AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 7

✅ Correct Answer

Butane can be used as a fuel.

💡 Key Knowledge

• Butane (C₄H₁₀) is a four-carbon alkane.
• Short-chain alkanes have low boiling points, ignite easily, and release large amounts of energy when burned, making them ideal fuels (e.g. in camping stoves and lighters).

✅ Correct Answer

Tick box 4: Propene

🧠 Exam Technique

Addition polymers are named simply by placing "poly" in front of the monomer name in brackets:

poly(monomer) → monomer

To find the monomer, remove the "poly" prefix and brackets: poly(propene) comes from propene.

❌ Common Errors

Selecting propane. Remember that addition polymers require an unsaturated monomer containing a C=C double bond (an alkene, ending in -ene), not an alkane.

📐 Step-by-Step Calculation

Formula:

Percentage Yield = (Actual Mass / Theoretical Mass) × 100

1. Identify values from the text:
   Actual mass made = 380 kg
   Maximum theoretical mass = 400 kg
2. Substitute into formula:
   (380 / 400) × 100 [1 mark]
3. Calculate final percentage:
   = 95% [1 mark]

❌ Distractor Data Trap

The question provides reactant masses: 243 kg of ethene and 157 kg of steam .

Notice that 243 + 157 = 400 kg (Conservation of Mass). You do not need to divide by the reactant masses individually; use the theoretical product mass (400 kg) directly.

✅ Correct Selections (Tick 2)

• ☑ Some ethanol changes back into ethene and steam. [1 mark]
• ☑ Some ethanol escapes from the apparatus. [1 mark]

💡 Understanding Yield Losses

• Reversible Reaction: The equation displays a reversible symbol ( ⇌ ). As product forms, some decomposes back into reactants.
• Separation/Transfer Loss: Ethanol is volatile (boils at 78 °C), so vapours easily escape during collection and purification.

❌ Incorrect Distractors

• "Ethanol is the only product" — true, but having no side products would normally help yield, not reduce it.
• "Ethanol is very unreactive" — ethanol is flammable and reactive.
• "Some ethanol reacts with steam" — steam reacts with ethene, not ethanol.

✅ Correct Balanced Equation

C₂H₅OH + 3 O₂ → 3 H₂O + 2 CO₂

🧠 Atom Balancing Step-by-Step

1. Count atoms on Right-Hand Side (RHS):
   Carbons = 2 × 1 = 2
   Hydrogens = 3 × 2 = 6
   Oxygens = (3 × 1) + (2 × 2) = 3 + 4 = 7
2. Balance Left-Hand Side (LHS):
   C₂H₅OH provides 2 C and (5 + 1) = 6 H (already balanced!).
   C₂H₅OH also provides 1 O.
3. Find remaining oxygen:
   Total needed = 7 O atoms.
   7 - 1 = 6 O atoms must come from O₂ molecules.
   6 ÷ 2 = 3 O₂.

❌ Common Error

Forgetting the single oxygen atom already inside the ethanol molecule ( C₂H₅OH ). Students who forget this often try to balance with 3.5 or write 7 .

✅ Advantages of Fermentation (Any 2)

• Low energy usage (operates at moderate temperatures, around 30–40 °C). [1 mark]
• Uses renewable raw materials (sugar cane/plant crops vs non-renewable crude oil). [1 mark]

✅ Disadvantages of Fermentation (Any 2)

• Low purity / produces impure ethanol (only ~15% yield before yeast is poisoned; requires fractional distillation). [1 mark]
• Slow rate of reaction (takes days vs a continuous, fast industrial reaction). [1 mark]