A-Level Chemistry May/June 2024 Q3(g): A fuel cell is an electrochemical cell that can be used to generate electrical energy b…

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  To find the overall cell reaction, we must identify the oxidation and reduction half-reactions. The half-reaction with the more positive Eº value is the reduction (at the cathode), and the one with the more negative Eº value is the oxidation (at the anode).

  • Reduction (Cathode): O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) (Eº = +0.40V)
  • Oxidation (Anode): The C₂O₄²⁻/CO₂ half-equation must be reversed. We also need to multiply it by 2 to balance the 4 electrons from the reduction half-reaction. 2C₂O₄²⁻(aq) → 4CO₂(g) + 4e⁻

  Overall Cell Reaction: Combine the two balanced half-equations and cancel the electrons: 2C₂O₄²⁻(aq) + O₂(g) + 2H₂O(l) → 4CO₂(g) + 4OH⁻(aq)

  Calculation of Eºcell: Use the formula $Eº_{cell} = Eº_{reduction} - Eº_{oxidation}$.

  $Eº_{cell} = (+0.40) - (-0.59)$

  $Eº_{cell} = +0.99 \, \text{V}$

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  How the marks are awarded

  • M1 — Deducing the correct, balanced overall cell reaction by identifying the correct oxidation and reduction half-equations, balancing the electrons, and combining them.
  • M2 — Correctly calculating the standard cell potential (Ecell) by subtracting the Eº value of the oxidation half-cell from the Eº value of the reduction half-cell, resulting in 0.99 V.

  Common mistakes

  • Incorrectly identifying the oxidation/reduction processes, leading to the Ecell calculation being reversed: (-0.59) - (0.40) = -1.00 V. A positive Ecell is required for a spontaneous reaction in a cell.
  • Failing to balance the electrons between the two half-equations, resulting in an incorrectly stoichiometrically balanced overall equation.
  • Incorrectly applying the Ecell formula, for example by adding the two Eº values: 0.40 + (-0.59) = -0.19 V.
  • Writing the overall equation in the reverse direction, which would represent the electrolysis reaction, not the fuel cell reaction.

  Examiner tip: For any electrochemical cell, always identify the half-reaction with the more positive Eº value as the reduction process and apply the formula Ecell = E(reduction) - E(oxidation).

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