OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 10

1 mark · Easy difficulty · Multiple Choice

Identify the correct explanation for the trend in boiling points of chlorine, bromine, and iodine down Group 7.

Practise this question

Question

Multiple choice question 10 asking for the correct explanation for the trend in boiling points of chlorine, bromine, and iodine down the group. Four options are provided: A, Bond enthalpy increases; B, Chemical reactivity decreases; C, Electronegativity decreases; D, London forces increase. An answer box and mark allocation of [1] are shown at the bottom.
Question text

10 What is the correct explanation for the trend in the boiling points of chlorine, bromine, and iodine

down the group?

A Bond enthalpy increases.

B Chemical reactivity decreases.

C Electronegativity decreases.

D London forces increase.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 10 is D, worth 1 mark under assessment objective AO1.1.

10 D 1 AO1.1

How to answer it

Halogen Boiling Points Trend

What this question tests: A solid understanding of intermolecular forces (specifically London forces), how they change down Group 7 (chlorine, bromine, iodine), and how these forces directly influence physical properties like boiling points.
Question 10 (Multiple Choice)

Trend in Halogen Boiling Points

Identifying the correct physical explanation

✅ Correct Answer: D

London forces increase. Down Group 7, molecules (Cl₂, Br₂, I₂) get larger, possess more electrons, and have larger electron clouds. This increases the strength of the induced dipole-dipole interactions (London forces) between molecules, requiring more thermal energy to break them during boiling.

💡 Key Knowledge

  • Halogens exist as simple covalent molecules (X₂) held together in the solid/liquid state by intermolecular London forces.
  • Boiling a molecular substance overcomes intermolecular forces, NOT covalent bonds within the molecule.
  • Going down Group 7: Atomic radius increases, number of shells increases, and total number of electrons increases.

🧠 Exam Technique

Watch out for distractor options like bond enthalpy (A) or electronegativity (C). Bond enthalpy refers to breaking covalent bonds inside the molecule, which is not what happens when a substance boils or melts!

❌ Common Errors

  • Confusing intermolecular London forces with covalent bonding (thinking that boiling a halogen breaks Cl-Cl, Br-Br, or I-I bonds).
  • Selecting option A because students know bond lengths change, forgetting that boiling involves intermolecular forces, not intramolecular ones.
Mark Scheme Allocation: 1 mark awarded for selecting D (AO1.1: Demonstrate knowledge and understanding of scientific ideas).

Topics

Module 3: Periodic table and energy · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.