AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 5

✅ Correct Selections (Tick Two)

• Ammonium phosphate [1 mark]
• Potassium nitrate [1 mark]

💡 Nutrient Breakdown

❌ Common Errors

• Ticking Ammonium nitrate: Students think because it has two words, it contains two different NPK elements; both "ammonium" and "nitrate" supply only nitrogen.
• Confusing Calcium with an NPK nutrient: Calcium is not one of N, P, or K.
• Ticking more than two boxes: Ticking 3 or more boxes results in penalty marks under standard marking rules.

🧠 Exam Technique

Always write down N - P - K in the margin. Check off which letter each part of the compound's chemical name provides before ticking:

• Ammonium = N
• Nitrate = N
• Phosphate = P
• Potassium = K

✅ Correct Answers

• Nitric acid: calcium nitrate [1 mark]
• Phosphoric acid: (calcium) triple superphosphate OR calcium dihydrogenphosphate [1 mark]

💡 The 3 Acid Reactions with Rock Phosphate

Phosphate rock is insoluble calcium phosphate. It must be treated with acids to make soluble fertiliser salts:

• + Nitric acid → Calcium nitrate (used directly as fertiliser) + Phosphoric acid
• + Phosphoric acid → Triple superphosphate (calcium dihydrogenphosphate)
• + Sulfuric acid → Single superphosphate (calcium phosphate + calcium sulfate)

❌ Common Errors

• Giving single superphosphate for phosphoric acid (single superphosphate comes from sulfuric acid).
• Writing "phosphoric acid" on the nitric acid line: although phosphoric acid is produced alongside calcium nitrate, the question asks for the soluble salts.
• Misspelling or giving vague names like "calcium superphosphate" without the word triple.

🧠 Memory Aid for Superphosphates

• Phosphoric acid produces Triple superphosphate (think: pure phosphorus source boosts it to "triple").
• Sulfuric acid produces Single superphosphate (match the S with S).

✅ Scoring Points (Any 3 comparative points + 1 conclusion)

• Scale: Industrial process is large(r) scale / Lab is small scale [1 mark]
• Speed: Industrial process is quicker / Lab process is slow / lab takes longer [1 mark]
• Process Type: Industrial is continuous / Lab is a batch process (or lab needs repeated setups) [1 mark]
• Reasoned Judgement: Clear justified conclusion that the industrial method is better suited for producing a large mass [1 mark]

🧠 Structuring an Evaluation Answer

When asked to evaluate methods to make a large mass, use direct comparison terms followed by a final decision:

1. State the industrial advantage: "The industrial process operates continuously on a much larger scale..."
2. Compare with the lab: "...whereas the lab method is a batch process that must be repeated many times to make significant quantities."
3. Compare speed: "The industrial method rapidly dries crystals down a heated column, while the lab requires slow cooling, crystallisation, and drying."
4. Final Conclusion: "Therefore, the industrial method is vastly superior for manufacturing large masses of fertiliser."

❌ Common Traps & Examiner Guidance

• Cost / Energy: The mark scheme states "ignore references to cost / energy". Do not waste lines writing about electricity bills or equipment costs unless the question provides financial data.
• Repeating the question stem: Writing simply "the industrial method produces a larger mass" scores 0 marks because that premise is already given in the question. You must say it is large-scale or continuous.
• Forgetting the conclusion: The 4th mark is exclusively for a reasoned judgement. Even with three perfect comparisons, omitting a summary decision caps your score at 3/4.

💡 Continuous vs. Batch Processing

• Continuous (Industry): Reactants flow in constantly and products flow out 24/7 without stopping. Fast, automated, huge yield.
• Batch (Laboratory): A set amount is mixed in glassware, followed by heating, waiting for crystals, filtering, and drying. Laborious, slow, small yield.