Edexcel A-Level Chemistry Paper 2, June 2018: Question 1

4 marks · Easy difficulty · Multiple Choice

Identify properties and reactions of alkanes, including free-radical substitution, incomplete combustion, reforming, and molecular formula determination from skeletal structures.

Practise this question

Question

A multiple-choice question with four parts about alkanes. Part (a) asks to classify the reaction of ethane and chlorine into chloroethane and hydrogen chloride with options A addition, B elimination, C initiation, D substitution. Part (b) asks to identify the black smoke from incomplete combustion of alkanes with options A carbon particulates, B oxides of nitrogen, C oxides of sulfur, D unburnt hydrocarbons. Part (c) shows a reaction of hexane converting into cyclohexane and hydrogen gas, with options A elimination, B hydrogenation, C isomerisation, D reforming. Part (d) shows the skeletal formula of decalin (two fused six-membered rings) and asks for its molecular formula among options A C10H22, B C10H20, C C10H18, and D C10H16.
Question text

1 This question is about alkanes.

(a) The reaction of ethane and chlorine in UV radiation produces chloroethane.

C2H6 + Cl2 → C2H5Cl + HCl

This reaction is classified as

(1)

A addition

B elimination

C initiation

D substitution

(b) The black smoke produced from the incomplete combustion of alkane fuels is

(1)

A carbon particulates

B oxides of nitrogen

C oxides of sulfur

D unburnt hydrocarbons

(c) A reaction of hexane is shown below.

+ H2

This is best described as

(1)

A elimination

B hydrogenation

C isomerisation

D reforming

(d) The skeletal formula of decalin is

2 The molecular formula of decalin is

*P52293A0224* (1)

A C10H22

B C10H20

C C10H18

D C10H16

(Total for Question 1 = 4 marks)

Mark scheme

Show the mark scheme The mark scheme indicates the correct answers for each part: 1(a) is D, 1(b) is A, 1(c) is D, and 1(d) is C, with brief explanations for why the other options are incorrect.

Question

Acceptable Answer Mark

Number

1(a) The only correct answer is D (1)

A is incorrect because it is not an addition reaction

B is incorrect because no multiple bond is formed

C is incorrect because initiation is only the first stage in the mechanism of the reaction

Question

Acceptable Answer Mark

Number

1(b) The only correct answer is A (1)

B is incorrect because oxides of nitrogen are not black solids

C is incorrect because oxides of sulfur are not black

D is incorrect because unburnt hydrocarbons are not black solids

Question

Acceptable Answer Mark

Number

1(c) The only correct answer is D (1)

A is incorrect because no alkenes are produced

B is incorrect because hydrogen is formed

C is incorrect because the molecular formulae of the organic compounds are not the same

Question

Acceptable Answer Mark

Number

1(d) The only correct answer is C (1)

A is incorrect because there are not 22 hydrogen atoms

B is incorrect because there are not 20 hydrogen atoms

D is incorrect because there are not 16 hydrogen atoms

(Total for Question 1 = 4 marks)

How to answer it

A-Level Chemistry Study Guide: Alkanes Properties & Reactions

What this question tests

This multiple-choice sequence assesses core foundational knowledge of alkane chemistry, including free-radical halogenation mechanisms, incomplete combustion products, catalytic reforming definitions, and interpreting skeletal formulae to determine molecular formulae.

Question 1(a)

Halogenation of Ethane

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

✅ Correct Answer

D: substitution

Awarded 1 mark for selecting D.

💡 Key Knowledge

  • Alkanes react with halogens in the presence of UV light via free-radical substitution.
  • A hydrogen atom in the alkane is replaced by a halogen atom.

❌ Common Errors

  • Choosing C (initiation): Initiation is only the first step of the mechanism, not the overall reaction type.
  • Choosing A or B: Alkanes do not undergo addition or simple elimination under these conditions.
Question 1(b)

Incomplete Combustion of Alkanes

✅ Correct Answer

A: carbon particulates

Awarded 1 mark for selecting A.

💡 Key Knowledge

  • Incomplete combustion occurs when oxygen is in limited supply.
  • It produces carbon monoxide and solid carbon particulates (soot/black smoke), rather than just carbon dioxide and water.

❌ Common Errors

  • Confusing particulates with oxides of nitrogen or sulfur, which arise from high-temperature combustion of nitrogen in air or sulfur impurities in fossil fuels respectively, not alkane soot.
Question 1(c)

Reactions of Hexane

Hexane → Cyclohexane + H₂

✅ Correct Answer

D: reforming

Awarded 1 mark for selecting D.

💡 Key Knowledge

  • Catalytic reforming involves converting straight-chain alkanes into cycloalkanes or branched-chain alkanes, often releasing hydrogen gas in the process.

❌ Common Errors

  • Selecting A (elimination): While hydrogen is eliminated, reforming is the specific industrial term used for converting straight-chain alkanes into cyclic hydrocarbons.
Question 1(d)

Molecular Formula from Skeletal Formula

✅ Correct Answer

C: C₁₀H₁₈

Awarded 1 mark for selecting C.

🧠 Exam Technique & Counting

  • Carbons (10): Decalin consists of two fused six-membered rings (cyclohexane rings sharing a C-C bond). Total vertices/carbons = 10.
  • Hydrogens (18): Apply the general formula for a bicyclic alkane with two rings, CₙH₂ₙ₋₂ (since 2 rings require the removal of 4 hydrogens total compared to an open-chain alkane). Therefore, for n = 10, 2(10) - 2 = 18 hydrogens. Alternatively, count junctions (2 CH groups with 1 H each) and unbranched CH₂ carbons (8 × 2 H) = 2 + 16 = 18.

❌ Common Errors

  • Assuming a saturated hydrocarbon must fit the standard open-chain alkane formula CₙH₂ₙ₊₂ (which would give C₁₀H₂₂). Remember that ring structures reduce the hydrogen count by 2 for each ring present.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.