Edexcel A-Level Chemistry AS Paper 2, June 2023: Question 2

14 marks · Medium difficulty · Calculations

Calculate reaction quantities involving alkane combustion and free-radical substitution of methane, and apply the ideal gas equation.

Practise this question

Question

Question text

2 This question is about alkanes and some of their reactions.

(a) Hexane is a saturated hydrocarbon.

State what is meant by the term ‘saturated hydrocarbon’.

Your answer should refer to both words in the term.

(2)

(b) When methane reacts with gaseous chlorine, the initiation step is bond breaking

in the chlorine molecule to form chlorine free radicals.

The equation for this step is shown.

Cl2(g) → 2Cl•(g)

Two equations for the propagation step are shown.

Cl•(g) + CH4(g) → CH3•(g) + HCl(g)

CH3•(g) + Cl2(g) → CH3Cl(g) + Cl•(g)

(i) Write the overall equation for this reaction of methane with chlorine.

State symbols are not required.

(1)

(ii) Calculate the maximum mass, in grams, of chloromethane produced when

7.00g of chlorine gas reacts completely with excess methane.

[Molar mass of CH Cl = 50.5 g mol–1]

(2)

(iii) Write an equation for a termination step for this reaction.

State symbols are not required.

(1)

(iv) Further substitution of chloromethane can occur, producing dichloromethane,*P71927A0328*

which is a liquid at room temperature.

Calculate the maximum volume, in cm3, of dichloromethane liquid that could

be produced from 12.5g of chloromethane.

Give your answer to an appropriate number of significant figures.

(3)

CH3Cl(g) + Cl2(g) → CH2Cl2(l) + HCl(g)

[Density of dichloromethane, CH Cl (l) = 1.32 g cm–3]

One of the components of commercial diesel fuel is a hydrocarbon with the

formula C12H26.

An equation for the complete combustion of C12H26 is shown.

C12H26(l) + 18½O2(g) → 12CO2(g) + 13H2O(l)

(i) Calculate the maximum volume, in m3, of carbon dioxide gas produced in

an engine by the complete combustion of 1.00 kg of C12H26 at 200°C, at a

pressure of 6.0 × 106 Pa.

[The ideal gas equation is pV = nRT

Gas constant, R = 8.31 J mol–1 K–1]

(4)

(ii) Biodiesel is derived from plants and petrodiesel is derived from crude oil.

Biodiesel is increasingly widely used, although it is more expensive and

provides less energy per kg when burned than petrodiesel.

Give one reason why biodiesel is now preferred to petrodiesel.

(1)

… *P71927A0428*

(Total for Question 2 = 14 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

2(a) An answer that makes reference to the following points: (2)

• saturated means all bonds (between C atoms) are (1) Allow answers that discuss the absence of double or triple

single bonds etc,

no more hydrogen can be added without breaking C−C

bonds

Allow saturated hydrocarbons only undergo substitution

reactions and not addition reactions

• hydrocarbon contains hydrogen and carbon (only) (1)

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(i) Example of equation (1)

• overall equation CH4 + Cl2 CH3Cl + HCl

Ignore state symbols, even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(ii) Example of calculation (2)

• calculation of moles of chlorine molecules (1) moles of chlorine molecules = 7.00 ÷ 71 = 0.098592 (mol)

Do not award use of 70 for the molar mass of chlorine molecule

• calculation of mass (g) of chloromethane 1 mole chlorine molecules produces 1 mole CH3Cl

produced (1) moles of CH3Cl = 0.098592 (mol)

= 0.098592 × 50.5 = 4.9789 (g)

= 4.98 / 5.0 / 5 g

TE on mol of chlorine

Ignore SF including 1 SF

Allow TE from equation in (b)(i)

Use 35.5 for the molar mass to give 9.96 g scores M2

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(iii) Example of equation (1)

• termination equation CH3• + CH3• C2H6 / 2CH3• CH3CH3

Cl• + Cl• Cl2

CH3• + Cl• CH3Cl

Free radical dots are required

Ignore state symbols, if shown, even if incorrect

Question

Acceptable Answer Additional Guidance Mark

Number

2(b)(iv) Example of calculation (3)

• calculation of mol CH3Cl (1) 12.5 ÷ 50.5 = 0.24752

• calculation of mass of CH2Cl2 produced (1) Mr CH2Cl2 = 85

0.24752 × 85 = 21.040 (g)

(1) 21.040 ÷ 1.32 = 15.939 (cm3)

• conversion of mass to volume of liquid using density

and

= 15.9 / 16 (cm3)

answer given to 2 or 3 SF only

Allow TE throughout

Question

Acceptable Answer Additional Guidance Mark

Number

2(c)(i) Example of calculation (4)

• calculation of moles of C12H26 (1) 1.00 kg = 1000 g

C H = 170 g mol−1

12 26

moles C12H26 = 1000 ÷ 170 = 5.8824 (mol)

• calculation of moles CO2 (from equation) (1) moles of CO2 = 5.8824 × 12

= 70.588 (mol)

• conversion of temperature to K (1) T = 200 + 273 = 473 K

and and

rearrangement of equation V = nRT/p

• calculation of volume of CO2 produced (1) V = 70.588 x 8.31 x 473

6 x 106

V = 0.046243 (m3)

Ignore SF except 1 SF

Allow TE throughout

Correct final answer with no working scores (4)

3.85 x 10−3 scores (3) due to omission of M2

Question

Acceptable Answer Additional Guidance Mark

Number

2(c)(ii) An answer that makes reference to the following point: Examples of acceptable answer (1)

• one reason for increasing use of biodiesel Renewable / sustainable resource (derived from plant matter)

Cleaner burning (because it contains oxygen in the molecule)

Better lubricant (than petrodiesel)

Uses up waste (cooking) oils

Plants grown locally so less dependent on imports

Allow references to carbon neutrality only if an explanation is

provided

Do not award if incorrect statements made such as biodiesel is

biodegradable are included with correct reasons such as

sustainability

(Total for Question 2 = 14 mark

How to answer it

Alkanes, Free Radical Substitution & Ideal Gases

What this question tests

This question assesses core organic chemistry and physical chemistry principles: defining saturated hydrocarbons, constructing free radical substitution mechanisms (overall and termination equations), stoichiometry and molar calculations, density conversions, manipulating the ideal gas equation ( pV = nRT ), and evaluating the environmental and sustainability aspects of biofuels versus fossil fuels.

Question 2(a)

Definition of a Saturated Hydrocarbon

State what is meant by 'saturated hydrocarbon'. (2 marks)

✅ Correct Answer

Point 1: Saturated means single bonds only (between carbon atoms).
Point 2: Hydrocarbon contains hydrogen and carbon (only).

💡 Key Knowledge

Both terms must be broken down explicitly to secure full marks.
Alternative acceptable phrasings for "saturated": no double/triple bonds, or no more hydrogen can be added without breaking C-C bonds.

❌ Common Errors

Students frequently lose marks by stating "contains only single bonds" without specifying that the single bonds are between carbon atoms, or by forgetting to state that a hydrocarbon contains only hydrogen and carbon.

Mark scheme breakdown: 1 mark for defining saturated (single bonds / no double bonds) and 1 mark for defining hydrocarbon (contains carbon and hydrogen only).

Question 2(b)(i)

Overall Equation for Halogenation

Write the overall equation for this reaction of methane with chlorine. (1 mark)

✅ Correct Answer

CH₄ + Cl₂ → CH₃Cl + HCl

🧠 Exam Technique

State symbols are explicitly not required as per the question stem. Simply combine your initiation and propagation steps to get the net reaction, cancelling out intermediate radicals ( Cl• and CH₃• ).

Mark scheme breakdown: 1 mark for the correct balanced equation. State symbols are ignored even if incorrect.

Question 2(b)(ii)

Stoichiometry & Mass Calculation

Calculate the maximum mass, in grams, of chloromethane produced when 7.00 g of chlorine gas reacts completely with excess methane. (2 marks)

📐 Step-by-Step Calculation

Find moles of Cl₂:
Molar mass of Cl₂ = 71.0 g mol⁻¹ (Do not use 70).
Moles = 7.00 ÷ 71.0 = 0.09859 mol.
Use reacting ratios (1 : 1):
1 mol of Cl₂ produces 1 mol of CH₃Cl.
Moles of CH₃Cl = 0.09859 mol.
Calculate mass of CH₃Cl:
Mass = moles × molar mass = 0.09859 × 50.5 = 4.98 g (or 5.0 g / 4.984 g).

❌ Common Calculation Traps

Using Mr = 35.5 for chlorine gas ( Cl₂ ) instead of 71.0.
Rounding intermediate values too early, leading to slight inaccuracies in the final significant figures.

Mark scheme breakdown: 1 mark for calculating moles of chlorine, 1 mark for calculating the mass of chloromethane produced.

Question 2(b)(iii)

Termination Step Equation

Write an equation for a termination step for this reaction. (1 mark)

✅ Correct Answer (Any one of):

CH₃• + CH₃• → C₂H₆ (or 2CH₃• → C₂H₆ )
Cl• + Cl• → Cl₂
CH₃• + Cl• → CH₃Cl

❌ Common Errors

Forgetting to include the free radical dots ( • ). Termination steps involve two radicals combining to form a single, stable molecule—omitting the radical dots means chemical species are incorrect.

Mark scheme breakdown: 1 mark for any valid termination equation containing free radical dots. State symbols are ignored.

Question 2(b)(iv)

Multi-step Calculation (Moles, Mass, Density & SF)

Calculate the maximum volume, in cm³, of dichloromethane liquid that could be produced from 12.5 g of chloromethane. (3 marks)

📐 Step-by-Step Calculation

Moles of CH₃Cl:
Moles = 12.5 ÷ 50.5 = 0.24752 mol.
Mass of CH₂Cl₂ produced:
Molar mass of CH₂Cl₂ = 12.0 + (2 × 1.0) + (2 × 35.5) = 85.0 g mol⁻¹.
Mass = 0.24752 × 85.0 = 21.040 g.
Volume conversion using density:
Density = Mass ÷ Volume ⇒ Volume = Mass ÷ Density
Volume = 21.040 ÷ 1.32 = 15.939 cm³.
Apply Significant Figures:
Given data uses 3 SF (12.5 g, 1.32 g cm⁻³).
Final Answer: 15.9 cm³ (or 16 cm³ for 2 SF).

🧠 Exam Technique

Pay close attention to the command: "Give your answer to an appropriate number of significant figures." Because input data is provided to 3 SF, give your final answer to 2 or 3 significant figures to earn the final mark.

Mark scheme breakdown: 1 mark for moles of CH₃Cl, 1 mark for mass of CH₂Cl₂, and 1 mark for converting mass to volume using density with a 2 or 3 SF final value.

Question 2(c)(i)

Ideal Gas Equation & Combustion Stoichiometry

Calculate the maximum volume, in m³, of carbon dioxide gas produced in an engine by the complete combustion of 1.00 kg of C₁₂H₂₆ at 200 °C, at a pressure of 6.0 × 10⁶ Pa. (4 marks)

📐 Step-by-Step Calculation

Moles of C₁₂H₂₆:
Mass in grams = 1.00 kg = 1000 g.
Molar mass of C₁₂H₂₆ = (12 × 12.0) + (26 × 1.0) = 170 g mol⁻¹.
Moles = 1000 ÷ 170 = 5.8824 mol.
Moles of CO₂ via balanced ratio:
From equation: 1 mol C₁₂H₂₆ produces 12 mol CO₂.
Moles of CO₂ = 5.8824 × 12 = 70.588 mol.
Convert units for Ideal Gas Equation ( pV = nRT ):
Pressure ( p ) = 6.0 × 10⁶ Pa
Temperature ( T ) = 200 + 273 = 473 K
Rearrange for volume: V = nRT / p
Calculate Volume ( V ):
V = (70.588 × 8.31 × 473) ÷ (6.0 × 10⁶)
V = 0.0462 m³ (or 0.046 m³)

❌ Common Calculation Traps

Forgetting to convert kg to g ( 1.00 kg = 1000 g ).
Forgetting to convert Celsius to Kelvin ( + 273 ).
Omitting the 1:12 stoichiometric multiplier from the combustion equation.

Mark scheme breakdown: 1 mark for moles of alkane, 1 mark for scaling to moles of CO₂, 1 mark for temperature conversion and rearranging pV=nRT , and 1 mark for the correct final volume calculation.

Question 2(c)(ii)

Biofuels vs. Fossil Fuels

Give one reason why biodiesel is now preferred to petrodiesel. (1 mark)

✅ Acceptable Answers (Any one):

It is a renewable / sustainable resource (derived from plants).
It is a cleaner burning fuel (contains oxygen in its molecule, promoting more complete combustion).
It acts as a better lubricant than petrodiesel.
It utilizes waste plant/cooking oils.
Crops are grown locally, reducing dependence on foreign oil imports.

❌ Common Errors

Stating simply that biodiesel is "carbon neutral" without further explanation is not accepted, because processing and transport release carbon. Examiners also do not award marks if contradictory statements are made (e.g. claiming biodiesel is biodegradable as a reason for engine fuel preference).

Mark scheme breakdown: 1 mark for a valid, chemically sound advantage of biodiesel over petrodiesel.

Topics

Physical Chemistry · Organic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.