OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify which of the given alkanes has the highest boiling point.

Practise this question

Question

Multiple-choice question 1 asking which alkane has the highest boiling point, with four options: A, CH3(CH2)5CH3; B, (CH3)3CCH(CH3)2; C, CH3(CH2)3CH(CH3)2; D, (CH3)2CHCH2CH(CH3)2. A box labeled 'Your answer' is provided below the options, and [1] mark is indicated in the bottom right corner.
Question text

1 Which alkane has the highest boiling point?

A CH3(CH2)5CH3

B (CH3)3CCH(CH3)2

C CH3(CH2)3CH(CH3)2

D (CH3)2CHCH2CH(CH3)2

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme showing question number 1 with the correct answer A, worth 1 mark under AO1.2.

AO

Question Answer Marks Guidance

element

1 A 1 AO1.2

How to answer it

Identifying Alkane Boiling Points

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of intermolecular forces in organic molecules, specifically how chain length and branching affect London dispersion forces and boiling points in alkanes (Module 4.1.1 Basic concepts of organic chemistry).

Question 1 Analysis

✅ Correct Answer

A ( CH₃(CH₂)₅CH₃ )

Marks: 1/1 | AO element: AO1.2

💡 Key Knowledge

  • Alkanes are non-polar molecules held together by London dispersion forces (induced dipole-dipole interactions).
  • Longer, unbranched carbon chains have a greater surface area of contact between molecules.
  • More branching leads to a more spherical molecular shape, reducing the surface contact area and weakening London forces.

🧠 Exam Technique

  • Count the total number of carbon atoms in each option to check for isomers or chain length differences.
  • Option A is heptane ( C₇H₁₆ ) with an unbranched chain, maximising surface contact. Options B, C, and D are heavily branched isomers with fewer carbons in the straight chain or lower total surface area.

❌ Common Errors

Students often mistake branched alkanes for having higher boiling points because branched structures can sometimes pack differently in solids, or they confuse boiling point trends with melting point crystal packing trends.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.