AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 5

✅ Correct Matches

• Nitrogen → Air (1 mark)
• Hydrogen → Natural gas (1 mark)

💡 Key Knowledge

• Air is approximately 78% nitrogen gas (N₂), separated by fractional distillation of liquid air.
• Hydrogen (H₂) is mainly obtained by reacting methane (found in natural gas) with steam.

❌ Common Errors

• Drawing two lines from one box (e.g. connecting Nitrogen to both Air and Natural Gas) immediately forfeits that mark.
• Confusing Hydrogen's source with water/air instead of natural gas in this specific match list.

✅ Correct Answer

Tick the first box:

Gas  |  Gas (1 mark)

💡 Key Knowledge

Both nitrogen (N₂) and hydrogen (H₂) enter the reactor as gases under high pressure (around 200 atmospheres) and high temperature (around 450 °C).

❌ Common Errors

Thinking hydrogen or nitrogen are pumped in as liquids because they are stored under pressure. At 450 °C, both substances are well above their boiling points and are strictly gases.

✅ Correct Answer

Tick the third box:

There is one product. (1 mark)

💡 Key Knowledge

• Atom economy = (Relative formula mass of desired product ÷ Total relative formula mass of all reactants) × 100%.
• When an addition reaction produces only one single product, every reactant atom ends up in that desired product, making the atom economy 100%.

🧠 Exam Technique

Do not confuse atom economy with percentage yield:

• "The reaction is reversible" explains why the yield is never 100%.
• "There is one product" explains why the atom economy is 100%.

✅ Correct Sentence Completions

1. "The ammonia can be removed from X because the ammonia is liquefied." (1 mark)
2. "The unreacted nitrogen and hydrogen are recycled." (1 mark)

💡 Key Knowledge

• Ammonia has a significantly higher boiling point (-33 °C) than nitrogen (-196 °C) and hydrogen (-253 °C).
• When cooled in condenser X, ammonia condenses into a liquid and is tapped off.
• Unreacted N₂ and H₂ remain gases and are piped back into the reactor (recycled) to save raw materials and money.

❌ Common Errors

• Writing "evaporated" instead of "liquefied" — cooling causes condensation/liquefaction, not evaporation!
• Writing words in the wrong order. The mark scheme explicitly states: must be in this order.

📐 Data Points to Plot

✅ Mark Breakdown

• 2 marks: All 6 points plotted correctly within ±½ small square tolerance.
• (1 mark if 4 or 5 points are plotted correctly).
• 1 mark: A smooth curved line of best fit drawn through the points.

🧠 Exam Technique for Line of Best Fit

• Notice the points follow a curve that flattens off at higher pressures — do not use a ruler to force a straight line through all points!
• Do not join dot-to-dot with straight segments. Draw a single, smooth curved line with a sharp pencil.
• Check scale carefully: on the y-axis, each large grid square (5 units) contains 10 small squares, meaning 1 small square = 0.5%. For example, 33% is 6 small squares above 30.

❌ Common Errors

• Thick, 'feathery', or double lines drawn when attempting the curve.
• Plotting 33 at 31.5 or 36 due to misreading the small squares on the vertical scale.
• Extending the curve awkwardly down to the origin (0, 0) when not required.

✅ Correct Answer

(Percentage yield) increases  (1 mark)

Also acceptable: "Yield gets higher" or "It increases".

💡 Why does this happen? (Le Chatelier's Principle)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• There are 4 moles of gas on the left, but only 2 moles on the right.
• Increasing pressure shifts equilibrium to the side with fewer gas molecules (the right), thereby increasing the yield of ammonia.

🧠 Exam Technique

The question says "Use Table 8". You do not need to explain why using equilibrium theory; simply state the clear direct trend shown by the numbers: as pressure goes from 50 to 300 atm, yield goes up from 20% to 64%.