AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2019: Question 4

💡 Key knowledge

• Phytomining uses plants to absorb copper compounds from low-grade ore.
• Electrolysis or displacement can then produce copper metal.
• moles = mass ÷ Mr
• A catalyst lowers activation energy but does not change overall energy change.
• Sustainable development means meeting current needs without harming future generations’ ability to meet theirs.

🧠 Exam technique

• For 4-mark description questions, give the process in order.
• For calculations, show the equation, substitute values, and give the final unit.
• In multiple-choice explanation questions, compare the start and end energy levels.
• Use short, precise science words: burnt, dissolved, electrolysis, displacement.

✅ Correct answer

1. Grow plants on low-grade ore containing copper compounds.
2. The plants absorb copper compounds from the soil.
3. Harvest the plants and burn them to produce ash.
4. Dissolve the ash in acid to make a solution of a copper compound, then obtain copper by electrolysis or by adding scrap iron to displace the copper.

💡 Key knowledge

The mark scheme accepts a few routes after burning:

• electrolysis of the copper compound solution
• or displacement using scrap iron

Named plants can be used, but are not required.

🧠 Exam technique

To get full marks, write the process in the correct order. Each key step can earn a mark, so don’t just say “copper is extracted from plants” — explain how.

❌ Common errors

• Leaving out the burning step.
• Not saying that the ash is dissolved in acid.
• Stopping at “plants absorb copper” without explaining how copper metal is recovered.
• Confusing phytomining with bioleaching.

📐 Calculations — step by step

1. Find the formula mass of CuSO₄:
   Cu = 63.5, S = 32, O₄ = 4 × 16 = 64
   Mr of CuSO₄ = 63.5 + 32 + 64 = 159.5
2. Calculate moles of CuSO₄ in 1 dm³:
   moles = mass ÷ Mr
   0.319 ÷ 159.5 = 0.002
3. Link to copper:
   There is 1 mole of Cu in 1 mole of CuSO₄, so the number of moles of copper = 0.002 mol = 2 × 10⁻³ mol.

✅ Correct answer

Number of moles of copper = 0.002 mol

Accept 2 × 10⁻³ mol.

🧠 Exam technique

• Use the given concentration as the “mass” in this question, because the mark scheme expects 0.319 ÷ 159.5 .
• State the formula mass clearly.
• Write the final answer with the correct unit: mol.
• GCSE examiners usually reward the method even if one earlier number is slightly wrong.

❌ Common errors

• Forgetting that CuSO₄ contains 1 copper atom, so copper and CuSO₄ are in a 1:1 ratio.
• Using the atomic mass of Cu only instead of the Mr of CuSO₄.
• Dropping the unit or writing mol dm⁻³ instead of mol .
• Not using standard form where needed.

✅ Correct answer

Both reaction profiles start at the same energy level and end at the same energy level.

💡 Key knowledge

A catalyst lowers the activation energy, but it does not change the energy of the reactants or products. So the overall energy change stays the same.

🧠 Exam technique

Look at the graph carefully: the two curves have different peaks, but their start and finish points match. That is the key idea.

❌ Common errors

• Choosing the statement about exothermic reactions — this does not explain the catalyst effect.
• Saying the activation energy is lower for the uncatalysed reaction.
• Thinking the catalyst changes the products.

✅ Correct answer

One mark for each of these points:

• The carbon dioxide used to make the ethanol is the same amount as the carbon dioxide given off when the ethanol is burned.
• So the process does not add extra carbon dioxide overall, making it sustainable.

💡 Key knowledge

The mark scheme also accepts: there is enough carbon dioxide in the atmosphere because it is constantly produced by burning fossil fuels.

🧠 Exam technique

For “suggest why” questions, give a reason linked to the carbon cycle or resource use. The best answers show balance: what is used is replaced.

❌ Common errors

• Saying only “it is renewable” without explaining why.
• Talking vaguely about “helping the environment” without mentioning carbon dioxide.
• Forgetting that burning ethanol produces carbon dioxide.

✅ Correct answer

Meets the needs of the current generation without compromising the ability of future generations to meet their own needs.

💡 Key knowledge

A simpler acceptable idea is: using resources so there are enough left for future generations.

🧠 Exam technique

Use the exact GCSE wording if you can. Examiners often award one mark for “current generation” and one mark for “future generations”.

❌ Common errors

• Only saying “protecting the environment”.
• Leaving out future generations.
• Writing a vague definition with no idea of meeting needs.

💡 Key facts to remember

• Phytomining: plants absorb copper compounds from low-grade ore.
• Recovery: burn plants, dissolve ash in acid, then use electrolysis or displacement.
• Mole calculation: moles = mass ÷ Mr .
• Catalysts: lower activation energy only.
• Sustainability: current needs met without harming future needs.

🧠 How to score well

• Write in the order the process happens.
• Use correct scientific terms.
• For maths, show working and include units.
• For graph questions, compare start and end points, not just the peaks.