Edexcel A-Level Chemistry Paper 3, June 2023: Question 1
7 marks · Medium difficulty · Short Open Response
Complete combustion and incomplete combustion equations, environmental impacts of sulfur impurities, and reforming reactions of alkanes.
Practise this questionQuestion
Question text
1 Alkanes are often used as fuels.
(a) Complete the equation for the complete combustion of hexane.
State symbols are not required.
(1)
C6H14 + … O2 → … CO2 + … H2O
(b) Identify two pollutants produced from the incomplete combustion of a pure
sample of pentane.
(2)
(c) Explain the problem which results from sulfur impurities in fuels.
(2)
(d) Adding cyclic hydrocarbons to fuels results in more efficient combustion.
These compounds can be made by reforming straight-chain alkanes.
Write an equation for the reforming of pentane into a cyclic hydrocarbon, using
skeletal formulae for the organic compounds.
State symbols are not required.
(2)
(Total for Question 1 = 7 marks)
Mark scheme
Show the mark scheme
Question Answer Additional Guidance Mark
Number
Example of equation (1)
1(a) C6H14 + 9½O2 → 6CO2 + 7H2O
• correct balancing
Accept decimals/improper fractions
Allow multiples 2:19:12:14
Ignore state symbols even if incorrect
Question
Answer Additional Guidance Mark
Number
An answer that makes reference to two of the following points: Do not award hydrogen / H2 (2)
1(b)
• carbon / C (1) Allow particulates / soot
• carbon monoxide / CO (1) Ignore carbon dioxide
• nitrogen monoxide / NO / nitrogen dioxide / NO2 / Ignore ‘unburnt hydrocarbons’
nitrogen oxides / NOx / pentane / C5H12 (1)
Number
An explanation that makes reference to the following points: (2)
1(c)
• acid rain is formed (1) Causes respiratory difficulties/bronchitis
Allow description of breathing difficulties
Ignore just vague references to harm the
environment
Ignore reference to greenhouse effect/global
warming
• because sulfur oxides / sulfur dioxide / SO2 / sulfur trioxide / SO3 Allow (formation of ) sulfuric acid
(formed which dissolve in water) (1) Allow SOx but not SO
Number
Examples of equations (2)
1(d) • skeletal formulae (1)
• hydrogen molecule (product) to balance the equation (1)
Award (1) for use of non-skeletal formulae in
balanced equation
Ignore state symbols even if incorrect
Allow formation of cyclopentene with 2H2
Do not award M2 if any other species is/are given in
the equation but ignore any catalysts written above the
arrow
(Total Question 1 = 7 marks)
How to answer it
Alkanes and Fuels Study Guide
This question assesses your understanding of alkane chemistry, focusing on complete and incomplete combustion reactions, environmental impacts of fuel impurities (such as sulfur dioxide and acid rain), and organic reforming processes using skeletal formulae.
Complete Combustion of Hexane
✅ Correct Answer
C₆H₁₄ + 9.5O₂ → 6CO₂ + 7H₂O
(Accepts fractions like 19/2 O₂ or whole number multiples like 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O)
💡 Key Knowledge
- Complete combustion occurs in a plentiful supply of oxygen.
- Products of complete combustion are always carbon dioxide (CO₂) and water (H₂O).
🧠 Exam Technique
Balance elements in a set order: balance the carbons first, then the hydrogens, and leave oxygen to balance last.
❌ Common Errors
Students sometimes struggle with balancing odd/even oxygen atoms on products versus reactants, or forget to multiply all coefficients if scaling up.
Pollutants from Incomplete Combustion
✅ Correct Answer
Any two of the following (1 mark each):
- Carbon / C (soot / particulates)
- Carbon monoxide / CO
- Nitrogen monoxide / NO (or NO₂, NOx)
- Unburnt pentane / C₅H₁₂
💡 Key Knowledge
Incomplete combustion happens when oxygen is limited, producing toxic gases (CO) and particulates (C). High temperatures inside engines can also cause atmospheric nitrogen to react with oxygen, forming nitrogen oxides.
❌ Common Errors
Do not write hydrogen (H₂) as a combustion pollutant. Students also frequently lose marks by giving vague descriptions instead of specific chemical species.
Environmental Impact of Sulfur Impurities
✅ Correct Answer
Point 1: Acid rain is formed.
Point 2: Sulfur oxides / sulfur dioxide (SO₂) dissolve in water.
💡 Key Knowledge
Crude oil fractions contain sulfur impurities. When burned, sulfur reacts with oxygen to form sulfur dioxide (SO₂), which dissolves in atmospheric moisture to produce acid rain (sulfurous/sulfuric acid).
🧠 Exam Technique
Examiners penalise vague statements like "it harms the environment" or "causes global warming". Be explicit: name acid rain and state that sulfur oxides dissolve in water.
Reforming Pentane into a Cyclic Hydrocarbon
✅ Correct Answer
An equation showing straight-chain pentane converting into a cyclic hydrocarbon (such as cyclopentane) plus hydrogen gas (H₂), drawn using skeletal formulae.
C₅H₁₂ (skeletal ring) → Cyclopentane (skeletal) + H₂
💡 Key Knowledge
Reforming involves converting straight-chain alkanes into branched-chain alkanes or cyclic hydrocarbons, which burn more efficiently in car engines (preventing knocking).
🧠 Exam Technique
Read the instruction carefully: "using skeletal formulae". Drawing structural or molecular formulae will lose you the first mark, even if the equation is balanced correctly.
❌ Common Errors
Including extra unnecessary molecules or forgetting to include H₂ as a byproduct to balance the hydrogen count (pentane has 12 H atoms, cyclopentane has 10 H atoms, so + H₂ is required).
Topics
Organic Chemistry · Topic 6: Organic Chemistry I
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.