AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 1

✅ Correct Answer

(The reaction is) reversible

Also allowed: "The products can react to reform the original reactants" or "the reaction goes both ways".

💡 Key Knowledge

The symbol ⇌ represents a reversible reaction:

nitrogen + hydrogen ⇌ ammonia
N₂ + 3H₂ ⇌ 2NH₃

The forward and reverse reactions happen at the same time in a closed system.

❌ Common Errors

• Writing "equal" or "in balance" instead of reversible.
• Confusing the symbol with an equilibrium state rather than the reversible nature of the reaction itself.

🧠 Exam Technique

Keep your answer concise. The single word "reversible" is all that is required for this mark.

✅ Correct Answer

• ☐ Chlorine
• ☑ Iron
• ☐ Oxygen

💡 Key Knowledge

In the Haber process:

• Catalyst: Iron ( Fe )
• Temperature: around 450 °C
• Pressure: around 200 atmospheres

A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, without being used up.

✅ Correct Answer

• ☑ Activation energy with a catalyst
• ☐ Activation energy without a catalyst
• ☐ Overall energy change with a catalyst
• ☐ Overall energy change without a catalyst

💡 How to Read the Profile

• Activation energy (Eₐ): The vertical distance from the reactants line to the peak of the curve.
• Higher peak: Reaction without a catalyst.
• Lower peak: Reaction with a catalyst.
• Label X measures from the reactants line to the lower peak, which means it must be the activation energy with a catalyst.

❌ Common Errors

Confusing activation energy with overall energy change. Overall energy change is measured from reactants to products, whereas activation energy always goes from reactants up to the crest of the curve.

🧠 Quick Rule

Catalysts never alter the overall energy change (reactants to products). They only lower the peak (activation energy).

✅ What You Must Draw

• Bar 1 (Phosphorus / P): Drawn up to exactly 22% (tolerance: ± ½ a small square) and clearly labelled "Phosphorus" or "P". [1 mark]
• Bar 2 (Potassium / K): Drawn up to exactly 25% (tolerance: ± ½ a small square) and clearly labelled "Potassium" or "K". [1 mark]

🧠 Working Out the Grid Scale

• From 0 to 10 on the y-axis spans 10 small squares.
• Therefore, 1 small square = 1%.
• 22%: 2 small squares above the 20 line.
• 25%: 5 small squares above the 20 line (midway between 20 and 30).

❌ Common Student Pitfalls

• Forgetting labels: Drawing the correct bar heights but failing to write "Phosphorus" and "Potassium" beneath them on the x-axis.
• Messy line drawing: Using a blunt pencil or drawing freehand without a ruler, leading to heights outside the ± ½ square tolerance.

✅ Correct Answer

Any one of the following:

• There are other elements in the fertiliser (besides phosphorus and potassium).
• It contains nitrogen.
• Also allowed: "There are other substances/compounds/water in the fertiliser."

💡 Key Knowledge: NPK Fertilisers

The prompt tells you: "NPK fertilisers contain the elements nitrogen, phosphorus and potassium."

Since the question gives values only for P (22%) and K (25%) (which total 47%), the remainder includes nitrogen (N) and other supporting ions (like sulfate, ammonium, or oxygen).

✅ Correct Answer

Fertiliser B

🧠 Deduction Steps

1. The text states:
   • Too little phosphorus causes slow growth.
   • Too little potassium causes brown leaf edges.
2. To prevent both problems, the fertiliser must contain BOTH phosphorus (P) AND potassium (K).
3. Looking at Table 1:
   • A: P = 0% (fails)
   • B: P = 16%, K = 23% (contains both!)
   • C: K = 0% (fails)
   • D: P = 0%, K = 0% (fails)

✅ Correct Answer

Fertiliser B

📐 Calculation: Sum of Essential Elements (N + P + K)

Add the three percentage columns across for each fertiliser:

• Fertiliser A: 14 + 0 + 39 = 53%
• Fertiliser B: 25 + 16 + 23 = 64% ← Highest!
• Fertiliser C: 21 + 23 + 0 = 44%
• Fertiliser D: 21 + 0 + 0 = 21%

❌ Common Pitfall

Do not just look at a single column for the highest individual number (e.g. Potassium in A is 39%). The question asks for the greatest total percentage, so you must sum all three numbers across the row.