Edexcel A-Level Chemistry AS Paper 2, June 2019: Question 1

8 marks · Medium difficulty · Short Open Response

Identify the unbranched alkane with six carbon atoms in kerosene, explain why its branched isomers have lower boiling temperatures, explain two reasons for cracking hydrocarbons, and determine the changes in organic molecule proportions resulting from reforming.

Practise this question

Question

An exam question about alkanes, fractional distillation, cracking, and reforming. Part (a)(i) asks for the unbranched alkane with six carbon atoms found in kerosene. Part (a)(ii) asks to explain why isomers have lower boiling temperatures. Part (b) asks to explain two major reasons for cracking hydrocarbons. Part (c) is a multiple-choice question with a table showing options A to D for the effect of reforming on the proportion of branched chain alkanes and cyclic hydrocarbons.
Question text

1 Some alkanes are used as fuels for transport. Crude oil, which is a mixture of

hydrocarbons, is the major source of these alkanes. Crude oil is processed by

fractional distillation, cracking and reforming.

(a) Fractional distillation produces fractions which contain molecules of a similar

boiling temperature. Molecules containing six carbon atoms are found in both

the petrol fraction and the higher boiling kerosene fraction.

(i) Identify, by name or structural formula, the unbranched alkane with six

carbon atoms found in kerosene.

(1)

(ii) Explain why isomers of the alkane in (a)(i) have lower boiling temperatures

and so are found in the petrol fraction.

(3)

(b) During cracking, carbon to carbon bonds are broken.

Explain the two major reasons for cracking hydrocarbons.

(3)

… *P55611A0228*

(c) Which of the following changes to the proportions of organic molecules within a

fraction results from the reforming process?

(1)

Proportion of branched chain Proportion of cyclic

alkanes hydrocarbons

A decrease decrease

B decrease increase

C increase decrease

D increase increase

(Total for Question 1 = 8 marks)

Mark scheme

Show the mark scheme The mark scheme for the question providing acceptable answers and additional guidance for parts 1(a)(i), 1(a)(ii), 1(b), and 1(c). Part 1(a)(i) accepts hexane or its structural formula. Part 1(a)(ii) awards marks for mentioning branched chains, lower surface area/less packing, and weaker London forces. Part 1(b) accepts formation of shorter chain alkanes and alkenes, usefulness for polymers, and higher demand/fuel value. Part 1(c) identifies option D as the correct answer.

How to answer it

Alkanes, Cracking and Reforming Study Guide

Edexcel AS Level Chemistry

What this question tests

This question assesses your knowledge of crude oil processing, structural isomerism in alkanes, boiling temperature trends linked to intermolecular forces (London forces), the economic and industrial purposes of cracking, and the outcomes of catalytic reforming.

Part (a)(i): Identifying Alkanes

Identifying an unbranched six-carbon alkane

✅ Correct Answer

hexane or structural formula CH₃CH₂CH₂CH₂CH₂CH₃ (or displayed/skeletal formula).

❌ Common Errors

Writing hexene instead of hexane (loses the mark as it names an alkene). Molecular formula C₆H₁₄ was ignored by examiners, but always provide names or clear structures when asked.

Mark: 1 mark

Part (a)(ii): Isomers and Boiling Temperatures

Explaining why branched isomers have lower boiling temperatures

✅ Correct Answer

  • Isomers found in the petrol fraction have branched chains (Point 1).
  • Branched chains have a lower surface area and cannot pack as closely together (Point 2).
  • This results in weaker intermolecular forces / London forces / van der Waals' forces between molecules, requiring less energy to overcome (Point 3).

💡 Key Knowledge

Boiling temperature depends heavily on molecular shape. Unbranched molecules pack tightly, maximizing surface contact points for instantaneous dipole-induced dipole forces. Branching introduces "bumps" that keep molecules apart.

🧠 Exam Technique

To secure all 3 marks, you must explicitly link branching to surface area / close packing, and finally connect that to the weaker intermolecular forces. Watch out: never say "breaking covalent bonds"—boiling only overcomes intermolecular forces!

Mark: 3 marks

Part (b): Reasons for Cracking

Explaining the two major reasons for cracking hydrocarbons

✅ Correct Answer

  • Shorter-chain alkanes and alkenes are formed (Point 1).
  • Alkenes are useful starting materials in organic synthesis / used for making polymers and plastics (Point 2).
  • Shorter-chain alkanes are in higher demand, have higher economic value, or make better fuels (Point 3).

❌ Common Errors

Students often lose marks by simply stating "cracking makes molecules more useful" without specifying why or naming the specific utility (e.g., polymers/plastics or higher-demand fuels).

Mark: 3 marks

Part (c): Reforming Process

Identifying changes to organic molecules during reforming

✅ Correct Answer

D (increase / increase)

Catalytic reforming converts straight-chain alkanes into branched-chain alkanes and cyclic hydrocarbons to improve petrol anti-knock properties. Therefore, the proportions of both branched-chain alkanes and cyclic hydrocarbons increase.

🧠 Exam Technique

Recall the definition of reforming: straight chains are turned into rings (cyclic) and branches. Both categories increase in proportion within the processed fraction.

Mark: 1 mark

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.