AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 4

✅ Correct answer

butane

This is worth 1 mark for the correct alkane name.

💡 Key knowledge

• Alkanes are hydrocarbons with only single bonds.
• Butane has 4 carbon atoms.
• Its formula is C₄H₁₀ .

🧠 Exam technique

• Count the carbon atoms in the model.
• Match the chain length to the alkane name.
• For a 1-mark naming question, spell the name clearly.

❌ Common errors

• Giving the formula instead of the name.
• Confusing butane with butene.
• Misspelling the alkane name.

✅ Correct answer

A hydrocarbon is a molecule made of carbon and hydrogen only.

💡 Key knowledge

• Hydrocarbons contain only carbon atoms and hydrogen atoms.
• They can be alkanes or alkenes.

🧠 Exam technique

A 1-mark definition needs the exact idea: carbon and hydrogen only. Extra detail is not needed.

❌ Common errors

• Saying “contains carbon and hydrogen and oxygen”.
• Writing “made of molecules” without defining the elements.
• Forgetting the word only.

✅ Correct answer

One full correct equation is:

C₁₁H₂₄ → C₅H₁₀ + 2C₂H₄ + C₂H₆

Other valid alternatives were allowed by the mark scheme, as long as atoms are balanced.

💡 Key knowledge

• Cracking breaks long-chain hydrocarbons into smaller molecules.
• Products can include alkenes and alkanes.
• You must conserve the number of each type of atom.

🧠 Exam technique

• Check carbon atoms first.
• Then check hydrogen atoms.
• Use coefficients to balance the equation.
• Make sure the products total the same atoms as the reactant.

❌ Common errors

• Forgetting to balance the atoms.
• Writing only one product from the allowed list.
• Changing the subscript in the formula instead of adding a coefficient.

✅ Correct answer

Any one of these linked explanations scores full marks:

• C₂H₆ is useful as a fuel because it is a smaller molecule and is more flammable.
• C₂H₄ / C₃H₆ / C₅H₁₀ are used to make polymers because they are more reactive than alkanes.

💡 Key knowledge

• Smaller hydrocarbons are often useful as fuels.
• Alkenes are important because they can be used to make plastics/polymers.
• Unsaturated molecules are more reactive than alkanes.

🧠 Exam technique

• For 2 marks, give a use and a reason.
• Use a cause-and-effect sentence: “X is useful because Y”.
• If you name a specific product, make sure the use matches it.

❌ Common errors

• Saying “it is high demand because it is important” — too vague.
• Mixing up alkanes and alkenes.
• Not linking the property to the use.

✅ What a strong answer includes

• Compares environmental factors for wood and plastic.
• Compares economic factors for wood and plastic.
• Uses the data to support a clear final judgement.
• Recognises that plastic is cheaper, but wood is generally better environmentally.

🧠 Exam technique

• Make direct comparisons: “plastic is higher than wood”.
• Include numbers from the table to prove your points.
• End with a judgement such as “overall, wood is more sustainable” or “overall, plastic is better economically”.
• Top answers are logically linked and cover both sides.

❌ Common errors

• Describing only one frame without comparing.
• Ignoring the economic data.
• Only listing facts with no conclusion.
• Saying plastic is sustainable just because it is cheaper.

💡 Remember

• Butane = C₄H₁₀
• Hydrocarbon = carbon and hydrogen only
• Cracking makes smaller alkanes and alkenes
• Alkenes are useful for polymers
• Wood is environmentally better here; plastic is cheaper

🧠 Best way to gain marks

• Use exact scientific words.
• Compare data directly.
• Give a reason, not just a statement.
• Always end evaluation questions with a judgement.