises a less reactive halide ion in aqueous solution, taking its place in the salt. For example, chlorine added to potassium bromide solution displaces bromine: Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange-brown as bromine forms. Iodine is displaced by chlorine or bromine, giving a brown solution. Fluorine is too hazardous for school tests. Displacement is evidence for the reactivity trend, and the colour change identifies which halogen has formed.
What earns the marks
- State that halogens react by gaining one electron to form 1⁻ ions, so a more reactive halogen is a stronger oxidising agent.
- Explain that reactivity decreases down Group 7 because atomic radius and shielding increase, so the outer shell attracts an electron less strongly.
- Write a balanced displacement equation, such as Cl₂ + 2KBr → 2KCl + Br₂, keeping formulae and state symbols correct.
- Describe the observation: a colour change in the aqueous solution as the displaced halogen forms, for example colourless to orange-brown with bromine.
- Identify a displacement reaction from given results by comparing the halogen added with the halogen in the salt.
- Explain that no reaction occurs when the added halogen is less reactive than the halogen in the salt, for example iodine added to potassium chloride solution.
Common mistakes and how to put them right
Writing that the more reactive halogen is displaced, rather than the less reactive halogen.
Put it right by saying the more reactive halogen displaces the less reactive halogen from its salt.
Using the formula of the halogen salt incorrectly, such as KBr₂ instead of KBr.
Put it right by checking that potassium forms 1⁺ ions and bromide forms 1⁻ ions, giving KBr.
Claiming that reactivity increases down Group 7.
Put it right by linking decreasing reactivity to increasing atomic radius and shielding, so the outer shell gains an electron less easily.
Exam tips
- Link every observation to the identity of the halogen formed, for example an orange-brown colour indicates bromine.
- Balance displacement equations by counting halogen atoms on both sides before checking potassium and other spectator ions.
- When comparing two halogens, name the more reactive one first and state clearly which halide it displaces.
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 is added to a colourless solution of potassium bromide. A displacement reaction occurs. Which halogen is displaced from the potassium bromide solution?
Listen
Group 7 halogens gain one electron to form 1⁻ ions, and reactivity decreases down the group as atomic radius increases and shielding rises, so the outer shell is gained less easily. A more reactive halogen therefore oxidises a less reactive halide ion in aqueous solution, taking its place in the salt. For example, chlorine added to potassium bromide solution displaces bromine: Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange-brown as bromine forms. Iodine is displaced by chlorine or bromine, giving a brown solution. Fluorine is too hazardous for school tests. Displacement is evidence for the reactivity trend, and the colour change identifies which halogen has formed. A strong answer earns marks for these points. State that halogens react by gaining one electron to form 1⁻ ions, so a more reactive halogen is a stronger oxidising agent. Explain that reactivity decreases down Group 7 because atomic radius and shielding increase, so the outer shell attracts an electron less strongly. Write a balanced displacement equation, such as Cl₂ + 2KBr → 2KCl + Br₂, keeping formulae and state symbols correct. Describe the observation: a colour change in the aqueous solution as the displaced halogen forms, for example colourless to orange-brown with bromine. A common slip: Writing that the more reactive halogen is displaced, rather than the less reactive halogen. Instead, saying the more reactive halogen displaces the less reactive halogen from its salt. Exam tip: Link every observation to the identity of the halogen formed, for example an orange-brown colour indicates bromine.
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