Edexcel A-Level Chemistry Paper 2, June 2017: Question 1
6 marks · Medium difficulty · Short Open Response
Identify reaction mechanisms, name branched alkanes from skeletal structures, explain fractional distillation of crude oil, and complete equations for cracking and reforming of alkanes.
Practise this questionQuestion
Question text
1 This is a question about alkanes.
(a) What is the reaction mechanism when ethane and chlorine react in UV light?
(1)
A electrophilic addition
B electrophilic substitution
C free radical addition
D free radical substitution
(b) What is the name of this alkane?
(1)
A 2-ethyl-3-propylpentane
B 4-ethyl-3-methylheptane
C 3-methyl-4-propylhexane
D 4-methyl-3-propylhexane
(c) Alkanes are obtained by processing crude oil.
(i) Explain why different alkanes in crude oil can be separated by fractional distillation.
(2)
(ii) Complete the equation for the cracking of octane to produce ethene and only
2 one other organic compound. State symbols are not required.
*P48059A0224* (1)
C8H18 r … + …
(iii) Write the equation for the reforming of hexane into cyclohexane, using
displayed formulae for the organic compounds. State symbols are not required.
(1)
(Total for Question 1 = 6 marks)
Mark scheme
Show the mark scheme
Question
Answer Mark
Number
1(a) The only correct answer is D (free radical substitution) (1)
A is not correct because electrophilic addition would be the reaction mechanism between alkenes,
such as ethene, and chlorine but the substance in the question is the alkane, ethane
B is not correct because electrophilic substitution involves the reactions of aromatic systems but this
question refers to the reaction of the alkane, ethane
C is not correct because the mechanism of reaction between an alkane such as ethane and chlorine
involves substitution and not addition
Question
Answer Mark
Number
1(b) The only correct answer is B (4-ethyl-3-methylheptane) (1)
A is not correct because the longest consecutive carbon chain involves the seven carbon atoms from
the top left of the molecular drawing down to the bottom right which means that the stem of the
name is not pentane but heptane, with the consequential effect on the numbering and length of side
chains
C is not correct because the longest consecutive carbon chain involves the seven carbon atoms from
the top left of the molecular drawing down to the bottom right which means that the stem of the
name is not hexane but heptane, with the consequential effect on the numbering and length of side
chains
D is not correct because the longest consecutive carbon chain involves the seven carbon atoms
from the top left of the molecular drawing down to the bottom right which means that the stem of
the name is not hexane but heptane, with the consequential effect on the numbering and length of
side chains
Question
Answer Additional Guidance Mark
Number
1(c)(i) An explanation that makes reference to the following (2)
points:
different alkanes have different Allow
boiling temperatures/points (1) Volatility for boiling temperature
Allow
Different alkanes condense at different
temperatures
Ignore melting temperatures if given with
boiling temperatures
Ignore densities
because of (different) chain length/molar mass Accept London /dispersion /van der Waals
/strength of intermolecular forces/ forces
number of electrons (1) Allow reference to size
A comparison such as ‘longer alkanes have
higher boiling points’ scores 2
Ignore any reference to surface area
Do not award references to cracking
Do not award reference to just weight/mass
Do not award incorrect trend
Question
Answer Additional Guidance Mark
Number
1(c)(ii) (1)
Correct equation Example of equation:
C8H18 → C2H4 + C6H14
OR
C8H18 → 2C2H4 + C4H10
OR
C8H18 → 3C2H4 + C2H6
Allow
CH2=CH2 for C2H4
Products can be given in either order
Do not award equations forming H2
Question
Answer Additional Guidance Mark
Number
1(c)(iii) (1)
Correct equation Example of equation:
Accept bonds to hydrogen atoms inside the ring, e.g.
Allow skeletal or structural formulae
for hexane and for cyclohexane
or CH3(CH2)4CH3 or (CH2)6
Ignore
C6H14 → C6H12 + H2
(Total for Question 1 = 6 marks)
How to answer it
Alkanes: Reactions, Nomenclature & Processing
This question assesses core organic chemistry knowledge regarding alkanes. Specifically, it tests your understanding of radical substitution mechanisms, IUPAC nomenclature rules for branched alkanes, fractional distillation principles based on intermolecular forces, and writing balanced equations for cracking and reforming reactions.
Part (a): Halogenation Mechanism
What is the reaction mechanism when ethane and chlorine react in UV light? (1 mark)
✅ Correct Answer
D: free radical substitution
💡 Key Knowledge
- Alkanes are unreactive due to strong, non-polar C-C and C-H sigma bonds.
- UV light provides the energy (homolytic fission) to break Cl-Cl bonds into free radicals.
❌ Common Errors
Confusing alkanes with alkenes. Alkenes undergo electrophilic addition, whereas alkanes undergo free radical substitution in the presence of UV light.
Part (b): IUPAC Nomenclature
What is the name of this alkane? (1 mark)
✅ Correct Answer
B: 4-ethyl-3-methylheptane
🧠 Exam Technique
Always find the longest continuous carbon chain first. In this structure, tracing from the top-left ethyl group down to the bottom-right carbon chain gives a chain of 7 carbons (heptane), not 5 or 6. Number from the end that gives the substituents the lowest locants (3,4 rather than 4,5).
❌ Common Errors
Choosing a straight-line horizontal or vertical path instead of the longest continuous chain. This leads to incorrect stem names like pentane or hexane.
Part (c)(i): Fractional Distillation
Explain why different alkanes in crude oil can be separated by fractional distillation. (2 marks)
✅ Correct Answers
1. Different alkanes have different boiling points (or volatilities).
2. This is due to differing chain lengths / molecular masses / strength of London (dispersion) forces.
💡 Key Knowledge
As molecular chain length increases, the number of electrons increases, leading to stronger London forces between molecules which require more thermal energy to overcome.
❌ Common Errors
Mentioning breaking of covalent bonds inside the molecules (fractional distillation only overcomes intermolecular forces, not covalent bonds). Vague references to "weight" or "size" without linking to intermolecular forces or boiling points often lose marks.
Part (c)(ii): Cracking of Alkanes
Complete the equation for the cracking of octane to produce ethene and only one other organic compound. (1 mark)
✅ Correct Answer
C₈H₁₈ → C₂H₄ + C₆H₁₄
Accept alternative valid cracking ratios such as 2C₂H₄ + C₄H₁₀ or 3C₂H₄ + C₂H₆ depending on the specific pairing, provided mass balance is maintained.
🧠 Exam Technique
Check your balancing by adding the subscript carbons and hydrogens of the products to ensure they equal the reactant (C = 8, H = 18 for octane).
❌ Common Errors
Including hydrogen gas ( H₂ ) as a product when the question specifically stipulates producing "ethene and only one other organic compound."
Part (c)(iii): Reforming of Alkanes
Write the equation for the reforming of hexane into cyclohexane, using displayed formulae for the organic compounds. (1 mark)
✅ Correct Answer
A balanced equation showing straight-chain hexane ( C₆H₁₄ ) drawn out fully with all C-H and C-C bonds displayed, reacting to form cyclohexane (a ring structure with all C-H and C-C bonds fully displayed) plus H₂ .
💡 Key Knowledge
Reforming involves converting straight-chain alkanes into cycloalkanes or branched chains, releasing hydrogen gas in the process to aromatize or cyclicize the structure.
❌ Common Errors
Using molecular or structural formulas (like C₆H₁₄ ) instead of fully displayed formulae. Every single bond and atom must be explicitly shown for the organic structures to gain the mark.
Topics
Organic Chemistry · Topic 6: Organic Chemistry I
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.