OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 10

1 mark · Easy difficulty · Multiple Choice

Identify which of the given molecules (H2S, CO2, CCl4, or C2F4) is polar.

Practise this question

Question

Multiple-choice question 10 asks: 'Which molecule is polar?' The options provided are A: H2S, B: CO2, C: CCl4, and D: C2F4, followed by an answer box worth 1 mark.
Question text

10 Which molecule is polar?

A H2S

B CO2

C CCl4

D C2F4

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table row showing question 10 with the correct answer as 'A' and awarded 1 mark.

10 A 1

How to answer it

Determining Molecular Polarity & Dipole Cancellation

What this question tests

Core Concepts: Electronegativity differences, polar bonds, VSEPR theory (shapes of molecules), and molecular symmetry determining overall dipole moments.

Exam Skill: Differentiating between a molecule possessing polar bonds versus being an overall polar molecule (where individual bond dipoles do not cancel out).

Question 10: Identifying the Polar Molecule

Multiple Choice [1 Mark]

✅ Correct Answer

A — H₂S

Hydrogen sulfide has a non-linear (bent) shape with two lone pairs on the sulfur atom. Because the molecule is asymmetrical, the polar bond dipoles do not cancel, producing a permanent molecular dipole.

Award [1] for selecting option A.

🧠 Exam Technique: Two-Step Polarity Check

  1. Step 1: Are there polar bonds? Check if there is a difference in electronegativity between bonded atoms (e.g. C=O, C-Cl, C-F, S-H).
  2. Step 2: Is the shape symmetrical?
    • Symmetrical (linear, trigonal planar, tetrahedral with identical outer atoms): Dipoles cancel → Non-polar molecule.
    • Asymmetrical (lone pairs present, e.g. bent, pyramidal): Dipoles do not cancel → Polar molecule.

📐 Molecule-by-Molecule Systematic Breakdown

Molecule Electron Pairs on Central Atom Shape & Bond Angle Symmetry & Dipole Status
A: H₂S 2 bonding pairs, 2 lone pairs (on S) Non-linear (bent), ~104.5° Asymmetrical: Dipoles reinforce each other → Net dipole exists (Polar)
B: CO₂ 2 double bonds (2 bonding regions), 0 lone pairs (on C) Linear, 180° Symmetrical: Two equal C=O dipoles pull in directly opposite directions → Dipoles cancel (Non-polar)
C: CCl₄ 4 bonding pairs, 0 lone pairs (on C) Tetrahedral, 109.5° Symmetrical: Four identical C–Cl dipoles act in opposing 3D directions → Dipoles cancel (Non-polar)
D: C₂F₄ 3 electron regions around each carbon (planar) Planar, ~120° Symmetrical: Highly polar C–F bonds pull equally and oppositely across the molecule → Dipoles cancel (Non-polar)

💡 Key Knowledge: The H₂S Analogy

  • Sulfur is in Group 16 (Group 6) directly below oxygen in the periodic table.
  • Just like H₂O, H₂S forms two single covalent bonds and retains two lone pairs on the sulfur atom.
  • Lone pair–lone pair repulsion pushes the S–H bonds together into a non-linear (bent) arrangement.
  • Sulfur is more electronegative than hydrogen, so each bond has a dipole (Sδ⁻–Hδ⁺). Because the bonds meet at an angle, their dipoles combine to create an overall molecular dipole towards the sulfur atom.

❌ Common Misconceptions & Traps

  • Assuming high bond polarity means a polar molecule: Fluorine is the most electronegative element, so students often guess C₂F₄. However, due to its symmetry, tetrafluoroethene has no overall dipole!
  • Assuming CO₂ is bent: Students sometimes confuse CO₂ with SO₂. CO₂ has no lone pairs on the central carbon atom and is strictly linear.
  • Treating H₂S as linear: Writing H₂S as H–S–H in a straight line on paper can trick you into thinking it cancels out. Always count electron pairs to determine 3D geometry!

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), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.