AQA A-Level Chemistry Paper 2, 2018: Question 1

10 marks · Medium difficulty · State/Explain/Describe

Write equations and conditions for alkane combustion, catalytic cracking, free-radical chlorination of butane, and ozone depletion by CFCs.

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Question

Question text

01 This question is about the reactions of alkanes.

01.1 Alkanes can be used as fuels.

Give an equation for the combustion of heptane (C7H16) in an excess of oxygen.

[1 mark]

01.2 Heptane can be obtained from the catalytic cracking of hexadecane (C16H34) at a high

temperature.

Identify a suitable catalyst for this process.

Give one condition other than high temperature.

Give an equation for the catalytic cracking of one molecule of hexadecane to produce

one molecule of heptane, one molecule of cyclohexane and one other product.

[3 marks]

Catalyst

Condition

Equation

01.3 Alkanes can be used in free-radical substitution reactions to produce

halogenoalkanes.

Give equations for the propagation steps in the reaction of butane to form

2-chlorobutane.

[2 marks]

01.4 Chlorofluorocarbons (CFCs) are a group of halogenoalkanes currently banned in

many countries. They cannot be used as solvents or refrigerants because of their

effect on the environment.

*02* The structure of a CFC is shown.

Identify the radical produced from this CFC that is responsible for the depletion of

ozone in the atmosphere.

Explain, with the aid of equations, why a single radical can cause the decomposition

of many molecules of ozone.

[4 marks]

Radical

Explanation

Mark scheme

Show the mark scheme

Question Answers Mark Additional Comments/Guidance

C7H16 + 11O2 7CO2 + 8H2O Ignore state symbols

01.1 M1

Allow multiples

Zeolite OR aluminosilicate M1 Allow porous pot / aluminium oxide / alumina / silica /

silicon dioxide

Slight/moderate pressure

01.2 M2 Slightly above atmospheric – allow 1-5 atmospheres

C H C H + C H + C H / 100-500kPa

16 34 7 16 6 12 3 6

If incorrect radical or ambiguous radical lose M1 but

Cl + CH3CH2CH2CH3 CH3CH2CHCH3 + HCl can award M2 for ecf in each equation.

01.3 + Cl CH CH CHClCH +

CH3CH2CHCH3 2 3 2 3 Cl M2 Allow equations in either order

Allow dot anywhere on the second carbon

Ignore extra initiation and termination steps

Cl. M1 Allow Cl or Chlorine in M1 and M4

Cl. + O ClO. + O M2 Penalise absence of dot once in the equations

Allow dot anywhere on the radical

ClO. + O Cl. + 2O M3

32 Apply the list principle in the equations and penalise

01.4

. M4 initiation from Cl2

Cl is regenerated (and causes a chain reaction in the

decomposition of ozone) Allow equations in either order.

Ignore Cl. acts as a catalyst

Total 10

How to answer it

Alkanes: Combustion, Cracking & Halogenation

📌 What this question tests

This question assesses fundamental organic mechanisms and industrial chemistry for alkanes:

Combustion Stoichiometry: Balancing complete combustion reactions in excess oxygen.
Catalytic Cracking: Industrial conditions (zeolite catalyst, moderate pressure) and stoichiometric balance of non-branched/cyclic products.
Free-Radical Substitution: Regioselective propagation steps targeting secondary halogenoalkanes (2-chlorobutane).
Environmental Halogenoalkane Chemistry: Catalytic breakdown of stratospheric ozone by chlorine free radicals derived from CFCs.

Question 01.1 • 1 Mark

Complete Combustion of Heptane

Writing balanced molecular equations for complete hydrocarbon combustion

✅ Correct Answer

C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O

[1 mark] for the correct balanced equation. Multiples are allowed (e.g. 2C₇H₁₆ + 22O₂ → 14CO₂ + 16H₂O). State symbols are not required.

🧠 Exam Technique

Carbon balance: 7 Carbons in C₇H₁₆ → produces 7CO₂ .
Hydrogen balance: 16 Hydrogens in C₇H₁₆ → produces 8H₂O (8 × 2 = 16).
Oxygen balance: (7 × 2) + 8 = 22 oxygen atoms needed → 11O₂ .

❌ Common Errors

Writing incomplete combustion products: The question states "in an excess of oxygen", so products must strictly be CO₂ and H₂O. Do not write CO or C (soot).
Miscounting oxygens: Forgetting that O₂ is diatomic and writing 22O₂ instead of 11O₂.

Question 01.2 • 3 Marks

Catalytic Cracking of Hexadecane

Conditions, catalysts, and balancing hydrocarbon cracking reactions

✅ Correct Answers

Catalyst: Zeolite (or aluminosilicate) [M1]

Condition: Slight / moderate pressure (or 1–5 atm / 100–500 kPa) [M2]

Equation: C₁₆H₃₄ → C₇H₁₆ + C₆H₁₂ + C₃H₆ [M3]

[3 marks total] 1 mark per bullet point. For the catalyst, aluminium oxide, silica, alumina, or porous pot are also accepted.

💡 Key Knowledge: Thermal vs Catalytic

Always distinguish the two types of cracking taught at A-Level:

Thermal: High temperature (400–900°C), high pressure (up to 70 atm). Produces mainly alkenes.
Catalytic: High temperature (~450°C), slight/moderate pressure, zeolite catalyst. Produces motor fuels (branched alkanes, cycloalkanes, aromatics).

📐 Deducing the Unknown Product

You are told hexadecane (C₁₆H₃₄) produces 1 heptane (C₇H₁₆) and 1 cyclohexane (cycloalkanes have general formula CₙH₂ₙ, so C₆H₁₂):

Remaining Carbons: 16 - (7 + 6) = 16 - 13 = 3 Carbons
Remaining Hydrogens: 34 - (16 + 12) = 34 - 28 = 6 Hydrogens
Unknown product = C₃H₆ (propene).

❌ Common Errors

Writing "high pressure": This is an immediate error. High pressure is required for thermal cracking, whereas catalytic cracking operates at slight/moderate pressure.
Incorrect formula for cyclohexane: Writing C₆H₁₄ (hexane) instead of C₆H₁₂ (cyclohexane ring has 2 fewer hydrogens).

Question 01.3 • 2 Marks

Free-Radical Substitution to Form 2-Chlorobutane

Regioselective propagation mechanisms

✅ Correct Equations

Step 1:
Cl• + CH₃CH₂CH₂CH₃ → CH₃CH₂C•HCH₃ + HCl

Step 2:
CH₃CH₂C•HCH₃ + Cl₂ → CH₃CH₂CHClCH₃ + Cl•

[2 marks total] 1 mark per correct step. Equations can be written in either order. Radicals can be displayed with dot on C2.

🧠 Exam Technique: Dot Placement

The question specifies 2-chlorobutane:

The radical formed in the first step must be a secondary radical: the radical dot must reside on carbon-2: CH₃CH₂C•HCH₃ .
If you place the dot on carbon-1 ( •CH₂CH₂CH₂CH₃ ), you will form 1-chlorobutane and lose M1!

❌ Common Errors

Forming CH₃CH₂CH₂CH₂Cl: Always re-read the target halogenoalkane isomer named in the stem.
Producing Cl• + H• instead of HCl: A chlorine radical abstracts a hydrogen atom to form stable covalent HCl , never free H• radicals.
Missing radical dots: Failing to clearly show the unpaired electron dot (•) loses the mark immediately.

Question 01.4 • 4 Marks

CFCs and Catalytic Ozone Depletion

Chain reactions, bond enthalpies, and ozone decomposition equations

✅ Correct Answer & Equations

Radical: Cl• (chlorine radical) [M1]

Equation 1: Cl• + O₃ → ClO• + O₂ [M2]

Equation 2: ClO• + O₃ → Cl• + 2O₂ [M3]

Explanation: Cl• is regenerated (it acts as a catalyst in a repeating chain reaction) [M4]

[4 marks total] 1 mark per point. Equations can be in either order. Dot omission penalised once across equations.

💡 Key Knowledge: Why Chlorine, Not Fluorine?

Although the molecule contains C-F bonds, the C-Cl bond is weaker (lower bond enthalpy) than the C-F bond:

UV radiation in the stratosphere breaks C-Cl homolytically:
R-Cl + hν → R• + Cl•
The C-F bond is too strong to be broken by this UV radiation, so fluorine radicals are not formed.

🧠 Overall Reaction Insight

Combining the two propagation steps gives the overall stoichiometric equation:

2O₃ → 3O₂

Notice that Cl• goes in at Step 1 and comes out unchanged at Step 2. This is the definition of a homogeneous catalyst, allowing one single Cl• radical to destroy thousands of ozone molecules before termination.

❌ Common Errors

Writing F• as the attacking radical: The C-F bond does not break under stratospheric UV light.
Stating only "it's a catalyst": The mark scheme specifically states "Ignore Cl• acts as a catalyst" on its own for M4. You must state that Cl• is regenerated!
Writing O instead of O₃: In AQA specifications, equations must show reaction with ozone ( O₃ ), not oxygen atoms (O).

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.2 Alkanes · 3.3.3 Halogenoalkanes

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