OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify the property that explains the low reactivity of alkanes from a multiple-choice list.

Practise this question

Question

Multiple choice question 15 asks which property explains the low reactivity of alkanes, with options A (Electron pair repulsion between sigma-bonds), B (Free rotation about sigma-bonds), C (High C-C bond enthalpy), and D (High polarity of the C-H bonds). An empty answer box is provided at the bottom left.
Question text

15 Which property explains the low reactivity of alkanes?

A Electron pair repulsion between σ-bonds

B Free rotation about σ-bonds

C High C–C bond enthalpy

D High polarity of the C–H bonds

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 15 with the correct answer C, awarding 1 mark.

15 C 1 1.1

How to answer it

Explaining the Low Reactivity of Alkanes

What this question tests: This question assesses your understanding of bonding, bond enthalpies, and polarity in saturated hydrocarbons (alkanes), and how these structural features explain their chemical inertness towards polar reagents and ionic species under normal conditions.
Question 15 (Multiple Choice)

The Correct Option & Breakdown

✅ Correct Answer: C

High C–C (and C–H) bond enthalpy makes alkanes unreactive because a significant amount of energy is required to break these strong covalent bonds during reactions.

💡 Key Knowledge

  • Alkanes contain only single covalent bonds (sigma σ-bonds).
  • Carbon and hydrogen have very similar electronegativities (C = 2.5, H = 2.1), making C–H bonds essentially non-polar.
  • Because they lack regions of high electron density (like pi-bonds) or significant permanent dipoles, they are not attacked by nucleophiles or electrophiles under standard conditions.

🧠 Exam Technique

When evaluating multiple-choice options about chemical reactivity, eliminate distractors by testing them against chemical principles. Ask yourself: Does this factor actually prevent a reaction from occurring? Free rotation (Option B) happens, but it has no bearing on bond-breaking energy requirements.

❌ Common Errors

  • Choosing D: Students often mistakenly think C–H bonds are polar enough to attract reagents. In reality, the electronegativity difference is too small to create a significant dipole.
  • Choosing A: Confusing the spatial arrangement/repulsion of sigma bonds with thermodynamic stability or bond strength.
Marks available: 1 mark | OCR Specification reference: 4.1.1 (Alkanes)

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

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.