OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 5

1 mark · Medium difficulty · Multiple Choice

Identify which compound has polar molecules from the given multiple-choice options.

Practise this question

Question

Multiple choice question 5 asks 'Which compound has polar molecules?'. The options are A: HCN, B: BCl3, C: CO2, and D: C2F4, with a blank answer box below.
Question text

5 Which compound has polar molecules?

A HCN

B BCl3

C CO2

D C2F4

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows that the correct answer for question 5 is A, worth 1 mark, under AO1.1.

5 A 1 AO1.1

How to answer it

Identifying Polar Molecules

What this question tests

This question assesses your understanding of covalent bonding, electronegativity, bond polarity, and 3D molecular shapes (VSEPR theory). To score the mark, you must determine whether individual bond dipoles cancel out or reinforce one another based on the symmetry of the molecule.

Question 5

Full Worked Solution & Analysis

✅ Correct Answer

A (HCN)

Hydrogen cyanide is a linear molecule with an asymmetrical distribution of electronegativity, resulting in a permanent overall dipole moment.

💡 Key Knowledge

  • Electronegativity: The power of an atom to attract the pair of electrons in a covalent bond.
  • Bond Dipoles: Formed when bonded atoms have different electronegativities (e.g., C-N, B-Cl, C-F).
  • Symmetry & Cancellation: Polar bonds do not automatically make a molecule polar. If the molecular geometry is symmetrical, individual bond dipoles cancel out completely (vector sum = 0).

🧠 Exam Technique

Quickly sketch or visualise the shape of each option:

  • HCN : Linear (asymmetrical) → Polar
  • BCl₃ : Trigonal planar (symmetrical) → Non-polar
  • CO₂ : Linear (symmetrical) → Non-polar
  • C₂F₄ : Planar / symmetrical alkene structure → Non-polar

❌ Common Errors

  • Assuming that the presence of polar bonds automatically means the molecule is polar.
  • Failing to recognise that symmetrical shapes (like trigonal planar BCl₃ or linear CO₂ ) cause dipole moments to cancel out in opposite directions.
  • Misreading molecular formulas under exam stress (e.g., confusing molecular shapes).
Mark Scheme Breakdown: 1 mark awarded for selecting A. (AO1.1 - Demonstration of knowledge and understanding of molecular structure and polarity).

Topics

Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.