AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2022: Question 7

17 marks · High Demand difficulty · Extended Answer

Analyze the Haber process, including raw materials, atom economy, separation of ammonia, plotting and extrapolating yield data against pressure, and explaining the choice of reaction conditions based on rate, equilibrium, and cost.

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Question text

07 Ammonia is produced in the Haber process.

The raw materials for the Haber process are nitrogen and hydrogen.

The equation for the reaction is:

N2(g) + 3H2(g) ⇌ 2NH3(g)

07.1 Give the sources of the nitrogen and of the hydrogen used in the Haber process.

[2 marks]

Nitrogen

Hydrogen

07.2 How does the equation for the reaction show that the atom economy of the

forward reaction is 100%?

[1 mark]

07.3 Figure 4 represents the Haber process.

Figure 4

Explain how the ammonia produced is separated from the unreacted nitrogen and

hydrogen in X.

[2 marks]

The Haber process uses a temperature of 450 °C and a pressure of

200 atmospheres.

Table 6 shows the percentage yield of ammonia produced at 450 °C using

different pressures.

Table 6

Pressure in Percentage (%) yield

atmospheres of ammonia

60 9

120 18

180 25

240 31

300 36

360 40

420 25 43

07.4 Complete Figure 5.

The first two points have been plotted.

You should:

• use a suitable scale for the x-axis

• plot the remaining data from Table 6

• draw a line of best fit.

[4 marks]

Figure 5

07.5 Determine the percentage yield of ammonia at 450 °C and 500 atmospheres.

Show your working on Figure 5.

[2 marks]

Percentage yield = %

07.6 The equation for the production of ammonia in the Haber process is:

N2(g) + 3H2(g) ⇌ 2NH3(g)

*25* The forward reaction is exothermic.

The conditions used are:

• a temperature of 450 °C

• a pressure of 200 atmospheres

• the presence of an iron catalyst.

Explain why these conditions are chosen for economical production of ammonia in the

Haber process.

You should include references to the rate of reaction and the position of equilibrium.

[6 marks]

Mark scheme

Show the mark scheme

Question 7

AO /

Question Answers Extra information Mark

Spec. Ref.

07.1 (nitrogen) air allow atmosphere 1 AO1

4.10.4.1

(hydrogen) natural gas allow methane 1

allow water / steam

AO /

Spec. Ref.

07.2 there is only one product 1 AO2

4.3.3.2

4.10.4.1

AO /

Spec. Ref.

07.3 (mixture is) cooled 1 AO1

4.10.4.1

(so that only) ammonia liquefies allow (so that only) ammonia 1

condenses

AO /

Spec. Ref.

07.4 scale labelled at 100, 200, 300 allow scale labelled at 50, 150, 1 AO2

and 400 (atm) 250 and 350 (atm) 4.10.4.1

all five points plotted correctly allow a tolerance of ± ½ a small 2

square

allow 1 mark for three / four

points plotted correctly

line of best fit 1

AO / 21

Spec. Ref.

07.5 extrapolation to 500 1 AO2

View with atmospheres 4.10.4.1

Figure 5

percentage value at 500 allow a tolerance of ± ½ a small 1

atmospheres square

AO /

Question Answers Mark

Spec. Ref.

07.6 Level 3: Relevant points (reasons/causes) are identified, given in 5–6 AO1

detail and logically linked to form a clear account. 4.6.1.3

4.6.1.4

Level 2: Relevant points (reasons/causes) are identified, and there 3–4 4.6.2.6

are attempts at logical linking. The resulting account is not fully 4.6.2.7

clear. 4.10.4.1

Level 1: Points are identified and stated simply, but their relevance 1–2

is not clear and there is no attempt at logical linking.

No relevant content 0

Indicative content

rate

• higher temperature gives higher rate because of more frequent

collisions

• higher temperature gives higher rate because more particles

have the activation energy

• higher pressure gives higher rate because of more frequent

collisions

• use of catalyst gives higher rate because the activation energy is

lowered

equilibrium

• higher temperature shifts the position of equilibrium to the left

because reaction is exothermic

• higher pressure shifts the position of equilibrium to the right

because more molecules on left-hand side

• use of catalyst has no effect on the position of equilibrium

other factors

• higher temperature (than 450°C) uses more energy so increases

costs

• higher pressure (than 200 atmospheres) uses more energy so

increases costs

• higher pressure (than 200 atmospheres) requires stronger

reaction vessels so increases costs

• use of a catalyst reduces energy costs

compromise

• the temperature chosen is a compromise between rate of

reaction and position of equilibrium

• the temperature chosen is a compromise between rate and cost

• the pressure chosen is a compromise between yield / rate and

cost

Total Question 7 17

How to answer it

Mastering the Haber Process: Yield, Rates, and Compromises

What this question tests

This question assesses your understanding of the Haber process for producing ammonia. Key areas tested include: identifying raw material sources, calculating atom economy from chemical equations, explaining industrial separation techniques, plotting and extrapolating graphical data, and constructing a high-scoring, balanced argument on how temperature, pressure, and catalysts are chosen as economic compromises.

Part 07.1: Raw Materials

Identify the sources of nitrogen and hydrogen used in the Haber process.

Recall & Facts [2 marks]

Correct Answers

Nitrogen: Air +1 mark
(Accept: atmosphere)
Hydrogen: Natural gas +1 mark
(Accept: methane / water / steam)

Common Errors

Writing "water" for nitrogen (confusing the two sources).
Stating "fuel" instead of specifically "natural gas" or "methane".
Vague answers like "the ground" or "the environment".

Part 07.2: Atom Economy

How does the equation show that the atom economy of the forward reaction is 100%?

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

Equation Analysis [1 mark]

Correct Answer

There is only one product (ammonia) formed in the reaction.

+1 mark

Key Knowledge

Atom economy is a measure of the amount of starting materials that end up as useful products. If a reaction has only one product, 100% of the reactant atoms must end up in that product, meaning no atoms are wasted!

Part 07.3: Separation of Ammonia

Explain how the ammonia produced is separated from the unreacted nitrogen and hydrogen in Condenser X.

Industrial Processes [2 marks]

Correct Answer

The mixture is cooled. +1 mark
This causes only the ammonia to liquefy / condense (so it can be run off). +1 mark

Exam Technique

Always link the physical change (cooling) to the specific substance that changes state (ammonia condensing). Nitrogen and hydrogen have extremely low boiling points, so they remain as gases and are recycled back into the reactor.

Parts 07.4 & 07.5: Graph Skills & Extrapolation

Plotting and using data to determine yield at 500 atmospheres.

Data & Graphing [6 marks total]

How to Plot and Complete Figure 5 [4 marks]

Scale the x-axis: Label the major grid lines at 100 , 200 , 300 , and 400 atmospheres. Each small square represents 10 atmospheres. +1 mark
Plot the remaining 5 points: Use the data from Table 6:
- (180 atm, 25%)
- (240 atm, 31%)
- (300 atm, 36%)
- (360 atm, 40%)
- (420 atm, 43%)
Plot within a tolerance of ± half a small square. +2 marks (1 mark if only 3 or 4 points are correct)
Line of Best Fit: Draw a smooth, single-line curve passing through the points. Do not use a ruler to connect dot-to-dot! +1 mark

07.5 Extrapolation [2 marks]

Step 1: Use a ruler to carefully extend (extrapolate) your smooth curve of best fit up to the 500 atmospheres line. +1 mark

Step 2: Read the corresponding percentage yield value on the y-axis. The expected value is approximately 47% to 49% (depending on your line of best fit). +1 mark

Common Graph Traps

Dot-to-dot lines: Drawing straight lines between points instead of a smooth curve will lose the line of best fit mark.
Missing extrapolation: Forgetting to physically draw the extended line on the graph to show your working.

Visualizing the Graph

The completed graph should show a curve that starts steep at lower pressures and gradually flattens out (curves to the right) as pressure increases. To find the yield at 500 atm, your drawn curve must continue past the last plotted point (420 atm, 43%) in the same curved trajectory up to the vertical grid line for 500 atm.

Part 07.6: The 6-Mark Compromise Question

Explain why a temperature of 450 °C, a pressure of 200 atmospheres, and an iron catalyst are chosen for the economical production of ammonia.

Level of Response [6 marks]

How to Structure a Level 3 (5-6 Mark) Answer

To get full marks, you must not just list facts. You must link your points logically, explaining how each condition affects both the rate of reaction and the position of equilibrium (yield), and why the final choice is an economic compromise.

1. Rate of Reaction

Higher temperature increases rate because particles have more kinetic energy, leading to more frequent collisions and more particles exceeding the activation energy.
Higher pressure increases rate because gas molecules are closer together, leading to more frequent collisions.
Iron catalyst increases rate by providing an alternative pathway with a lower activation energy.

2. Position of Equilibrium

The forward reaction is exothermic. Therefore, a higher temperature shifts the equilibrium to the left (reducing the yield of ammonia).
There are fewer molecules of gas on the right (2 molecules of NH₃ vs 4 molecules of reactants). Therefore, a higher pressure shifts the equilibrium to the right (increasing the yield of ammonia).
The catalyst has no effect on the position of equilibrium.

3. Costs & Compromise

Temperature (450 °C): A compromise. Low temp gives high yield but is too slow. High temp is fast but gives a very low yield.
Pressure (200 atm): A compromise. High pressure gives high yield and fast rate, but building safe, strong pipes to withstand extremely high pressures is too expensive.
Catalyst: Allows a lower temperature to be used while maintaining a fast rate, saving energy and reducing costs.

Common Pitfalls where Students Lose Marks

Confusing Rate and Yield: Stating that a catalyst increases the yield of ammonia. (Catalysts only make the reaction *faster*, they do not change the amount of product made at equilibrium!).
Vague Cost Statements: Saying "high pressure is dangerous" without linking it to the economic cost of building stronger, reinforced reaction vessels.
Missing the "Exothermic" link: Failing to state that the forward reaction is exothermic when explaining why high temperatures lower the yield.

Topics

Chemistry · C10: Using Resources · C3: Quantitative Chemistry · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.