AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 9

✅ Model Answer (3 Marks)

1. The forward reaction is endothermic (absorbs heat). [1 mark]
2. Therefore, a high temperature shifts the position of equilibrium to the right. [1 mark]
3. This increases the yield of products (hydrogen / carbon monoxide). [1 mark]
   (Alternative for 3rd mark: and a high temperature increases the rate of reaction).

🧠 Exam Technique: 3-Step Structure

Whenever asked why a specific temperature is chosen for a reversible reaction, follow this strict formula:

• Step 1: State if forward reaction is endo- or exothermic.
• Step 2: State which way equilibrium shifts (left or right).
• Step 3: State the outcome on yield (or mention rate).

❌ Common Errors

• Forgetting to link the shift to the direction of energy change (failing to mention "endothermic").
• Confusing rate with yield. A high temperature always increases rate, but it only increases yield if the forward reaction is endothermic!

✅ Model Answer (2 Marks)

• A low pressure shifts the equilibrium position to the right. [1 mark]
• Because there are more moles (or molecules) of gas on the right-hand side (4 moles of gas on the right vs 2 moles of gas on the left). [1 mark]

💡 Key Knowledge: Pressure Rules

Count the balanced gaseous balancing numbers (stoichiometry):

1 CH₄ + 1 H₂O = 2 moles  ⇌  1 CO + 3 H₂ = 4 moles

• Decreasing pressure shifts equilibrium to the side with more gas moles to restore pressure.
• Increasing pressure shifts equilibrium to the side with fewer gas moles.

✅ Correct Checkbox

☑ No effect on equilibrium position.

💡 Why Doesn't a Catalyst Shift Equilibrium?

A catalyst increases the rate of the forward and reverse reactions by the exact same amount by providing an alternative pathway with a lower activation energy. It helps equilibrium be reached faster, but does not change the yield.

📐 Step-by-Step Calculation

Step 1: Recall the percentage of nitrogen in air.
Air is approximately 80% nitrogen (AQA also accepts 78%).

Step 2: Set up the expression.
Volume of air = (Volume of N₂ × 100) / % of N₂ in air
Volume = (50 × 100) / 80 [1 mark]

Step 3: Calculate the final volume.
Volume = 5000 / 80 = 62.5 dm³ [1 mark]
(If using 78%: 50 / 0.78 ≈ 64.1 dm³)

❌ Common Error

Many students multiply 50 by 0.80 to get 40 dm³! That would be calculating how much nitrogen is inside 50 dm³ of air.

Here you are working backwards: you need 50 dm³ of nitrogen, so you need more air than 50 dm³ to obtain it!

✅ Model Answer (3 Marks)

1. The mixture is cooled. [1 mark]
2. Ammonia condenses into a liquid (and is tapped off). [1 mark]
3. Nitrogen and hydrogen remain as gases (because they have much lower boiling points) and are recycled. [1 mark]

🧠 Examiner's Insight

To secure all 3 marks, you must mention all three chemicals:

• What happens to ammonia? It condenses into a liquid.
• What happens to nitrogen AND hydrogen? They stay as gases.
• What is the process condition? Cooling.

✅ Model Answer (Any 3 from):

• Abundance: Nitrogen is abundant / plentiful in the air (~80%). [1 mark]
• Renewability: Nitrogen is renewable / the released nitrogen replaces that used to make ammonia. [1 mark]
• Resource type: Natural gas (used to make hydrogen) is a finite / non-renewable resource. [1 mark]
• Global Warming: The production of hydrogen releases carbon dioxide (CO₂), which contributes to climate change / global warming, whereas nitrogen extraction does not. [1 mark]
• Emissions: The use of nitrogen releases fewer greenhouse gases. [1 mark]

🧠 Top-Grade Revision Tip

When an exam prompt provides context (e.g. "Nitrogen is released by bacteria... methane releases carbon dioxide"), use those specific hints directly in your answer!

Always make direct comparisons between the two feedstocks (e.g. finite methane vs renewable nitrogen; CO₂ released vs no greenhouse gases).