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2.4 Group 7 – The Halogens

GCSE Chemistry · AQA · Chapter 2: The Periodic Table

A free GCSE Chemistry lesson for AQA (8462) from OBA School: properties of the halogens, their reactions with metals and hydrogen, displacement reactions and trends down Group 7. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the halogens as non-metals made of diatomic molecules, and recall their colours and states
  • Describe the trends in boiling point and reactivity going down Group 7
  • Describe and write equations for reactions of halogens with metals and with hydrogen
  • Explain and predict displacement reactions, including ionic equations (Higher tier)

Key words

  • Halogens: The Group 7 elements: fluorine, chlorine, bromine, iodine and astatine.
  • Diatomic: A molecule made of two atoms, e.g. Cl₂.
  • Halide ion: The 1− ion a halogen atom forms when it gains one electron, e.g. Cl− (chloride), Br− (bromide).
  • Metal halide: An ionic salt made when a metal reacts with a halogen, e.g. sodium chloride.
  • Displacement: A reaction in which a more reactive element takes the place of a less reactive one in a compound.

Properties of the halogens

The halogens are non-metals. Each atom has seven electrons in its outer shell. Two atoms share a pair of electrons to make a diatomic molecule with a single covalent bond: F₂, Cl₂, Br₂, I₂. They are all toxic and have coloured vapours.

  • low melting and boiling points (typical of small molecules)
  • poor conductors of heat and electricity, like most non-metals
  • all are toxic, so they are used in a fume cupboard or well-ventilated room
  • Melting and boiling points; Trend increase (gas → liquid → solid at room temperature)
  • Relative molecular mass; Trend increases
  • Reactivity; Trend decreases: fluorine is the most reactive

Reactions with metals and with hydrogen

Halogens react with metals to form metal halides. These are ionic salts containing 1− halide ions. For example, hot iron wool glows as it reacts with chlorine to form orange-brown iron(III) chloride.

2Na(s) + Cl₂(g) → 2NaCl(s)

2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)

Mg(s) + Br₂(l) → MgBr₂(s)

With non-metals, halogens form molecules with covalent bonds. With hydrogen they form hydrogen halides, which are gases. These dissolve in water to make acidic solutions; hydrogen chloride makes hydrochloric acid.

H₂(g) + Cl₂(g) → 2HCl(g)

H₂(g) + F₂(g) → 2HF(g)

How fiercely a halogen reacts with hydrogen shows the reactivity trend very clearly:

  • fluorine; Reaction with hydrogen explodes on mixing, even when cold and in the dark; Reactivity most reactive
  • chlorine; Reaction with hydrogen explodes when the mixture is lit by bright light (sunlight or UV); only slow in the dark; Reactivity
  • bromine; Reaction with hydrogen needs heating to a high temperature with a catalyst; Reactivity
  • iodine; Reaction with hydrogen needs strong heating with a catalyst; slow, and the reaction is reversible so it never fully finishes; Reactivity least reactive

Key fact: Test for chlorine: it bleaches damp litmus paper (blue litmus may turn red first, then white).

Displacement reactions

A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt. You see a colour change as the less reactive halogen is released.

  • chlorine water; potassium chloride (aq) —; potassium bromide (aq) ✓ turns orange (bromine forms); potassium iodide (aq) ✓ turns brown (iodine forms)
  • bromine water; potassium chloride (aq) ✗ no reaction; potassium bromide (aq) —; potassium iodide (aq) ✓ turns brown (iodine forms)
  • iodine solution; potassium chloride (aq) ✗ no reaction; potassium bromide (aq) ✗ no reaction; potassium iodide (aq) —

Colours in water: chlorine water is very pale green (almost colourless), bromine water is yellow to orange, and iodine solution is brown. Halide salt solutions (KCl, KBr, KI) are all colourless.

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

Key fact: Fluorine would displace all the other halogens, but it reacts violently with water itself, so it cannot be used in these test-tube reactions.

(Higher tier) The potassium ions do not change, so they are left out of the ionic equation. Chlorine atoms gain electrons (reduction) and bromide ions lose electrons (oxidation).

Worked example: ionic and half equations (Higher tier)

Ionic equation: Cl₂ + 2Br− → 2Cl− + Br₂
Reduction: Cl₂ + 2e− → 2Cl− (gain of electrons)
Oxidation: 2Br− → Br₂ + 2e− (loss of electrons)

Answer: chlorine is reduced; bromide ions are oxidised

Predicting properties

Exam tip: In displacement questions, decide first which halogen is more reactive (higher up the group). Only a halogen higher in the group can displace one below it.

Worked example: predicting astatine

Astatine is below iodine. Iodine is a dark grey solid that boils at 184 °C.
Boiling points increase down the group, so astatine is a solid with a boiling point above 184 °C, probably black.
Reactivity decreases down the group, so astatine is the least reactive halogen. It would not displace iodine from potassium iodide.

Answer: a dark solid, less reactive than iodine

Common mistakes

  • Writing halogens as single atoms (Cl, Br) in equations. They are diatomic: Cl₂, Br₂.
  • Mixing up the element and its ion: chlorine (Cl₂) is the element; chloride (Cl−) is the ion in a salt.
  • Saying reactivity increases down Group 7. It decreases (the opposite of Group 1).
  • Thinking iodine can displace chlorine. A less reactive halogen cannot displace a more reactive one.
  • Writing “bromine is oxidised” in the chlorine/bromide reaction. It is the bromide ions that are oxidised.

Revision questions

Try these, then check your answers below each one.

Question 1. Write the formula of a bromine molecule.

Answer: Br2

Working: Bromine exists as diatomic molecules: Br₂.

Question 2. Which halogen reacts with hydrogen only when heated strongly with a catalyst, and even then slowly and reversibly? (Choose from: Iodine, Chlorine, Fluorine, Bromine.)

Answer: Iodine

Working: Fluorine explodes with hydrogen even in the cold and dark; chlorine explodes in bright light; bromine needs heating with a catalyst; iodine, the least reactive here, reacts slowly and reversibly even when heated.

Question 3. True or false? Iodine solution displaces bromine from potassium bromide solution.

Answer: False

Working: False. Iodine is less reactive than bromine (it is lower in Group 7), so it cannot displace it.

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