AQA A-Level Chemistry Paper 3, June 2025: Question 28

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement about the fractional distillation and cracking of alkanes.

Practise this question

Question

Question 28 asks: 'Alkanes are obtained from crude oil by fractional distillation. Which statement is correct?' Four multiple-choice options are given: A: 'Alkanes obtained are converted to alkenes by thermal cracking', B: 'Alkanes with longer chains rise to the top of the fractionating column', C: 'The diesel fraction contains sulfur compounds that react to form SO3 in car exhausts', and D: 'The boiling points of alkane isomers increase as the carbon chains become more branched'.
Question text

28 Alkanes are obtained from crude oil by fractional distillation.

Which statement is correct?

[1 mark]

A Alkanes obtained are converted to alkenes by thermal cracking.

B Alkanes with longer chains rise to the top of the fractionating column.

The diesel fraction contains sulfur compounds that react to form SO3 in

C

car exhausts.

The boiling points of alkane isomers increase as the carbon chains become

D

more branched.

Mark scheme

Show the mark scheme Mark scheme for question 28 showing the answer is A (1 mark, AO1): 'Alkanes obtained are converted to alkenes by thermal cracking.'

28 A 1 (AO1) Alkanes obtained are converted to alkenes by thermal cracking.

How to answer it

Fractional Distillation, Cracking, and Properties of Alkanes

📌 What this question tests

Core specification knowledge (AQA 3.3.1.1 & 3.3.1.2 - Alkanes):

  • The process and products of fractional distillation of crude oil.
  • The conditions and products of thermal cracking versus catalytic cracking.
  • Factors affecting the boiling points of alkanes (chain length and branching / intermolecular forces).
  • Combustion of sulfur-containing impurities and atmospheric pollutants ( SO₂ ).

Question 28 Analysis

Multiple Choice Question Breakdown (1 Mark, AO1)

✅ Correct Answer: A

"Alkanes obtained are converted to alkenes by thermal cracking."

Thermal cracking takes place at high temperature (approx. 700–1200 K) and high pressure (up to 7000 kPa). It breaks C–C bonds by homolytic fission to produce a high percentage of alkenes (used to make polymers) along with some shorter-chain alkanes.

💡 Key Knowledge Recap

  • Thermal Cracking: High temp (700–1200 K), high pressure (up to 7000 kPa) → produces mostly alkenes.
  • Catalytic Cracking: High temp (approx. 720 K), slight pressure, zeolite catalyst → produces branched alkanes, cycloalkanes, and aromatic hydrocarbons (motor fuels).
  • Fractionating Column: Temperature is hottest at the bottom and coolest at the top. Short chains have lower boiling points and rise to the top.
  • Branching: More branched alkanes have smaller surface contact areas → weaker van der Waals forces → lower boiling points.

❌ Why Options B, C, and D are Incorrect

  • Option B is FALSE: Longer carbon chains have greater molecular masses, more electrons, and stronger van der Waals forces, giving them higher boiling points. Consequently, they condense near the bottom where temperatures are higher. Shorter chains rise to the top.
  • Option C is FALSE: Combustion of sulfur impurities inside vehicle internal combustion engines produces sulfur dioxide (SO₂), not sulfur trioxide (SO₃):
    S + O₂ → SO₂
    (SO₂ may later be oxidised to SO₃ in the upper atmosphere, but car exhausts directly emit SO₂).
  • Option D is FALSE: As branching increases, molecules cannot pack together as closely, reducing the surface area of molecular contact. This decreases the strength of van der Waals forces, meaning branched isomers have lower boiling points than their straight-chain counterparts.

🧠 Exam Technique: Elimination Strategy

When approaching 1-mark multiple-choice questions on crude oil and alkanes:

  1. Identify standard conditions & products: Remember the mnemonic "Thermal = Alkenes (TA)" and "Catalytic = Branched / Aromatics / Cars (CB)".
  2. Sanity-check physical trends: Always picture packing and surface area when evaluating boiling points of isomers. Branching always disrupts packing and lowers van der Waals forces.
  3. Eliminate distractors quickly: B contradicts basic fractional distillation principles; D contradicts standard intermolecular force theory.

🎯 Mark Allocation

1 Mark total:

  • Mark given for selecting A.
  • Assessment Objective: AO1 (Demonstrate knowledge and understanding of scientific ideas, processes, techniques and procedures).

Topics

Organic Chemistry · 3.3.2 Alkanes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.