Why do boiling temperatures rise down Group 7?

More electrons per molecule, so stronger London forces needing more energy to overcome.

State and colour of Cl₂, Br₂, I₂ at room temperature.

Cl₂ pale green gas; Br₂ red-brown liquid; I₂ grey-black solid.

Why does electronegativity decrease down Group 7?

Larger atomic radius and more shielding, so the nucleus attracts the bonding pair less strongly.

Why does oxidising power decrease down Group 7?

The atom is larger with more shielding, so it gains an electron less readily.

Ionic equation for chlorine and bromide ions.

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

Which species is oxidised in Cl₂ + 2I⁻ → 2Cl⁻ + I₂?

Iodide ions (−1 to 0); chlorine is reduced (0 to −1).

Define disproportionation.

A reaction in which the same element is simultaneously oxidised and reduced.

Equation for chlorine with water.

Cl₂ + H₂O ⇌ HCl + HClO (Cl 0 to −1 and +1).

Equation for chlorine with cold dilute NaOH.

Cl₂ + 2NaOH → NaCl + NaClO + H₂O (bleach).

Equation for chlorine with hot NaOH.

3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (Cl −1 and +5).

Predict the state and reactivity of astatine.

Black solid; weaker oxidising agent than iodine, so no displacement of iodide.

Group 7: the halogensEdexcel A-Level Chemistry: Flashcards

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What intermolecular force acts between halogen molecules?

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What intermolecular force acts between halogen molecules?: London (instantaneous dipole–induced dipole) forces.
Why do boiling temperatures rise down Group 7?: More electrons per molecule, so stronger London forces needing more energy to overcome.
State and colour of Cl₂, Br₂, I₂ at room temperature.: Cl₂ pale green gas; Br₂ red-brown liquid; I₂ grey-black solid.
Why does electronegativity decrease down Group 7?: Larger atomic radius and more shielding, so the nucleus attracts the bonding pair less strongly.
Why does oxidising power decrease down Group 7?: The atom is larger with more shielding, so it gains an electron less readily.
Ionic equation for chlorine and bromide ions.: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Colours of Br₂ and I₂ in cyclohexane.: Bromine orange; iodine violet.
Which species is oxidised in Cl₂ + 2I⁻ → 2Cl⁻ + I₂?: Iodide ions (−1 to 0); chlorine is reduced (0 to −1).
Define disproportionation.: A reaction in which the same element is simultaneously oxidised and reduced.
Equation for chlorine with water.: Cl₂ + H₂O ⇌ HCl + HClO (Cl 0 to −1 and +1).
Equation for chlorine with cold dilute NaOH.: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (bleach).
Equation for chlorine with hot NaOH.: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (Cl −1 and +5).
Predict the state and reactivity of astatine.: Black solid; weaker oxidising agent than iodine, so no displacement of iodide.

Exam questions on Group 7: the halogens

A technician compares the Group 7 elements chlorine, bromine and iodine in a fume cupboard at room temperature (about 20 °C).
Explain why iodine has a higher boiling temperature than chlorine.2 marks
A student investigates displacement reactions of the halogens. She has aqueous solutions of potassium chloride, potassium bromide and potassium iodide, together with chlorine water, bromine water, aqueous iodine and the organic solvent cyclohexane, which is less dense than water and immiscible with it.
The student adds aqueous iodine to potassium bromide solution and sees no reaction. Explain this, referring to the relative oxidising power of bromine and iodine.2 marks
A water company treats drinking water with chlorine to kill bacteria. A separate factory makes household bleach by passing chlorine into sodium hydroxide solution.
Chlorine reacts with water as it dissolves. Write an equation for the reaction and explain, in terms of oxidation numbers, why it is a disproportionation reaction.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).