AQA A-Level Chemistry AS Paper 2, June 2025: Question 16
1 mark Ā· Easy difficulty Ā· Multiple Choice
Identify which statement correctly describes the properties of crude oil fractions collected higher up in a fractionating column compared to those collected lower down.
Practise this questionQuestion
Question text
16 Crude oil is separated into fractions in a fractionating column at an oil refinery.
Which statement about the fractions collected higher up in the column, compared with
those collected lower down, is correct?
[1 mark]
A They are collected at a higher temperature.
B They have stronger intermolecular forces.
C They are more volatile.
D They are more viscous.
Mark scheme
Show the mark scheme
16 C 1 (AO1) They are more volatile.
How to answer it
Fractional Distillation of Crude Oil & Trends in Hydrocarbons
This question assesses basic recall and understanding of fractional distillation of alkanes (AQA Organic Chemistry: 3.3.1.2 Fractional distillation of crude oil):
- The temperature gradient across a fractionating column (cool at top, hot at bottom).
- How carbon chain length relates to boiling points and intermolecular London (van der Waals) forces.
- Physical property trends of fractions (volatility, viscosity, flammability, and boiling point).
Analysis & Correct Answer
ā Correct Answer
C: They are more volatile.
Fractions collected near the top of the column have shorter hydrocarbon chains, lower boiling points, and vaporise/evaporate more easily (they are more volatile).
ā¢ 1 mark (AO1): Correct option chosen is C.
š” Key Knowledge
In a commercial fractionating tower:
- Top of column: Cooler (~20 Ā°C ā 40 Ā°C), collects short-chain alkanes (low boiling points, weak van der Waals forces, high volatility, low viscosity).
- Bottom of column: Hotter (~350 Ā°C ā 400 Ā°C), collects long-chain alkanes (high boiling points, strong van der Waals forces, low volatility, high viscosity).
Trend Summary: Column Top vs Bottom
| Property | Higher up (Top of column) | Lower down (Bottom of column) |
|---|---|---|
| Collection Temperature | Lower (Cooler) | Higher (Hotter) |
| Carbon Chain Length | Shorter (e.g. CāāCā in refinery gases) | Longer (e.g. Cāā+ in fuel oil/bitumen) |
| Intermolecular Forces (van der Waals) | Weaker (fewer electrons, smaller surface area) | Stronger (more electrons, larger contact area) |
| Volatility (ease of evaporation) | Higher (More volatile) | Lower (Less volatile) |
| Viscosity (resistance to flow) | Lower (Runny / less viscous) | Higher (Thick / more viscous) |
š§ Exam Technique: Evaluating the Distractors
- A is incorrect: The top is cooler than the bottom, so top fractions condense and collect at a lower temperature.
- B is incorrect: Top fractions are smaller molecules with fewer electrons, meaning they have weaker London/van der Waals forces.
- D is incorrect: Short-chain hydrocarbons flow very easily, so they are less viscous, not more.
ā Common Misconceptions & Errors
- Confusing the gradient: Forgetting that columns are heated from the base, meaning temperature decreases with height.
- Misunderstanding "volatility": Confusing volatility with reactivity. Volatility strictly means a substance's tendency to vaporise (lower boiling point = higher volatility).
- Misreading the comparison direction: Mixing up "collected higher up compared to lower down" with "collected lower down compared to higher up". Always underline the subject of the comparison.
Topics
Organic Chemistry Ā· 3.3.2 Alkanes
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.