AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2021: Question 5

✅ Correct answer

At high temperatures in the engine, nitrogen reacts with oxygen to produce nitrogen dioxide.

💡 Key knowledge

• Car engines get very hot during combustion.
• Nitrogen from the air can react with oxygen at high temperature.
• This forms nitrogen oxides, including NO₂.

🧠 Exam technique

• To gain all 3 marks, include: high temperature, nitrogen, and reacts with oxygen.
• Use the exact idea from the mark scheme: no need for a long explanation.

❌ Common errors

• Saying only “fuel burns and makes NO₂” is too vague.
• Leaving out temperature loses a key mark.
• Confusing nitrogen dioxide with carbon dioxide.

📐 Calculations

1. Mean = total ÷ number of values
2. Total for 7 days = 33 × 7 = 231
3. Add the known 6 values: 35 + 37 + 37 + 34 + 37 + 29 = 209
4. X = 231 − 209 = 22

X = 22 micrograms per m³

✅ Correct answer

X = 22 micrograms per m³

🧠 Exam technique

• Show the mean calculation or the rearranged total method.
• Units matter: write micrograms per m³ .
• Using brackets clearly helps avoid sign errors.

❌ Common errors

• Forgetting there are 7 days.
• Adding the known values incorrectly.
• Giving the answer without units.

✅ Correct answer

Countryside data has the smallest values, so ± 2 is a higher proportion of the value.

💡 Key knowledge

• Same absolute uncertainty has a bigger effect on smaller readings.
• In the countryside the values are only about 6 to 8, so ± 2 is a large percentage.

🧠 Exam technique

• Compare relative uncertainty, not just the size of the uncertainty itself.
• Use the idea of “higher proportion” or “bigger percentage”.

❌ Common errors

• Saying it is most significant because the uncertainty is “largest” — it is the same for all values.
• Not comparing it to the actual size of the countryside readings.

✅ Correct answer

2 NO₂ → N₂ + 2 O₂

💡 Key knowledge

• Atoms must be conserved in a balanced equation.
• Two NO₂ molecules give 2 nitrogen atoms and 4 oxygen atoms.
• Products must also contain 2 nitrogen atoms and 4 oxygen atoms.

🧠 Exam technique

• Check atom counts one element at a time.
• If you can balance the coefficients, you do not need to change the chemical formulae.
• All marks are for the correct balanced equation.

❌ Common errors

• Writing N + O₂ instead of N₂ + O₂.
• Forgetting to balance oxygen.
• Changing the subscript in NO₂ instead of the coefficient.

✅ Correct answer

A resource which will run out.

💡 Key knowledge

• Finite means not unlimited.
• Platinum is finite because Earth’s supply is limited.

🧠 Exam technique

• One short sentence is enough for 1 mark.
• “Non-sustainable resource” is also accepted.

❌ Common errors

• Saying “expensive” or “rare” without mentioning it will run out.
• Describing it as renewable.

✅ Correct answer

CO₂ is also emitted when extracting and processing raw materials, during manufacturing, and when the car is disposed of at the end of its useful life.

💡 Key knowledge

• Life cycle assessment looks at the environmental impact at every stage of a product’s life.
• For a car, that includes raw material extraction, manufacture, use, and disposal.
• The “use” stage is only one part of the total carbon footprint.

🧠 Exam technique

• To get all 3 marks, mention at least three stages.
• Link each stage to CO₂ being released, not just to the stage name.
• Use the phrase life cycle assessment to show you are answering the question fully.

❌ Common errors

• Only talking about fuel burned while driving.
• Listing stages without saying why they add to the footprint.
• Forgetting disposal at the end of useful life.

💡 Core facts

• High temperatures in engines can make nitrogen react with oxygen.
• Mean = total ÷ number of values.
• Uncertainty matters more when readings are small.
• Balanced equations must conserve atoms.
• Finite resource = will run out.
• Carbon footprint includes the whole life cycle.

📐 Calculation habit

• Write the formula first.
• Show every step.
• Keep units in the final answer.
• Check your answer makes sense.

❌ Biggest traps

• Mixing up NO₂ with CO₂.
• Forgetting brackets in mean calculations.
• Using absolute uncertainty instead of relative uncertainty.
• Not balancing the equation fully.
• Ignoring the manufacturing and disposal stages of a life cycle.