AQA A-Level Chemistry Paper 2, 2021: Question 2
19 marks · Medium difficulty · Long Answer
This question explores fuels, particularly petrol and its production. It begins by testing understanding of the term “fraction” in crude oil processing. It then focuses on cracking hexadecane into smaller hydrocarbons like hexane and cyclopentane, and naming an appropriate catalyst. It moves into the environmental impact of burning fuels, explaining how carbon dioxide absorbs infrared radiation. A reaction mechanism is analysed for carbon capture using 2-aminoethanol, followed by structural and bonding analysis of protonated and unprotonated amine groups. Finally, bioethanol as a carbon-neutral alternative fuel is evaluated, including its production, relevant equations, and environmental trade-offs.
Practise this questionQuestion
Question text
02 This question is about fuels.
02.1 The petrol fraction obtained from crude oil can be used as fuel in cars.
State the meaning of fraction, as used in the term petrol fraction.
[1 mark]
02.2 Hexadecane (C16H34) can be cracked at high temperature to form petrol.
Complete the equation to show the cracking of one molecule of hexadecane to form
hexane and cyclopentane only.
Give the name of a catalyst used in this cracking reaction.
[3 marks]
C16H34 → +
Catalyst
02.3 Carbon dioxide is formed when petrol is burned.
Carbon dioxide acts as a greenhouse gas when it absorbs infrared radiation.
Give a reason why carbon dioxide absorbs infrared radiation.
[1 mark]
02.4 Compound Z (HOCH2CH2NH2) can be used to remove carbon dioxide from the
mixture of waste gases produced in some power stations.
Figure 1 shows part of a suggested mechanism for the reaction of Z with
carbon dioxide.
Figure 1
Draw two curly arrows to complete the mechanism in Figure 1.
Name compound Z (HOCH2CH2NH2)
Deduce the role of Z in step 2 of the mechanism.
[4 marks]
Name
Role
02.5 HOCH2CH2NH2 can be represented as XNH2
[HOCH CH NH ]+ can be represented as [XNH ]+
22 3 3
Draw the shape of XNH and of [XNH ]+
State whether the H–N–H bond angle in XNH2 is greater than, the same as, or smaller
than that in [XNH ]+
Explain your answer.
[4 marks]
Shape of XNH Shape of [XNH ]+
Bond angle
Explanation
02.6 Bioethanol is used as an alternative to fossil fuels.
This statement appeared on a website.
“The fact that bioethanol is a carbon-neutral fuel outweighs the environmental
disadvantages of producing bioethanol.”
Evaluate this statement.
In your answer you should include:
• an outline of how bioethanol is produced
• relevant equations
• analysis of the environmental impacts.
[6 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidelines Mark
A group of (hydrocarbons/compounds) with similar boiling points Allow compounds that boil in a similar range of 1
temperatures
02.1
Compounds with similar (carbon) chain length with
C5-C12 range or within range
zeolite Allow Aluminosilicate or aluminium oxide M1
02.2 All formulae correct M2
Balanced equation C16H34 → C6H14 + 2 C5H10 M3
The difference in energy between the ground and
C=O bonds vibrate at the same frequency as IR first excited vibrational state of CO2 is equal to the 1
energy of the infrared radiation.
02.3
Allow
Bond vibrations match frequency of IR radiation
C=O bonds vibrate in range 1680-1750cm-1
C=O bonds are polar
– A-LEVEL CHEMISTRY – –
Curly arrow from N lp to H M1
Curly arrow from N-H bond to N+ M2
02.4
Allow 2-hydroxyethylamine M3
2-aminoethanol 2-hydroxyethanamine
ethanolamine
Base Allow proton acceptor / removes H+ / electron pair
M4
donor
Allow these shapes with lines instead of wedges M1
and dashed lines
allow with/without lone pair
02.5
M2
allow with/without charge
Smaller Allow comparison of correct numbers
M3
lone (or non-bonding) pair repulsion greater than bond pair repulsion – A-LEVEL CHEMISTRY – – M4
This question is marked using Levels of Response. Refer to the Mark Indicative Chemistry content
Scheme Instructions for Examiners for guidance. Stage 1 names of processes
Level 3 All stages are covered and each stage is generally 1a Photosynthesis (is the natural process in plants
5-6 marks correct and virtually complete. that takes CO2 from the air)
Answer is communicated coherently and shows a logical 1b Fermentation (is the process used to make
progression from Stage 1 to Stages 2 and 3 bioethanol releasing some CO2)
Covers at least 2 points for stage 1, 2 for stage 2 and 3 1c Combustion (is the process where bioethanol is
for stage 3. burned and releases CO2)
Level 2 All stages are covered but stage(s) may be incomplete or Stage 2 Equations
3-4 marks may contain inaccuracies 2a 6CO + 6H O → C H O + 6O
22 6 12 6 2
02.6 OR two stages are covered and are generally correct 6
and virtually complete. 2b C6H12O6 → 2C2H5OH + 2CO2
Answer is communicated mainly coherently and shows a 2c 2C2H5OH + 6O2 → 6H2O + 4CO2
logical progression from Stage 1 to Stages 2 and 3. Stage 3 Carbon neutrality and environmental
Covers at least 1 point for stage 1 to stages 2 and 3. issues
Level 1 Two stages are covered but stage(s) may be incomplete 3a Deforestation / Sacrifice land that could be
1-2 marks or may contain inaccuracies OR only one stage is used for food
covered but is generally correct and virtually complete. 3b Loss of biodiversity / habitat
Answer includes isolated statements but these are not 3c 6CO2 in and 6CO2 out but It isn’t actually C
presented in a logical order. neutral as fuel is used in production, distribution,
etc
0 mark Insufficient correct chemistry to gain a mark
How to answer it
Study Guide: Fuels, Cracking and the Chemistry of Bioethanol
Q1. What is a Fraction?
Answer: A fraction is a group of hydrocarbons with similar boiling points or carbon chain lengths.
Q2. Cracking Hexadecane
Balanced equation: C16H34 → C6H14 + 2 C5H10
Catalyst: Zeolite (Allow: aluminium oxide / aluminosilicate)
Q3. Why CO₂ Absorbs Infrared Radiation
Answer: C=O bonds vibrate at the same frequency as IR radiation , allowing CO₂ to absorb energy as it traps heat.
Q4. Reaction of 2-Aminoethanol with CO₂
- Name of compound: 2-aminoethanol
- Curly arrows:
- From lone pair on nitrogen to the H in the ammonium group
- From N–H bond to the positively charged nitrogen
- Role in step 2: Base (proton acceptor)
Q5. Shape and Bond Angles: XNH₂ vs [XNH₃]+
Shape:
- XNH₂: Pyramidal
- [XNH₃]+: Trigonal planar
Bond Angle: XNH₂ has a larger bond angle
Explanation: Lone pair on XNH₂ causes greater repulsion than bond pairs in [XNH₃]+
Q6. Is Bioethanol Really Carbon Neutral?
Stage 1: How is it made?
- Photosynthesis:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - Fermentation:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ - Combustion:
2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O
Stage 2: Environmental impacts
- Bioethanol is considered carbon-neutral because plants absorb CO₂ before it’s released on burning.
- But: Energy used in farming, transport and processing may release additional CO₂.
- May involve deforestation or loss of food crops → loss of biodiversity
Topics
Organic Chemistry · 3.3.2 Alkanes · 3.3.5 Alcohols · 3.3.14 Organic Synthesis
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.