OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 2

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement regarding chemical bonds in organic molecules such as ethene and ethane.

Practise this question

Question

Multiple choice question 2 asking which statement about bonds is correct, with options A to D covering bond polarity, sigma and pi bonds, bond angles, and orbital overlap. Option A states the C=C bond in ethene is more polar than the C-C bond in ethane. Option B states a sigma-bond is stronger than a pi-bond. Option C states the H-C-H bond angle in ethane is greater than the H-C-H bond angle in ethene. Option D states a sigma-bond is formed from sideways overlap of p orbitals. An answer box is provided at the bottom.
Question text

2 Which statement about bonds is correct?

A The C=C bond in ethene is more polar than the C−C bond in ethane.

B A σ-bond is stronger than a π-bond.

C The H−C−H bond angle in ethane is greater than the H−C−H bond angle in ethene.

D A σ-bond is formed from sideways overlap of p orbitals.

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is B.

2 B 1 1.1

How to answer it

Evaluating Covalent Bonds, Orbital Overlap, and Bond Angles

What this question tests

This question assesses your foundational understanding of covalent bonding, specifically the differences between sigma ( σ ) and pi ( π ) bonds, orbital hybridisation, electronegativity and bond polarity, and VSEPR theory relating to bond angles in hydrocarbons like ethane and ethene.

Question Analysis & Correct Answer

Multiple Choice Question (Part 2)

Identifying the correct statement about chemical bonds

✅ Correct Answer: Option B

A σ -bond is stronger than a π -bond.

Sigma bonds involve direct end-on-end (head-on) orbital overlap, leading to a high electron density directly between the nuclei. This electrostatic attraction makes σ -bonds significantly stronger than π -bonds, which are formed by sideways overlap of p-orbitals with a lower degree of orbital overlap.

💡 Key Knowledge Breakdown

  • Sigma ( σ ) bonds: Formed by direct head-on overlap of orbitals (s-s, s-p, or p-p along the internuclear axis).
  • Pi ( π ) bonds: Formed by the sideways (parallel) overlap of adjacent p-orbitals above and below the bonding axis.
  • Polarity: Both C–C and C=C bonds are non-polar because carbon and hydrogen have very similar electronegativities (and C=C in ethene is between identical carbon atoms).
  • Bond Angles: Ethane is tetrahedral (109.5°), whereas ethene is trigonal planar (120°). Therefore, the H–C–H bond angle in ethane (109.5°) is smaller than in ethene (120°), making statement C false.

🧠 Exam Technique

In multiple-choice questions involving multiple statements, evaluate each option systematically rather than guessing. Cross out obviously incorrect statements first:

  • Option A is false because C and C have identical electronegativities (non-polar bond).
  • Option D is incorrect because sideways overlap creates π -bonds, not σ -bonds.
  • Option C gets the bond angles backwards (ethane is 109.5°, ethene is 120°).

❌ Common Errors & Misconceptions

  • Confusing orbital overlap types: Students frequently mix up head-on overlap ( σ ) with sideways overlap ( π ). Remember: σ comes first in single bonds, and double bonds contain one σ and one π .
  • Misremembering bond angles: Forgetting that double bonds restrict rotation and alter geometry (trigonal planar 120° vs tetrahedral 109.5°).
Mark Scheme Allocation: 1 mark awarded for selecting B. Specification reference: 2.1.2 (Compounds, formulae and equations) / 2.2.2 (Bonding).

Topics

Module 4: Core organic chemistry · Module 2: Foundations in chemistry · 4.1 Basic concepts and hydrocarbons · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.