AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2021: Question 7
11 marks · Standard Demand difficulty · Extended Answer
Answer a series of questions about crude oil, including its formation, properties of alkanes, a comparison of methane and hexane, cracking of decane, and the test for alkenes.
Practise this questionQuestion
Question text
07 Crude oil is a resource found in rocks.
Most of the compounds in crude oil are hydrocarbons.
07.1 Complete the sentence.
[1 mark]
Crude oil is formed by the decomposition of .
07.2 Alkanes are hydrocarbons.
Give the name of the alkane molecule that has three carbon atoms.
[1 mark]
07.3 Figure 7 shows two alkane molecules.
Figure 7
Table 4 shows the melting points and boiling points of methane and hexane.
Table 4
Melting point in °C Boiling point in °C
Methane −183 −162
Hexane −95 69
Compare the structure and properties of methane and hexane.
[6 marks]
Hydrocarbons are cracked to produce more useful alkanes and alkenes.
07.4 Decane (C10H22) is cracked to produce two products.
Complete the equation for the reaction.
[1 mark]
C10H22 → + C2H4
07.5 C2H4 is an alkene.
What is the test for alkenes?
Give the result of the test if an alkene is present.
[2 marks]
Test
Result
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
07.1 plankton allow microscopic plants / 1 AO1
animals 5.7.1.1
or
(ancient) biomass
07.2 propane allow C3H8 1 AO1
5.7.1.1
AO /
Question Answers Mark
Spec. Ref.
07.3 Level 2: Scientifically relevant features are identified; the way(s) in 4–6 AO3
which they are similar / different is made clear and (where
appropriate) the magnitude of the similarity / difference is noted.
Level 1: Relevant features are identified and differences noted. 1–3 AO1
AO2
No relevant content 0
5.2.2.4
Indicative content 5.7.1.1
• methane has 1 carbon atom, hexane has 6 5.7.1.3
• methane has 4 hydrogen atoms, hexane has 14
• both contain C – H bonds
• only hexane contains C – C bonds
• both are hydrocarbons
• hexane has a higher melting point than methane (or converse)
• hexane has a higher boiling point than methane (or converse)
• methane is a gas at room temperature
• hexane is a liquid at room temperature
• both are small molecules
• hexane has larger molecules than methane
• weak forces between molecules
• forces between hexane molecules stronger than between
methane molecules
• hexane is more viscous than methane
• both are flammable
• methane is more flammable than hexane (or converse)
• possible products of combustion from both are: carbon, carbon
monoxide, carbon dioxide, water
18 • neither conduct electricity
07.4 C8H18 1 AO2
5.3.1.1
5.7.1.4
07.5 bromine (water) 1 AO1
5.7.1.4
turns (from orange / brown) to MP2 is dependent on MP1 1
colourless allow decolourises
ignore clear
Total 11
How to answer it
Crude Oil, Alkanes, and Cracking
What this question tests
This question assesses your understanding of organic chemistry basics: how crude oil forms, naming and comparing the physical properties of alkanes, completing cracking equations, and the chemical test for alkenes.
Formation of Crude Oil
Correct Answer
Plankton or (ancient) biomass.
Key Knowledge
- Crude oil is a finite resource.
- It is formed over millions of years from the remains of tiny sea creatures (plankton) buried in mud.
Naming Alkanes
Correct Answer
Propane (or C₃H₈)
Exam Technique
Memorise the first four alkanes using a mnemonic like "Monkeys Eat Peeled Bananas":
- 1 Carbon: Methane
- 2 Carbons: Ethane
- 3 Carbons: Propane
- 4 Carbons: Butane
Comparing Methane and Hexane (6 Marks)
Structure and Properties
How to get 6 marks
This is a "Level 2" question. To get 4-6 marks, you must compare both the structure and the properties. Don't just list facts about one molecule; use comparative words like "higher than" or "larger than".
1. Comparing Structure
- Methane: 1 carbon, 4 hydrogens. Small molecule.
- Hexane: 6 carbons, 14 hydrogens. Larger molecule.
- Similarities: Both are hydrocarbons; both contain C-H single bonds.
- Differences: Only hexane contains C-C bonds.
2. Comparing Properties
- Boiling Point: Hexane has a much higher boiling point than methane.
- State: At room temperature (20°C), methane is a gas (it boiled at -162°C) but hexane is a liquid (it boils at 69°C).
- Intermolecular Forces: Hexane has stronger forces between molecules because it is a larger molecule.
Common Error: Bonding vs Forces
Never say "hexane has stronger covalent bonds" to explain boiling points. The covalent bonds inside the molecule do not break when it boils. It is the intermolecular forces (forces between molecules) that are overcome.
Cracking Equations
Step-by-Step Calculation
Cracking is a thermal decomposition reaction. The total number of atoms on the left must equal the total on the right.
Equation: C₁₀H₂₂ → ________ + C₂H₄
- Carbons: Start with 10. Subtract the 2 in the alkene.
10 - 2 = 8 Carbons - Hydrogens: Start with 22. Subtract the 4 in the alkene.
22 - 4 = 18 Hydrogens
Final Answer: C₈H₁₈
Test for Alkenes
Correct Answer
Test: Bromine water
Result: Turns from orange/brown to colourless.
Common Error: "Clear"
Examiners will often reject the word "clear". Water is clear, but it is also colourless. Bromine water starts as an orange liquid; when it reacts with an alkene, it loses its colour. Use the word colourless to be safe.
Topics
Chemistry · C7: Organic Chemistry
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.