OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 1

1 mark · Medium difficulty · Multiple Choice

Identify which of the given substances contains polar molecules from a multiple-choice list.

Practise this question

Question

Multiple choice question 1 asking which substance contains polar molecules, with options A (C2H4), B (CO2), C (NCl3), and D (SF6), an answer box, and a mark allocation of [1].
Question text

1 Which substance contains polar molecules?

A C2H4

B CO2

C NCl3

D SF6

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table showing question number 1 with the correct answer C, awarding 1 mark under AO element 1.2 with no additional guidance.

AO

Question Answer Marks Guidance

element

1 C 1 1.2

How to answer it

Identifying Polar Molecules

📌 What this question tests

This question assesses your understanding of electronegativity differences, 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 1: Multiple Choice Analysis

Identifying Polar Molecules in C₂H₄, CO₂, NCl₃, and SF₆

✅ Correct Answer: C (NCl₃)

Nitrogen trichloride ( NCl₃ ) has a pyramidal shape (similar to ammonia, NH₃ ) with a lone pair of electrons on the central nitrogen atom. Because the bonds are arranged asymmetrically, the individual bond dipoles do not cancel out, resulting in a net dipole moment and a polar molecule.

💡 Key Knowledge

  • Electronegativity: Measures an atom's ability to attract bonding electrons. Differences create polar bonds.
  • Symmetry & Cancellation: Even if a molecule contains polar bonds, it can be non-polar if it is symmetrical enough for dipoles to cancel.
  • Lone Pairs: Central atoms with lone pairs often create asymmetric shapes (e.g., trigonal pyramidal, bent), leading to polarity.

🧠 Exam Technique

For each option, quickly sketch or visualise the 3D shape:

  • C₂H₄ : Planar, highly symmetrical, dipoles cancel.
  • CO₂ : Linear, symmetrical, dipoles cancel.
  • NCl₃ : Pyramidal with a lone pair, asymmetric.
  • SF₆ : Octahedral, perfectly symmetrical, dipoles cancel.

❌ Common Errors

  • Assuming a molecule is polar just because it contains polar bonds (forgetting to check molecular geometry).
  • Confusing bond polarity with overall molecular polarity in symmetrical molecules like CO₂ or SF₆ .
Mark Allocation: [1 mark total] awarded for correctly selecting option C.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.