2020 P1 Q28 - Determine Moles of O2 Released at Anode
Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 28.
We are required to determine the moles of oxygen gas released at the anode for electrolysis of dilute sulfuric acid.
Let's first consider the species that can be oxidised at the anode.
We have 2 species to consider, sulfate ions and water.
We need to choose the relevant half equations in the data booklet to compare E value.
Since oxidation occurs at anode, these 2 species must be found on the right hand side of the half equation.
Comparing the following 2 half equations,
S2O82- + 2e ↔ 2SO42- +2.01V
O2 + 4H+ + 4e ↔ 2H2O +1.23V
Water has the more negative E value, hence more likely oxidised at the anode.
Therefore water will be oxidised according to the following half equation:
2H2O → O2 + 4H+ + 4e
We can now calculate moles of oxygen gas via the following steps:
- calculate moles of electrons using Faraday's equations
- calculate mole of O2 by comparing mole ratio of O2 to electron
Therefore the answer to this question is option C.
Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2020 A Level H2 Chemistry Paper 1.
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