OCR A-Level Chemistry AS Depth in chemistry (02), June 2025: Question 4

17 marks · Medium difficulty · Structured Questions

State and explain the trend in boiling points of alkanes, perform combustion calculations and draw an enthalpy profile diagram, and describe the isomerism and free-radical substitution of alkanes.

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Question

Question 4 displays Table 4.1 containing data for straight chain alkanes: propane, butane, pentane, and hexane, showing molecular formulae, boiling points (-42, 0, 36, and 69 degrees Celsius), and enthalpy change of combustion (-2219, estimated, -3509, -4163 kJ/mol). Part (a) asks to state and explain the boiling point trend (4 marks). Part (b) focuses on pentane combustion: writing the balanced equation, calculating the mass of CO2 per 1.00 kJ of energy released, completing an enthalpy profile diagram, and estimating the enthalpy of combustion of butane to 2 significant figures. Part (c) asks for skeletal formulae and systematic names of two structural isomers of C5H12. Part (d) focuses on free-radical bromination of propane, requiring the overall equation, definition of homolytic fission, equations for propagation steps, and an explanation for why two structural isomers of C3H7Br form.
Question text

4 The alkanes belong to a homologous series of hydrocarbons.

Table 4.1 shows information about some straight chain alkanes.

Table 4.1

Alkane Molecular Boiling point Enthalpy change of

formula / °C combustion / kJ mol–1

Propane C3H8 –42 –2219

Butane C4H10 0 To estimate in part (b)(iv)

Pentane C5H12 36 –3509

Hexane C6H14 69 –4163

(a) State and explain the trend in boiling points of the straight chain alkanes in Table 4.1.

… [4]

(b) When alkanes undergo complete combustion carbon dioxide and water are produced.

(i) Write the balanced equation for the complete combustion of one mole of pentane.

… [1]

(ii) Use the information in Table 4.1 to calculate the mass of carbon dioxide formed when 1.00 kJ of

energy is released during the complete combustion of pentane.

Mass of carbon dioxide … g [2]

(iii) Complete the enthalpy profile diagram for the complete combustion of pentane.

On your diagram:

• Label the enthalpy change as ΔH.

• Include the formulae of the reactants and products.

• Label the activation energy as Ea.

Enthalpy

Progress of reaction

[2]

(iv) Use the data in Table 4.1 to estimate a value for the enthalpy change of combustion of butane.

Give your answer to 2 significant figures.

Estimated enthalpy change of combustion of butane … kJ mol–1 [1]

(c) Besides pentane, there are two other structural isomers of C5H12.

Draw the skeletal formulae of these two other structural isomers and state the systematic name

of each.

Isomer 1 Isomer 2

Skeletal

formula

Systematic

name

[2]

(d) In the presence of ultraviolet radiation, propane reacts with bromine to form a mixture of products.

Two of these products are structural isomers of C3H7Br.

(i) Write an equation, using molecular formulae, for the formation of C3H7Br from propane.

… [1]

(ii) The first step in the mechanism of the reaction is the homolytic fission of a Br–Br bond.

Explain what is meant by homolytic fission.

… [1]

(iii) Complete the equations for the propagation steps in the mechanism.

Use molecular formulae for organic species and dots (•) for unpaired electrons on radicals.

C3H8 + Br• … + …

… + … C3H7Br + …

[2]

(iv) Explain why two structural isomers of C3H7Br are formed.

… [1]

Mark scheme

Show the mark scheme Mark scheme for Question 4 provides marking points across parts (a) to (d). (a) Boiling point increases as chain length increases due to more surface contact, stronger induced dipole-dipole interactions, and requiring more energy to break (4 marks). (b)(i) C5H12 + 8O2 -> 5CO2 + 6H2O (1 mark). (b)(ii) Mass of CO2 = (5 x 44.0)/3509 = 0.0627 g or 0.063 g (2 marks). (b)(iii) Enthalpy profile showing exothermic curve with Ea from reactants to peak and ΔH arrow pointing downwards to products below reactants (2 marks). (b)(iv) -2900 kJ/mol (1 mark). (c) Skeletal structures and names for 2-methylbutane and 2,2-dimethylpropane (2 marks). (d)(i) C3H8 + Br2 -> C3H7Br + HBr (1 mark). (d)(ii) Bond breaking where each bonding atom receives one electron from the pair (1 mark). (d)(iii) Propagation steps: C3H8 + Br• -> •C3H7 + HBr and •C3H7 + Br2 -> C3H7Br + Br• (2 marks). (d)(iv) Bromine can substitute at different positions along the carbon chain (1 mark).

Question Answer Mark Guidance

4 (a) 4 ANNOTATE WITH TICKS AND CROSSES

Comparisons needed throughout

ORA throughout

IGNORE references to Enthalpy of Combustion

Trend:

Boiling point increases (down the series) ALLOW the following for ‘chain length increases’

AND • Longer molecule/alkane

As chain length increases • Number of carbons increases (question says they are

straight chain alkanes)

DO NOT ALLOW reference to presence of branching

Explanation:

Surface area alone is not sufficient, must have idea of contact.

More (surface) contact / interaction (between molecules)

IGNORE comments about packing

ALLOW more electrons (as chain length increases)

More /stronger induced dipole(–dipole) interactions/ London

(dispersion) forces (between molecules) IGNORE van der Waals’/vdw forces

DO NOT ALLOW reference to other intermolecular forces e.g.

permanent dipole(-dipole) or hydrogen bonding.

More energy to break induced dipole(–dipole)

interactions/London forces/intermolecular forces/intermolecular ALLOW ‘more energy to break intermolecular forces’ if

bonds intermolecular forces are not identified or incorrect.

IGNORE harder to overcome/break intermolecular forces (no

reference to energy)

IGNORE just ‘bonds’ intermolecular/London forces required

DO NOT ALLOW Covalent bonds break

4 (b) (i) C5H12 + 8O2 → 5CO2 + 6H2O 1 IGNORE state symbols

DO NOT ALLOW multiples

4 (b) (ii) FIRST CHECK THE ANSWER ON THE ANSWER LINE 2 ALLOW ECF from incorrect mole ratio in equation (b)(i)

If answer = 0.063 OR 0.0627(g) award 2 marks

Mass CO2 produced from one mole C5H12

= 5 x 44.0 OR 220 (g) calculator value 0.06269592476

Mass of CO2 produced per 1.00 kJ IGNORE significant figures, marks can be awarded based on

220 correct method e.g. ALLOW 0.06 for final answer provided

= 3509 = 0.0627 (g)

working is shown

Alternative approaches:

The calculation has 3 steps which can be done in any order:

• X 5

• X 44

• Divide by 3509

Need 2 steps for first mark, and final step scores second mark.

Moles of CO2 per 1.00 kJ

= (1.00/3509) x 5 OR 1.4249… x 10-3

Mass of CO2 produced per 1.00 kJ

= 1.4249… x 10-3 x 44 = 0.0627 (g)

OR

Mass of CO2 per mol per 1.00 kJ

= 44/3509 OR 0.012539…

Mass of CO2 produced per 1.00 kJ

= 0.012539… x 5 = 0.0627 (g)

4 (b) (iii) 2 ANNOTATE ANSWER WITH TICKS AND CROSSES ETC

17 IGNORE state symbols

ALLOW 1 mark for a correctly labelled endothermic diagram

IGNORE stoichiometry even if incorrect

ALLOW ECF from incorrect reactant and products in equation

(b)(i)

For Ea, ALLOW AE OR AE

Reactants, products and Ea ALLOW arrowheads at each end of Ea line OR no arrowhead

Reactants on LHS C5H12 + O2 BUT DO NOT ALLOW arrowhead down

AND

Products on RHS CO2 + H2O Ea line must reach maximum (or near to maximum) on curve

AND

Activation energy correctly labelled / Ea

DO NOT ALLOW –ΔH

H DO NOT ALLOW double headed arrow on ΔH

H labelled with product below reactant ALLOW ΔH arrow even with small gap at the top and bottom, i.e.

AND line does not quite reach reactant or product line.

Arrow downwards ALLOW –3509 for ΔH

4 (b) (iv) –2900 (kJ mol–1) 1 Must be 2 SF with negative sign

4 (c) 2

ALL 4 points → 2 marks

2 OR 3 points → 1 mark

Structures must be skeletal

(2-)methylbutane (2,2-)dimethylpropane

Numbers in names are not required but if given must be

correct

IGNORE lack of hyphens, extra hyphens, or addition of

commas

DO NOT ALLOW the following for methyl: methy, meth, methly

4 (d) (i) C3H8 + Br2 → C3H7Br + HBr 1 IGNORE state symbols

4 (d) (ii) (covalent/Br-Br) bond breaks 1 ALLOW the breaking of (a covalent/Br-Br) bond where each atom

AND keeps one of the bonding electrons

each (bonding) atom / Br receives one electron from the ALLOW when a bond breaks one electron from the bond goes to

each product / species / radical

bonding / shared pair

DO NOT ALLOW ‘molecule’ or ‘compound’ or ‘particle’ or

‘element’ for ‘atom’

IGNORE homolytic fission equations

4 (d) (iii) C3H8 + Br• → •C3H7 + HBr 2 ALLOW dot at any position on the radical

•C3H7 + Br2 → C3H7Br + Br• ALLOW 1 mark if both equations correct but any dots omitted

from radicals

4 (d) (iv) Br can substitute at different positions along (carbon) chain 1 ALLOW AW

e.g

ALLOW Br can replace an end H or a middle H

ALLOW Br can substitute at any C

OR

two different radicals form CH3CH2CH2 AND CH3CHCH3 IGNORE position of radical dot on radical structures

IGNORE references to minor/major product

DO NOT ALLOW stability of carbocations/haloalkanes

How to answer it

Alkanes: Physical Trends, Combustion, Isomerism & Free-Radical Substitution

📋 What This Question Tests

This question assesses core physical and organic chemistry fundamentals from Module 2 and Module 4 of OCR AS Chemistry:

  • Intermolecular forces & physical trends: Explaining boiling points using London dispersion forces and surface contact area.
  • Energetics & quantitative stoichiometry: Writing balanced combustion equations, calculating product mass from enthalpy change (ΔH), and plotting exothermic reaction profiles.
  • Data estimation & reporting: Identifying patterns in homologous series to estimate missing thermochemical data to the requested significant figures.
  • Structural isomerism: Drawing precise skeletal formulas and applying IUPAC nomenclature rules for branched alkanes.
  • Free-radical substitution mechanism: Defining homolytic bond fission, writing accurate propagation equations with unpaired electron dots (•), and explaining structural isomer formation.

Part (a) — Boiling Point Trends of Straight Chain Alkanes

4 Marks • Trend Identification and Intermolecular Explanation

✅ Model Answer (4 Marks)

  • Trend: Boiling point increases as the carbon chain length (or number of carbon atoms) increases. [1]
  • Surface contact: Longer molecules have more points of surface contact between adjacent molecules. [1]
  • Force strength: This results in stronger (or more) induced dipole–dipole interactions (London dispersion forces). [1]
  • Energy: More energy is required to overcome these stronger intermolecular forces. [1]

🧠 Exam Technique & Mark Scheme Logic

To secure all 4 marks, your explanation must be comparative throughout:

  • Always link: Chain length ↑ → Surface contact ↑ → London forces ↑ → Thermal energy needed to separate molecules ↑.
  • Use the term "surface contact" or "surface interaction". Simply stating "larger surface area" without mentioning contact was penalised in examiner reports.

❌ Common Errors to Avoid

  • Breaking covalent bonds: Never say "covalent C–C bonds break when boiling". Boiling only overcomes weak intermolecular forces.
  • Mentioning branching: The question explicitly specifies straight chain alkanes. Do not discuss branched chains.
  • Vague terminology: OCR mark schemes strictly state: IGNORE van der Waals / vdw forces. Always use induced dipole–dipole interactions or London forces.

Part (b)(i) & (b)(ii) — Combustion Equation & Stoichiometric Energy Calculation

1 Mark + 2 Marks • Complete Combustion & Enthalpy Stoichiometry

✅ (b)(i) Balanced Equation (1 Mark)

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Note: Multiples are not allowed because the prompt specifies "one mole of pentane". State symbols are not required.

📐 (b)(ii) Step-by-Step Calculation

Question: Calculate mass of CO₂ produced when 1.00 kJ of energy is released.

  1. Find energy per mole of pentane:
    From Table 4.1, ΔH_c = -3509 kJ mol⁻¹. Thus, 1 mol C₅H₁₂ releases 3509 kJ.
  2. Find moles/mass of CO₂ per mole of pentane:
    From balanced equation: 1 mol C₅H₁₂ → 5 mol CO₂.
    M(CO₂) = 12.0 + (2 × 16.0) = 44.0 g mol⁻¹.
    Mass of CO₂ per mole of pentane = 5 × 44.0 g = 220 g. [1 mark]
  3. Scale down to 1.00 kJ:
    Mass of CO₂ = 220 g / 3509 kJ = 0.0627 g (or 0.063 g). [1 mark]

💡 Alternative Valid Methods

You can also work via moles of fuel per kJ:

  • Moles pentane per 1.00 kJ = 1 / 3509 = 2.8498 × 10⁻⁴ mol
  • Moles CO₂ = 5 × (2.8498 × 10⁻⁴) = 1.4249 × 10⁻³ mol
  • Mass CO₂ = 1.4249 × 10⁻³ × 44.0 = 0.0627 g
Error Carried Forward (ECF): Allowed if you had an incorrect stoichiometry in (b)(i).

Part (b)(iii) & (b)(iv) — Enthalpy Profile & Data Estimation

2 Marks + 1 Mark • Exothermic Reaction Profiles & Homologous Trends

✅ (b)(iii) Enthalpy Profile Diagram (2 Marks)

  • Mark 1: Products line drawn below reactants line. Reactants labelled as C₅H₁₂ + O₂ (or 8O₂) and products labelled as CO₂ + H₂O (or 5CO₂ + 6H₂O). Activation energy, Ea, shown with a single-headed or double-headed arrow pointing upwards from the reactant level to the peak of the curve.
  • Mark 2: Enthalpy change, ΔH, labelled with a single-headed arrow pointing strictly downwards from the reactant level to the product level.
Enthalpy ▲ ╭--▲--╮ (Peak) │ / │ \ │ / │ Ea \ │ ────────┴─────┼───── C₅H₁₂ + 8O₂ (Reactants) │ (Reactant level)│ │ │ │ ΔH (arrow pointing DOWN) │ ▼ ▼ │ ──────── 5CO₂ + 6H₂O (Products) │ └──────────────────────────────────────► Progress of reaction

✅ (b)(iv) Estimating Butane ΔH_c (1 Mark)

  • Propane (C₃H₈): -2219 kJ mol⁻¹
  • Pentane (C₅H₁₂): -3509 kJ mol⁻¹
  • Butane (C₄H₁₀) is midway between propane and pentane:

Value = (-2219 + -3509) / 2 = -2864 kJ mol⁻¹

Rounding to 2 significant figures gives:

-2900 kJ mol⁻¹

⚠️ Trap: You MUST include the negative sign (-) and quote strictly to 2 significant figures. An answer of 2900 or -2864 scores 0.

Part (c) — Structural Isomerism of Pentane (C₅H₁₂)

2 Marks • Skeletal Formulae and Systematic Nomenclature

✅ Skeletal Formulae & IUPAC Names

Mark allocation: 4 points correct (2 structures + 2 names) = 2 marks; 2 or 3 points correct = 1 mark.

Isomer Skeletal Description Systematic Name
Isomer 1 4-carbon zigzag main chain with a single vertical branch at carbon 2:
/\_ with branch | at C2
 2-methylbutane
(methylbutane is accepted)
Isomer 2 Central carbon with 4 arms radiating outwards (plus sign / X-cross shape):
>< shape (central quaternary C)
 2,2-dimethylpropane
(dimethylpropane is accepted)

❌ Common IUPAC & Skeletal Slip-ups

  • Showing carbon letters: Skeletal formula must never show letter 'C' for carbons along the chain or at vertices.
  • Showing hydrogens on carbons: Hydrogen atoms bonded to carbon must not be explicitly written.
  • Spelling errors: "methy", "meth", or "methly" are strictly rejected by the examiner.
  • Redundant numbering: Calling isomer 1 "3-methylbutane" is incorrect because numbering must give the substituent the lowest possible locant.

Part (d) — Free-Radical Substitution of Propane

5 Marks Total • Overall Equation, Definitions, Mechanism Steps & Isomer Origin

✅ (d)(i) Overall Formation Equation (1 Mark)

C₃H₈ + Br₂ → C₃H₇Br + HBr

Must use molecular formulas as required by question. HBr is a compulsory byproduct.

💡 (d)(ii) Definition: Homolytic Fission (1 Mark)

The breaking of a covalent bond where each bonding atom receives one electron from the shared pair (forming two radicals).

⚠️ Do NOT write 'molecule', 'compound', or 'ion' instead of atom.

📐 (d)(iii) Propagation Steps (2 Marks)

Complete the two successive propagation reactions:

Step 1: C₃H₈ + Br• → •C₃H₇ + HBr

Step 2: •C₃H₇ + Br₂ → C₃H₇Br + Br•
  • The radical dot (•) may be placed anywhere on the organic radical (e.g. •C₃H₇ or C₃H₇•).
  • If dots are omitted from radicals across both steps, a maximum of 1 mark can be awarded.

🧠 (d)(iv) Why Two Structural Isomers Form (1 Mark)

Accepted Explanations:

  • Bromine can substitute at different positions along the carbon chain (at carbon-1 or carbon-2 / end hydrogen vs middle hydrogen).
  • OR: Two different intermediate radicals can form: 1-propyl radical ( CH₃CH₂CH₂• ) and 2-propyl radical ( CH₃CH•CH₃ ).
❌ Examiner Warning: Do NOT explain this using the stability of carbocations. Free-radical substitution proceeds via neutral radicals, not carbocations!

Topics

Module 4: Core organic chemistry · Module 3: Periodic table and energy · Module 2: Foundations in chemistry · 4.1 Basic concepts and hydrocarbons · 3.2 Physical chemistry · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.