VCE Unit 3 Chemistry SAC
Topic: Rates, Equilibrium & Electrolysis (Haber Process + URFC Case Studies)
Total Marks: 40 | Time Allowed: 50 minutes
SECTION A: The Haber Process (20 marks)
Case Study – The Haber Process
Section A
A case study: the Haber process developed by Fritz Haber and carl Bosch in the early 20th century is a method for
synthesizing ammonia from nitrogen and hydrogen gasses by using metal catalysts. The Haber process revolutionized
agriculture by providing a reliable and cost-effective way to produce ammonia. Before its development ammonia was
primarily obtained from natural sources such as guano which was limited and expensive the Haber process made
ammonia production scalable transforming the agricultural industry, although the process is exothermic and requires
a significant energy supply to achieve optimum temp at 450C. the Haber process also played a significant role in
WW1 and WW2 because it ability to produce ammonia efficiently made it critical for the production of explosives.
Multiple Choice (10 marks – 1 mark each)
Circle the correct option.
1. What type of reaction is the Haber Process?
A. Endothermic
B. Exothermic
C. Combustion
D. Electrolytic
2. Which catalyst is typically used in the Haber Process?
A. Copper
B. Nickel
C. Iron
D. Platinum
3. What would increase the yield of ammonia according to Le Chatelier’s Principle?
A. Increasing temperature
B. Increasing pressure
C. Decreasing nitrogen concentration
D. Removing hydrogen
4. What is the balanced chemical equation for the Haber Process?
A. N₂ + 3H₂ ⇌ 2NH₃
B. N₂ + H₂ ⇌ NH₃
C. N + 3H ⇌ NH₃
D. N + H₂ ⇌ NH
5. Why is 450°C used in the Haber Process?
A. It gives maximum yield
B. It is the melting point of ammonia
C. It balances rate and yield
D. It favours the reverse reaction
6. In the Haber Process, if hydrogen is continuously removed, the reaction will shift:
A. Left
B. Right
C. Stop
D. None of the above
7. What is the role of the catalyst in the Haber Process?
A. Increase yield
B. Increase activation energy
C. Provide alternative pathway
D. Shift equilibrium
8. What happens to the value of K when temperature increases in an exothermic reaction?
A. Increases
B. Decreases
C. Stays the same
D. Becomes zero
9. Which factor does NOT affect the rate of the Haber Process?
A. Catalyst
B. Pressure
C. Temperature
D. Colour of the gas
10. What is the purpose of recycling unreacted nitrogen and hydrogen in the process?
A. Lower temperature
B. Conserve energy
C. Maximise efficiency
D. Cool the reaction chamber
Short Answer Questions (10 marks)
Q1. (3 marks)
Explain the conflict between optimal rate and yield in the Haber Process. Refer to temperature and Le Chatelier’s
Principle.
Answer:
Q2. (3 marks)
Sketch a concentration-time graph for the Haber Process reaching equilibrium. Label all relevant parts.
Answer:
(Draw below with axes labelled)
Q3. (4 marks)
At equilibrium, the concentrations are as follows:
[N₂] = 1.0 mol/L, [H₂] = 2.0 mol/L, [NH₃] = 3.0 mol/L
Write the expression for K and calculate its value. Include units.
Answer:
SECTION B: Unitised-Regenerative Fuel Cells (URFCs) (20 marks)
Case Study – URFCs
Section B
A chemical innovation: researchers at Washington University in St. Louis (USA) have recently developed Unitised-
Regenerative Fuel Cells (URFC), which represent an innovative chemical approach to energy storage, combining the
functions of a fuel cell & an electrolyser (electrolysis device) within a single device (Jefferson, 2021). URFCs operate
by using electricity to split water into hydrogen & oxygen gases during electrolysis mode, storing energy chemically
(Figure 1). Conversely, in a fuel cell mode, they recombine these gases to generate electricity and water. This dual
functionality offers significant benefits, including reduced system complexity, lower weight & volume compared to
separate systems, and a potential for high round-trip efficiency, making them promising for applications like
renewable energy integration, portable power, and specialised uses in aerospace & underwater vehicles.
Short Answer + Calculations (20 marks)
Q1. (3 marks)
Describe how URFCs use water in both electrolysis mode and fuel cell mode.
Answer:
Q2. (3 marks)
Explain the advantage of using URFCs over separate electrolyser and fuel cell systems.
Answer:
Q3. (4 marks)
Write the half-equations at the electrodes and the overall equation for URFC in acidic conditions during:
• Electrolysis mode
• Fuel cell mode
Answer:
Electrolysis Mode:
Anode: _______________________________________________________
Cathode: _____________________________________________________
Overall: ______________________________________________________
Fuel Cell Mode:
Anode: _______________________________________________________
Cathode: _____________________________________________________
Overall: ______________________________________________________
Q4. (4 marks)
Faraday’s Law:
If a current of 2.00 A is passed through an acidic water electrolyser for 1 hour, calculate the volume of hydrogen gas
(at STP) produced.
(Molar volume at STP = 22.4 L/mol, F = 96,500 C/mol e⁻)
Answer:
Q5. (6 marks)
The URFC cell operates at high efficiency when using a polymer electrolyte membrane (PEM).
a. (3 marks) Explain the role of the PEM in both modes.
b. (3 marks) Identify two green chemistry principles demonstrated by URFC technology and justify your choices.
Part B – Short Answer and Calculations (2 marks each)
11. Describe two conditions used in the Haber process and how they affect the rate and yield of ammonia.
12. Explain Le Chatelier’s Principle and how it applies when pressure is increased in the Haber process.
13. The equilibrium constant (K) for the Haber process decreases with increasing temperature. Explain why.
14. Write the balanced chemical equation for the Haber process and indicate the states.
15. A reaction vessel initially contains 1.00 mol of N₂ and 3.00 mol of H₂. At equilibrium, 0.60 mol of NH₃ is formed.
Calculate the equilibrium constant K.
Part C – Theory and Calculations (2 marks each)
16. Describe how a PEM (polymer electrolyte membrane) electrolysis system works and its advantages.
17. Compare the electrolyser and fuel cell functions of a URFC as shown in the case study diagram.
18. Discuss the environmental impact of using URFCs compared to conventional energy systems.
19. Identify two strengths and two limitations of URFCs as an energy solution.
20. Compare PEM electrolysis and artificial photosynthesis as methods for producing hydrogen. Which is more
sustainable and why?
