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.

2020 p1 q28 question

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.

2020 p1 q28 species at 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.

2020 p1 q28 deduce water oxidised at anode

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

2020 p1 q28 calculate moles of O2

Therefore the answer to this question is option C.

2020 p1 q28 answer

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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