AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2023: Question 5

Correct Answer

CuFeS₂ + 4O₂ → CuSO₄ + FeSO₄

Key Knowledge

To balance any equation, the number of atoms of each element must be identical on both sides of the reaction arrow:

• Right side (Products): 1 Copper (Cu), 1 Iron (Fe), 2 Sulfur (S), and 8 Oxygen (O) atoms (4 in CuSO₄ + 4 in FeSO₄).
• Left side (Reactants): 1 Cu, 1 Fe, 2 S are already present. We need 8 Oxygen atoms.
• Since oxygen gas is diatomic ( O₂ ), we need 4 molecules of O₂ to get 8 oxygen atoms ( 4 × 2 = 8 ).

Exam Technique

Always count the atoms of elements that are already fixed first. Here, the products CuSO₄ and FeSO₄ have no blanks in front of them, meaning their ratio is fixed at 1:1. Use them as your anchor to count the total oxygen atoms needed on the left.

Common Errors

Writing O₈ or 8O on the line. Oxygen gas exists naturally as a diatomic molecule ( O₂ ). You can only balance equations by putting numbers in front of formulas, never by changing the small subscript numbers!

How to secure full marks

Even if you make an arithmetic error in Step 1, write down your working! The mark scheme allows Error Carried Forward (ECF). If you use your incorrect Mᵣ value correctly in Step 2 and Step 3, you can still score 2 out of 3 marks.

Common Calculation Traps

Forgetting subscripts: A common mistake is forgetting that there are two Sulfur atoms in CuFeS₂ . Students often calculate Mᵣ as 63.5 + 56 + 32 = 151.5, which loses the first mark. Always look closely at the chemical formula subscripts!

Correct Answers (Two acceptable routes)

Route 1 (Precipitation Test):

• Test: Add sodium hydroxide solution
• Result: Blue precipitate

Route 2 (Flame Test):

• Test: Flame test
• Result: Green flame (or blue-green flame)

Key Knowledge

Copper(II) ions ( Cu²⁺ ) react with hydroxide ions ( OH⁻ ) to form an insoluble metal hydroxide precipitate:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

This precipitate has a highly characteristic blue color which is used to identify the presence of copper in solution.

Exam Technique

Be precise with chemical names. Do not just write "add sodium" or "add hydroxide". You must state the full name of the reagent: sodium hydroxide. "NaOH" is also fully acceptable.

Common Errors

Confusing the flame test color of Copper (green/blue-green) with other metal ions like Barium (apple green) or Calcium (brick red). Ensure you memorize your flame test colors thoroughly before the exam!

Correct Answer

• The use of bacteria [1 mark]
• To produce leachate solutions that contain metal/copper compounds [1 mark]

Key Knowledge

As high-grade copper ores run out, we must use low-grade ores. Traditional mining is too expensive and damaging for low-grade ores, so we use biological methods:

• Bioleaching: Bacteria feed on low-grade ores. This process produces a solution called a leachate, which contains copper ions that can then be easily extracted using scrap iron or electrolysis.

How to get both marks

To get full marks, your answer must connect the biological agent (bacteria) to the physical outcome (producing a leachate solution containing the target metal compounds).

Common Errors

Confusing Bioleaching with Phytomining: This is the most common mix-up in GCSE Chemistry!

• Bioleaching = Bacteria
• Phytomining = Plants (which absorb metals through roots and are then burned to ash)