OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 16

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 16 asks: Which property explains the low reactivity of alkanes? Options are A: Low C-C bond enthalpy., B: Low bond enthalpy of pi-bonds., C: Low polarity of sigma-bonds., D: Low reactivity of carbon and hydrogen. A box is provided for the answer.
Question text

16 Which property explains the low reactivity of alkanes?

A Low C–C bond enthalpy.

B Low bond enthalpy of π‑bonds.

C Low polarity of σ‑bonds.

D Low reactivity of carbon and hydrogen.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer for question 16 is C, worth 1 mark.

16 C 1

How to answer it

Reactivity and Bonding in Alkanes

What this question tests

This question assesses your core understanding of alkane bonding, specifically sigma (sigma) bonds, electronegativity differences between carbon and hydrogen, and how molecular structure dictates chemical inertness.

Question 16

Exam Breakdown

✅ Correct Answer: C

Alkanes are unreactive because they possess strong sigma-bonds with very low polarity due to the similar electronegativities of carbon and hydrogen.

💡 Key Knowledge

  • Alkanes only contain single covalent bonds (sigma-bonds formed by end-on overlap of orbitals).
  • Carbon (2.5) and Hydrogen (2.1) have very similar electronegativity values, meaning the C-H bond is essentially non-polar.
  • Sigma-bonds also have high bond enthalpies, requiring significant energy to break.

🧠 Exam Technique

  • Eliminate options featuring pi-bonds (alkanes do not have double bonds).
  • Distinguish between bond enthalpy and bond polarity: while C-C and C-H bonds have high bond enthalpies, the primary reason alkanes lack sites for electrophilic or nucleophilic attack is their low polarity.

❌ Common Errors

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  • Choosing option B by confusing alkanes with alkenes (which contain reactive pi-bonds).
  • Selecting option A incorrectly thinking C-C bond enthalpy is the primary driver; C-C bonds are actually broken during cracking at high temperatures.
Mark Allocation: 1 mark available for selecting option C. Multiple-choice question with no working required.

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), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.