OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 5

1 mark · Easy difficulty · Multiple Choice

Identify which of the given molecules is not planar based on its shape and bond angles.

Practise this question

Question

Multiple choice question 5 asks which molecule is not planar, with four options: A (C2H4), B (C2H6), C (H2CO), and D (HCN). A response box is provided next to 'Your answer', with 1 mark indicated at the bottom right.
Question text

5 Which molecule is not planar?

A C2H4

B C2H6

C H2CO

D HCN

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme shows question number 5 with the correct answer as B, awarding 1 mark.

5 B 1

How to answer it

Identifying Non-Planar Molecules

What this question tests

This question assesses your understanding of Electron Pair Repulsion Theory (VSEPR), standard bond angles, and 3D molecular geometry. Specifically, you must determine whether atoms lie in a single flat plane based on their shape and hybridization around central atoms.

Question 5

Determining Planarity in Simple Covalent Molecules

✅ Correct Answer

B (C₂H₆)

Ethane has a tetrahedral arrangement around each carbon atom (bond angle approximately 109.5°), making the molecule 3D rather than flat (planar).

💡 Key Knowledge

  • C₂H₄ (Ethene): Trigonal planar around each carbon (120°). All 6 atoms lie in the same plane.
  • C₂H₆ (Ethane): Tetrahedral around both carbons (109.5°). Not planar.
  • H₂CO (Methanal): Trigonal planar around the carbon atom (120°).
  • HCN (Hydrogen cyanide): Linear geometry (180°). All atoms naturally lie on a straight line, which is inherently planar.

🧠 Exam Technique

  • Quickly sketch or visualize the VSEPR shape for each option before jumping to a conclusion.
  • Remember that any linear or single-bonded/double-bonded system involving three or fewer atoms total (like HCN or H₂CO) is automatically coplanar. Look for tetrahedral (sp³ hybridized) centers like CH₃ groups, which break planarity.

❌ Common Errors

  • Confusing double-bond planarity (ethene/methanal) with single-bonded saturated carbons (ethane).
  • Misinterpreting linear molecules like HCN as non-planar; remember that a straight line is technically contained within a 2D plane.
Mark Allocation: [1 mark] awarded for selecting option B.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.