Group 7 — Halogens

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CognitoGroup 7 halogens — trends and displacement reactions

Key facts & methods

Properties and trends in Group 7

Group 7 elements: F, Cl, Br, I, At. All have 7 electrons in outer shell. Form −1 ions.
All exist as diatomic molecules: F₂, Cl₂, Br₂, I₂.
States at room temperature: F₂ = pale yellow gas. Cl₂ = yellow-green gas. Br₂ = orange-brown liquid. I₂ = grey-black solid.
Trend going down Group 7: reactivity DECREASES. Boiling points increase.
Reason: outer shell further from nucleus → harder to gain an electron → less reactive.

Reactions of halogens

Halogens react with metals to form salts: 2Na + Cl₂ → 2NaCl.
With hydrogen: H₂ + Cl₂ → 2HCl. Less vigorous going down the group.
Displacement reactions: a more reactive halogen displaces a less reactive halide from solution.
Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromine — chlorine is more reactive).
Cl₂ + 2KI → 2KCl + I₂ (chlorine displaces iodine).
Br₂ + 2KI → 2KBr + I₂ (bromine displaces iodine).
But Br₂ cannot displace Cl⁻, and I₂ cannot displace Br⁻ or Cl⁻.

Testing for halide ions

Add dilute nitric acid then silver nitrate solution to the solution being tested.
Cl⁻ → white AgCl precipitate. Br⁻ → cream AgBr precipitate. I⁻ → yellow AgI precipitate.
Silver chloride dissolves in dilute NH₃; silver bromide dissolves in concentrated NH₃; silver iodide is insoluble in NH₃.
Nitric acid is added first to remove any carbonate or sulfate ions that would also give precipitates.

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markTrend in reactivityAQA 8462 style

Which halogen is the most reactive?

A Iodine

B Bromine

C Chlorine

D Fluorine

3 marksDisplacement reactionAQA 8462 P1 style

Chlorine solution is added to potassium bromide solution. Write a word equation and a balanced symbol equation, and describe what you would observe. (3 marks)

Hint: Word equation: chlorine + potassium bromide → potassium chloride + bromine. Balance: Cl₂ needs 2KBr. Observation: colourless → orange-brown.

+30 XP

1 markDisplacement predictionAQA 8462 style

Bromine solution is added to potassium iodide solution. What would you observe?

A No reaction — bromine and iodine have similar reactivity

B The solution turns dark brown/violet as iodine is displaced from the potassium iodide

C The solution turns pale yellow-green as chlorine is released

D A white precipitate forms

3 marksCompare Group 1 and Group 7 trendsAQA 8462 style

Compare the trend in reactivity going down Group 1 (alkali metals) with the trend going down Group 7 (halogens). Explain the reason for each trend. (4 marks)

Hint: State the trend for each group, then explain in terms of: position of outer electron relative to nucleus, how easily it is lost (Group 1) or gained (Group 7).

+30 XP

Group 7 — Halogens

Properties and trends in Group 7

Reactions of halogens

Testing for halide ions

Module complete! 🎉