2020 P1 Q27 - Hydrogen Oxygen Fuel Cell in Alkaline Medium

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 27.

2020 p1 q27 question

We are given a hydrogen-oxygen fuel cell with NaOH as electrolyte.

We need to determine the direction of electron flow in the external circuit and the E value for the oxygen electrode.

Let us first deduce the anode and cathode from the redox reaction between hydrogen and oxygen.

2020 p1 q27 deduce anode cathode for fuel cell

Hydrogen is oxidised to water so electrode X at the hydrogen side is the anode (negatively charged).

Oxygen is reduced to water so electrode Y at the oxygen side is the cathode (positively charged).

So in the external circuit, electron will flow from the negative electrode X to positive electrode Y.

2020 p1 q27 determine electron flow

Next let us determine the correct half equation and E value for the oxygen electrode.

Since oyxgen is reduced, it should be found on the left hand side of the half equation found in the data booklet.

The electrolyte is NaOH(aq), so the reaction is in alkaline medium.

Hence we want the half equation where O2 is reduced to a product with stable oxidation state, in the presence of OH-.

2020 p1 q27 deduce E of oxygen electrode in alkaline medium

Comparing the options, the following half equation is the best choice since OH- is present and oxidation state of oxygen in product OH- is -2 and stable.

O2 + 2H2O + 4e ↔ 4OH-        +0.40V

Finally we can compare the options and determine the answer to this question is option A.

2020 p1 q27 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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