Fuel cellsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A hydrogen–oxygen fuel cell with an acidic electrolyte is used to power a small vehicle. Standard electrode potentials: 2H⁺(aq) + 2e⁻ ⇌ H₂(g) 0.00 V; O₂(g) + 4H⁺(aq) + 4e⁻ ⇌ 2H₂O(l) +1.23 V.

   (a)

   Which half-equation takes place at the negative electrode of this fuel cell?

   [1 mark]

   • AO₂ + 4H⁺ + 4e⁻ → 2H₂O
   • B2H⁺ + 2e⁻ → H₂
   • CH₂ → 2H⁺ + 2e⁻
   • DH₂ + 2e⁻ → 2H⁻

   (b)

   What is E°cell for this fuel cell?

   [1 mark]

   • A+2.46 V
   • B+1.23 V
   • C0.00 V
   • D−1.23 V

   (c)

   Explain why the electrolyte, which allows H⁺ ions to pass from one electrode to the other, is needed in this fuel cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hydrogen–oxygen fuel cell uses aqueous potassium hydroxide as the electrolyte.

   (a)

   Which half-equation takes place at the positive electrode of this fuel cell?

   [1 mark]

   • AO₂ + 2H₂O + 4e⁻ → 4OH⁻
   • B2H₂O + 2e⁻ → H₂ + 2OH⁻
   • CO₂ + 4H⁺ + 4e⁻ → 2H₂O
   • D4OH⁻ → O₂ + 2H₂O + 4e⁻

   (b)

   Which statement about the overall reaction in this fuel cell is correct?

   [1 mark]

   • AHydroxide ions are used up overall, so the electrolyte becomes less alkaline
   • BHydroxide ions are formed overall, so the electrolyte becomes more alkaline
   • CThe overall reaction forms hydrogen peroxide
   • DThe overall reaction is 2H₂ + O₂ → 2H₂O and no hydroxide ions are used up overall

   (c)

   Write the half-equation for the reaction at the negative electrode of this fuel cell and use oxidation numbers to show that it is an oxidation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A methanol fuel cell uses an acidic electrolyte. The half-equations are: anode CH₃OH(l) + H₂O(l) → CO₂(g) + 6H⁺(aq) + 6e⁻; cathode O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l). Use F = 96 500 C mol⁻¹ and M(CH₃OH) = 32.0 g mol⁻¹.

   (a)

   Combine the half-equations to deduce the overall equation for the reaction in the methanol fuel cell.

   [3 marks]

   (b)

   Calculate the charge, in C, that 1.00 g of methanol can supply, and the time for which this could keep a current of 0.50 A flowing.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city bus company is comparing buses powered by fuel cells with buses powered by diesel engines. The fuel-cell options are a hydrogen–oxygen cell and a methanol–oxygen cell, both with an acidic electrolyte.

   (a)

   Explain how a hydrogen–oxygen fuel cell with an acidic electrolyte generates an electric current, and why it can keep supplying current for as long as fuel is provided, unlike a conventional battery.

   [6 marks]

   (b)

   Evaluate the use of methanol rather than hydrogen as the fuel for the fuel-cell buses.

   [6 marks]

   Total for question 4: 12 marks

End of questions