AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2025: Question 4

✅ Correct Lines

• Nitrogen → Air [1 mark]
• Hydrogen → Natural gas [1 mark]

💡 Key Knowledge

• Air is approximately 78% nitrogen, extracted easily via fractional distillation.
• Natural gas consists mainly of methane (CH₄), which is reacted with steam to produce hydrogen.

❌ Common Errors

• Drawing multiple lines from one element box (marks are automatically lost if two lines leave "Nitrogen" or "Hydrogen").
• Confusing the source of hydrogen with water or limestone.

✅ Correct Box

Tick: Cooling [1 mark]

💡 How It Works

Gases leaving the reactor contain nitrogen, hydrogen, and ammonia. Ammonia condenses at a much higher temperature than nitrogen and hydrogen:

• On cooling, ammonia liquefies and is tapped off at the bottom.
• Unreacted N₂ and H₂ remain gases and are recycled back into the reactor.

🧠 Exam Technique

Look carefully at the state labels on the diagram: the feed going into X is a gas mixture, and the output is "Liquid ammonia". Converting a gas to a liquid is condensation, caused by cooling.

✅ Correct Answer

The iron in the reactor is used as a catalyst. [1 mark]

💡 Why a Catalyst is Used

The iron catalyst speeds up both the forward and reverse reactions equally, reaching dynamic equilibrium faster without being used up itself. It allows the reaction to run at an acceptable rate at 450 °C.

❌ Common Misconceptions

• Writing that iron increases the percentage yield (catalysts do not change the position of equilibrium).
• Spelling: phonetic approximations are allowed, but learn the correct spelling: catalyst.

✅ Correct Answer

It increased (or rose / went up). [1 mark]

🧠 Exam Technique

Table 1 shows:

• 1990: 120 arbitrary units
• 2020: 165 arbitrary units

Keep the description simple and direct. Avoid unnecessary over-elaboration when only a one-word trend is required.

🧠 Exam Technique: Error Carried Forward (ECF)

Even if you made a subtraction mistake in Step 1 or Step 2, you could still earn the 3rd and 4th marks by correctly dividing your mass change by your number of years!

❌ Common Calculation Traps

• Wrong start year: Using 1990 (120) instead of 2000 (130). Always re-read the date range specified in the prompt!
• Flipping the division: Dividing years by mass (20 / 35 = 0.57) instead of mass by years.

✅ Correct Box

Tick: Change in world population [1 mark]

💡 Why is Population the Cause?

As the human population increases, more food is required. Most synthetic ammonia is used to manufacture nitrogen-based fertilisers (such as ammonium nitrate), which increase crop yields to feed the growing population.

❌ Distractors

Acid rain and global temperature changes do not drive an increase in the commercial manufacture of fertiliser.

✅ Correct Selections (Tick Two)

• Ca(NO₃)₂ (Calcium nitrate) [1 mark]
• (NH₄)₂SO₄ (Ammonium sulfate) [1 mark]

🧠 Exam Technique: Use Your Periodic Table

Look up the chemical symbol for nitrogen: it is simply N.

• Ca(NO₃)₂ → Contains N ✅
• Ca₃(PO₄)₂ → Contains Ca, P, O (No N) ❌
• KCl → Contains K, Cl (No N) ❌
• K₂SO₄ → Contains K, S, O (No N) ❌
• (NH₄)₂SO₄ → Contains N ✅

❌ Common Slip-Ups

Confusing N (Nitrogen) with Na (Sodium) or confusing P (Phosphorus) with nitrogen. Fertilisers are often described as "NPK" fertilisers, but this question asked strictly for compounds containing nitrogen.