AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 5
12 marks · Standard Demand difficulty · Short Answer
Answer questions on unsaturated hydrocarbons, including estimating boiling points, reactions of alkenes, tests for chlorine, and calculating gas volume from mass.
Practise this questionQuestion
Question text
05 This question is about unsaturated hydrocarbons.
05.1 Table 6 shows the boiling points of some unsaturated hydrocarbons.
Table 6
Formula of unsaturated hydrocarbon Boiling point in °C
C2H4 – 104
C3H6 – 47
C4H8 X
C5H10 30
C6H12 63
Estimate the boiling point X in Table 6.
[1 mark]
°C
05.2 C2H4 reacts with water vapour (steam).
Name the compound produced when C2H4 reacts with water vapour.
[1 mark]
C4H8 reacts with chlorine (Cl2).
05.3 Describe the test for chlorine.
Give the result of the test.
[2 marks]
Test
Result
05.4 Which is the displayed formula of the organic compound produced when C4H8
reacts with chlorine?
[1 mark]
Tick ( ) one box.
05.5 Name the unsaturated hydrocarbon with the formula C3H6
[1 mark]
05.6 Why does C3H6 tend to burn in air with a smoky flame?
[1 mark]
05.7 C3H6 is a gas at room temperature and pressure.
Calculate the volume of 2.1 g of C3H6 at room temperature and pressure.
Give your answer in cm3.
The volume of one mole of any gas at room temperature and pressure is 24 dm3.
Relative atomic masses (Ar): H = 1 C = 12
[5 marks]
Volume = cm3
Mark scheme
Show the mark scheme
Question 5
AO /
Question Answers Extra information Mark
Spec. Ref.
05.1 – 6 (°C) allow a value in the range 1 AO2
(+)4 to –16 (°C) 4.7.1.3
AO /
Spec. Ref.
05.2 ethanol allow C2H5OH 1 AO2
4.7.2.2
AO /
Spec. Ref.
05.3 (test) 1 AO1
damp litmus paper 4.8.2.4
(result)
litmus paper is bleached ignore litmus paper turns red 1
or
litmus paper turns white ignore litmus paper turns red
AO /
Spec. Ref.
05.4 1 AO2
4.7.2.2
AO /
Spec. Ref.
05.5 propene 1 AO1
4.7.2.1
AO /
Spec. Ref.
05.6 (because of) incomplete ignore insufficient oxygen 1 AO1
combustion 4.7.2.2
16 AO /
Spec. Ref.
05.7 (conversion 24 dm3 =) allow conversion of volume 1 AO2
24 000 cm3 anywhere in method 4.3.1.2
4.3.2.1
4.3.5
(Mr = (3 × 12) + (6 × 1) =) 1 4.7.2.1
2.1 allow correct use of an 1
(moles = = ) 0.05 incorrectly determined M
42 r
(volume =) 24 000 × 0.05 allow correct use of an 1
incorrectly determined number
of moles
allow correct use of an incorrect
/ no conversion of volume
= 1200 (cm3) 1
alternative approach
(conversion 24 dm3 =) allow conversion of volume
24 000 cm3 (1) anywhere in method
(Mr = (3 × 12) + (6 × 1) =)
42 (1)
42 allow correct use of an
(ratio of mass = = ) 20 (1) incorrectly determined M
2.1 r
24 000 allow correct use of an incorrect
(volume =) (1) / no conversion of volume
= 1200 (cm3) (1)
Total Question 5 12
How to answer it
AQA GCSE Chemistry: Unsaturated Hydrocarbons & Calculations
This question assesses core knowledge of alkenes (naming, homologous series trends, and addition reactions with steam and halogens), gas testing (chlorine), combustion characteristics, and quantitative skills in calculating molar gas volume at room temperature and pressure (r.t.p.).
Question 05.1: Estimating Boiling Point
1 Mark • AO2 • Specification Reference: 4.7.1.3
✅ Accepted Answers
- -6 °C
- Allowance: Any value in the range of +4 °C to -16 °C.
🧠 Exam Technique: Interpolating Data
Look at the differences between successive members:
- C₂H₄ (-104) to C₃H₆ (-47) = increase of 57 °C
- C₅H₁₀ (30) to C₆H₁₂ (63) = increase of 33 °C
- The jump from C₃H₆ to C₅H₁₀ covers 2 steps (total increase = 77 °C). Dividing roughly gives steps of around 38-40 °C.
- -47 + 40 = -7 °C (or 30 - 37 = -7 °C), neatly within the range!
Question 05.2: Reaction with Steam (Hydration)
1 Mark • AO2 • Specification Reference: 4.7.2.2
✅ Correct Answer
- Ethanol (formula C₂H₅OH is also allowed).
💡 Key Knowledge: Alkene + Steam
Alkenes react with water vapour (steam) in the presence of an acid catalyst to form alcohols:
Ethene + Steam → Ethanol
C₂H₄ + H₂O → C₂H₅OH
Question 05.3: Chemical Test for Chlorine Gas
2 Marks • AO1 • Specification Reference: 4.8.2.4
✅ Correct Marking Points
- Test: (Damp) litmus paper [1 mark]
- Result: Litmus paper is bleached / turns white [1 mark]
❌ Common Errors & Examiner Notes
- "Turns red": The mark scheme explicitly states ignore litmus paper turns red. Chlorine is acidic and briefly turns blue litmus red, but the diagnostic result is bleaching (turning white).
- Dry paper: Forgetting that litmus paper must be damp. Chlorine gas reacts with water on the paper to produce the bleaching agent (chlorous/hydrochloric acid).
Question 05.4: Displayed Formula of Halogenation Product
1 Mark • AO2 • Specification Reference: 4.7.2.2
✅ Correct Selection
Tick the 2nd box down:
Cl Cl H H
| | | |
H-C--C--C--C-H
| | | |
H H H H
(1,2-dichlorobutane)
💡 Addition Reaction Mechanism
- Halogens add across the carbon-carbon double bond ( C=C ).
- The double bond breaks to form a single bond, and one halogen atom adds to each of the two adjacent carbon atoms.
- Therefore, butene ( C₄H₈ ) + Cl₂ must have exactly two chlorine atoms attached to neighbouring carbon atoms.
Questions 05.5 & 05.6: Alkene Properties
2 Marks Total • AO1 • Specification References: 4.7.2.1, 4.7.2.2
05.5: Naming C₃H₆ (1 Mark)
Answer: propene
- Prefix: prop- = 3 carbon atoms.
- Suffix: -ene = contains a C=C double bond (alkene).
- General formula matches CₙH₂ₙ where n = 3.
05.6: Smoky Flame Explanation (1 Mark)
Answer: Due to incomplete combustion.
- Alkenes have a higher carbon-to-hydrogen ratio than alkanes.
- When burned in air, there is insufficient oxygen to fully oxidise all carbon, leaving unburnt carbon particles (soot), which glow yellow and produce smoke.
Question 05.7: Calculating Gas Volume at R.T.P.
5 Marks • AO2 • Specification References: 4.3.1.2, 4.3.2.1, 4.3.5, 4.7.2.1
📐 Step-by-Step Calculation
Mᵣ = (3 × 12) + (6 × 1) = 36 + 6 = 42
Moles = Mass ÷ Mᵣ
Moles = 2.1 g ÷ 42 = 0.05 mol
1 dm³ = 1000 cm³
24 dm³ = 24 × 1000 = 24,000 cm³
Volume = Moles × Molar Volume
Volume = 0.05 mol × 24,000 cm³
Volume = 1200 cm³ (or 1.2 dm³ converted to 1200 cm³)
❌ Common Traps to Avoid
- Unit Error: Giving the final answer as 1.2 without converting to cm³ as explicitly required by the question prompt.
- Atomic Mass Confusion: Multiplying atomic numbers rather than relative atomic masses ( Aᵣ : C = 12, H = 1).
- Wrong Conversion Factor: Dividing by 1000 instead of multiplying by 1000 when converting dm³ to cm³.
💡 Alternative Ratio Method
You can also solve this via direct mass ratio:
- 1 mole (42 g) occupies 24,000 cm³.
- Ratio of mass = 42 ÷ 2.1 = 20.
- Volume = 24,000 ÷ 20 = 1200 cm³.
Topics
Chemistry · C3: Quantitative Chemistry · C7: Organic Chemistry · C8: Chemical Analysis
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.