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A-Level Chemistry May/June 2024 Q2(e): An electrochemical cell uses Ag2O as the positive electrode and Zn as the negative elec…
A-Level Chemistry · Paper 9701/41 · May/June 2024 · Question 2(e) · [2 marks]
An electrochemical cell uses Ag2O as the positive electrode and Zn as the negative electrode immersed in an alkaline electrolyte. The overall cell reaction is shown. Ag2O + Zn + H2O → 2Ag + Zn(OH)2 Complete the half-equation for the reaction at each electrode.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Positive electrode (cathode):
Reduction occurs at the positive electrode. Silver is reduced from an oxidation state of +1 in Ag₂O to 0 in Ag.
- Start with the main species:
Ag₂O → 2Ag - Balance the oxygen atoms by adding OH⁻ ions to the right:
Ag₂O → 2Ag + 2OH⁻ - Balance the hydrogen atoms by adding H₂O to the left:
Ag₂O + H₂O → 2Ag + 2OH⁻ - Balance the charge. The left side has a charge of 0. The right side has a charge of -2. Add 2 electrons to the left side to balance.
Final half-equation: Ag₂O + H₂O + 2e⁻ → 2Ag + 2OH⁻
Negative electrode (anode):
Oxidation occurs at the negative electrode. Zinc is oxidised from an oxidation state of 0 in Zn to +2 in Zn(OH)₂.
- Start with the main species:
Zn → Zn(OH)₂ - Balance the oxygen and hydrogen atoms by adding OH⁻ ions to the left:
Zn + 2OH⁻ → Zn(OH)₂ - Balance the charge. The left side has a charge of -2. The right side has a charge of 0. Add 2 electrons to the right side to balance.
Final half-equation: Zn + 2OH⁻ → Zn(OH)₂ + 2e⁻
How the marks are awarded
- M1 — The first mark is awarded for the correct half-equation at the positive electrode: Ag₂O + H₂O + 2e⁻ → 2Ag + 2OH⁻. All species, state symbols (if included), balancing, and charge must be correct.
- M2 — The second mark is awarded for the correct half-equation at the negative electrode: Zn + 2OH⁻ → Zn(OH)₂ + 2e⁻. All species, balancing, and charge must be correct.
Common mistakes
- Attempting to balance the equations using H⁺ ions, which is only appropriate for acidic or neutral conditions, not the stated alkaline electrolyte.
- Incorrectly placing electrons, for example, showing electrons being gained during oxidation or lost during reduction.
- Failing to balance the atoms or charges correctly, for instance, having an unequal number of oxygen atoms or an overall charge that is not zero on both sides of the arrow.
- Reversing the equations for the electrodes, assigning the oxidation half-equation to the positive electrode and reduction to the negative electrode.
Examiner tip: When balancing half-equations in an alkaline electrolyte, remember to use OH⁻ ions and H₂O to balance oxygen and hydrogen atoms, not H⁺ ions.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
- Start with the main species:
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