Edexcel A-Level Chemistry Paper 2, June 2022: Question 2
4 marks · Medium difficulty · Short Open Response
Define a hydrocarbon, explain the difference in boiling temperatures between 2,2-dimethylpropane and pentane, and identify the product of heterolytic bond fission in an alkane.
Practise this questionQuestion
Question text
2 This is a question about hydrocarbons.
(a) State what is meant by the term hydrocarbon.
(1)
(b) Explain why 2,2-dimethylpropane has a much lower boiling temperature than its
isomer pentane.
Detailed descriptions of the forces involved are not required.
(2)
(c) The heterolytic bond fission of a sigma (σ) bond in an alkane would produce
(1)
A only carbocations
B only free radicals
C free radicals and ions
D ions
(Total for Question 2 = 4 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
2(a) An answer which makes reference to: (1)
• a compound of hydrogen and carbon only Allow absence of ‘only’
Allow
substance/molecule/chain/species
for compound
Do not award reference to a
carbon and/or a hydrogen
Do not award ‘an element made
of carbon and hydrogen’
Do not award a mixture of carbon
and hydrogen
Do not award contains carbon
and hydrogen molecules
Question
Answer Additional Guidance Mark
Number
2(b) An explanation which makes reference to the following points: Accept reverse argument (2)
• branching results in fewer/weaker London forces (1) Allow van der Waals / instantaneous
dipole-induced dipole / dispersion
forces
Ignore just intermolecular forces
Do not award ‘fewer electrons’
Do not award if covalent bonds broken
• due to less surface area/points of contact (1) Allow reference to less close packing of
molecules together
Question
Answer Mark
Number
2(c) The only correct answer is D (ions) (1)
A is not correct because both anions and cations are produced
B is not correct because homolytic fission produces free radicals
C is not correct because homolytic fission produces free radicals and heterolytic fission also produces
anions
(Total Question 2 = 4 marks)
How to answer it
Question 2: Hydrocarbons Study Guide
What this question tests
This question assesses foundational organic chemistry knowledge, specifically defining core terminology (hydrocarbons), applying intermolecular forces (London forces and branching) to physical properties like boiling points, and understanding bond fission mechanisms (heterolytic vs. homolytic fission).
State what is meant by the term hydrocarbon.
✅ Correct Answer
A compound consisting of hydrogen and carbon only.
💡 Key Knowledge
- The inclusion of the word "only" is critical. Omitting it means the definition could technically include compounds with other functional groups (like alcohols or halogenoalkanes).
❌ Common Errors
- Saying "an element made of carbon and hydrogen" (hydrocarbons are compounds, not elements).
- Stating "a mixture of carbon and hydrogen" (they are chemically bonded compounds, not mixtures).
- Forgetting the word "only".
Explain why 2,2-dimethylpropane has a much lower boiling temperature than its isomer pentane.
✅ Correct Answer
- Branching results in fewer / weaker London forces (or van der Waals / instantaneous dipole-induced dipole forces).
- Due to less surface area or fewer points of contact between molecules (or less close packing).
🧠 Exam Technique
- This is a classic 2-mark "structure-property" explanation. Link the shape (branching/spherical) directly to surface area, and then surface area to the strength of intermolecular forces.
- Accept the reverse argument for pentane (longer chain = greater surface area = stronger London forces).
❌ Common Errors
- Stating that covalent bonds are broken during boiling (boiling only overcomes intermolecular forces, not covalent bonds).
- Claiming branched molecules have "fewer electrons" (isomers have the exact same molecular formula and number of electrons!).
- Just saying "weaker intermolecular forces" without explaining *why* (surface area / packing).
The heterolytic bond fission of a sigma bond in an alkane would produce...
✅ Correct Answer
D - ions
💡 Key Knowledge
- Heterolytic fission: The bond breaks unevenly; one bonded atom takes both electrons from the shared pair, forming a cation (+) and an anion (-), which are both ions.
- Homolytic fission: The bond breaks evenly; each atom takes one electron from the shared pair, forming free radicals.
❌ Common Errors
- Choosing A (only carbocations) - heterolytic fission produces *both* a cation and an anion (positive and negative ions), not just positive ones.
- Choosing B or C - confusing heterolytic fission with homolytic fission (which produces free radicals).
Topics
Organic Chemistry · Physical Chemistry · Topic 6: Organic Chemistry I · Topic 2: Bonding and Structure
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.