AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 7
8 marks · Standard Demand difficulty · Extended Answer
Explain why methane and oxygen do not react at room temperature, predict the energy released by butane combustion from a pattern, and calculate the C–C bond energy in propane combustion using bond energies.
Practise this questionQuestion
Question text
07 Methane, ethane, propane and butane all react with oxygen to produce carbon dioxide
and water.
07.1 Suggest why a mixture of methane and oxygen does not react at room temperature.
Answer in terms of particles.
[2 marks]
07.2 Table 3 shows the energy released when methane, ethane and propane react
with oxygen to produce carbon dioxide and water.
Table 3
Compound reacted with oxygen
Methane Ethane Propane
Formula of compound CH4 C2H6 C3H8
Energy released in kJ/mol 680 1160 1640
Predict the energy released when butane (C4H10) reacts with oxygen to produce
carbon dioxide and water.
[1 mark]
Energy released = kJ/mol
07.3 Propane reacts with oxygen to produce carbon dioxide and water.
The displayed formula equation for the reaction is:
*22* The reaction is exothermic.
In the reaction, the energy released when forming new bonds is 1640 kJ/mol greater
than the energy needed when breaking bonds.
Table 4 shows bond energies.
Table 4
Bond H–C C–C O=O C=O O–H
Bond energy in kJ/mol 410 X 500 740 460
Calculate the C––C bond energy (X).
[5 marks]
X = kJ/mol
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
07.1 particles collide 1 AO3
5.5.1.2
(but at room temperature)
particles have insufficient 1
energy
or
(but) have energy less than the
activation energy (so collisions
are not successful)
07.2 2120 (kJ/mol) 1 AO3
5.5.1.2
5.5.1.3
07.3 allow C–C for X AO2
5.5.1.3
(bonds broken = allow (bonds broken =
(8 × 410) + 2 X + (5 × 500) (8 × 410) + (5 × 500)
= 5780 + 2 X = 5780 1
(bonds formed =
(6 × 740) + (8 × 460)
= 8120 1
(bonds broken – bonds formed =
energy released)
(5780 + 2 X) – 8120 = – 1640 allow correct use of incorrect 1
values from step 1 and/or step 2
(2 X =) 700 allow correct use of incorrect 1
value from step 3
(X =) 350 (kJ/mol) 1
Total 8
How to answer it
Combustion of Alkanes: Reactivity, Energy and Bond Enthalpy
What this question tests
You need to explain why reactions do not happen at room temperature using particles and activation energy, spot a pattern in energy released by alkanes, and calculate a missing bond energy using: energy to break bonds − energy released when forming bonds . Full marks come from clear calculations, correct bond counting, and using the right units.
Question 07
Methane, ethane, propane and butane react with oxygen to produce carbon dioxide and water.
Part (a) 07.1
Suggest why a mixture of methane and oxygen does not react at room temperature. Answer in terms of particles. [2 marks]
✅ Correct answer
Particles collide, but at room temperature they have insufficient energy to react. Their collisions are not successful because they do not have enough energy to overcome the activation energy.
💡 Key knowledge
- Reactions happen when particles collide.
- Not every collision leads to a reaction.
- Particles need enough energy to get over the activation energy barrier.
🧠 Exam technique
- Use particle language because the question asks for it.
- For 2 marks, give two linked ideas: collision + insufficient energy / activation energy.
- Best answers are short and precise.
❌ Common errors
- “They don’t collide” — wrong, they do collide.
- “They are too cold” without explaining why that matters.
- Forgetting the phrase activation energy or saying “needs heat” without particle detail.
Part (b) 07.2
Predict the energy released when butane (C₄H₁₀) reacts with oxygen to produce carbon dioxide and water. [1 mark]
✅ Correct answer
2120 kJ/mol
💡 Key knowledge
- As the number of carbon atoms increases, the energy released increases.
- Methane: 680 kJ/mol
- Ethane: 1160 kJ/mol
- Propane: 1640 kJ/mol
🧠 Exam technique
- The pattern goes up by 480 kJ/mol each time.
- Propane → butane: 1640 + 480 = 2120
- Because it is a 1-mark prediction, a correct pattern-based value is enough.
📐 Calculation / pattern
- Find the change between each alkane: 1160 - 680 = 480
- Check again: 1640 - 1160 = 480
- Add the same increase to propane: 1640 + 480 = 2120 kJ/mol
❌ Common errors
- Writing energy absorbed instead of released.
- Trying to invent a different pattern.
- Leaving off units: write kJ/mol.
Part (c) 07.3
Propane reacts with oxygen to produce carbon dioxide and water. Calculate the C—C bond energy (X). [5 marks]
✅ Correct answer
X = 350 kJ/mol
💡 Key knowledge
- Bond energy calculations use: bonds broken − bonds formed = energy change.
- Because the reaction is exothermic, the energy change is negative.
- Propane contains 8 H—C bonds and 2 C—C bonds.
🧠 Exam technique
- Always count bonds carefully from the displayed formula.
- Use the given “energy released is 1640 kJ/mol greater than the energy needed to break bonds” to set up the equation correctly.
- Show every step to access method marks.
❌ Common errors
- Counting 3 C—C bonds in propane instead of 2.
- Forgetting that there are 5 O=O bonds broken.
- Using energy formed − energy broken in the wrong direction.
- Dropping the negative sign for an exothermic reaction.
📐 Calculation: step-by-step
- Bonds broken in propane and oxygen:
(8 × 410) + (2 × X) + (5 × 500)
= 3280 + 2X + 2500 = 5780 + 2X - Bonds formed in carbon dioxide and water:
(6 × 740) + (8 × 460)
= 4440 + 3680 = 8120 - Use the exothermic energy change:
(bonds broken) − (bonds formed) = −1640
(5780 + 2X) − 8120 = −1640 - Solve the equation:
2X − 2340 = −1640
2X = 700 - Final answer:
X = 350 kJ/mol
🧠 How to get full marks
The mark scheme gives marks for: counting the correct bonds, calculating bonds formed, setting up the exothermic equation correctly, rearranging to find 2X , and then giving X = 350 kJ/mol . Even if one earlier number is wrong, you can still gain method marks if your working is consistent.
❌ Common calculation traps
- Mixing up bonds broken and bonds formed.
- Forgetting there are 2 C—C bonds in propane.
- Not multiplying by the number of molecules in the equation.
- Writing the final answer without kJ/mol.
Quick revision takeaways
💡 Collision theory
Particles must collide with enough energy to react. Below activation energy, collisions are unsuccessful.
💡 Pattern in alkane combustion
Energy released increases as the carbon chain gets longer: 680, 1160, 1640, 2120 kJ/mol.
💡 Bond energy method
Count all bonds broken, count all bonds formed, then use the exothermic sign correctly.
Topics
Chemistry · C5: Energy Changes · C7: Organic Chemistry · C2: Bonding, Structure and the Properties of Matter
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.