2021 P1 Q25 - Redox Reaction between Chromite and Oxygen

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 25.

2021 p1 q25 question

We need to determine the moles of oxygen molecules required to react with one mole of chromite ions.

First we need to determine which species is oxidised and reduced in this redox reaction.

There is an increase in oxidation state of chromium from +3 in chromite Cr2O42- to +6 in chromate CrO42-.

Hence chromite is oxidised while oxygen is reduced.

Since the reaction is done in molten sodium hydroxide, oxygen should be reduced to hydroxide ions.

2021 p1 q25 deduce who is oxidised and reduced

There are 2 ways to determine the mole ratio between oxygen molecule and chromite.

1. Balance Redox Equation

We can balance this redox reaction using half equation method to determine the mole ratio.

2021 p1 q25 balance redox equation

Check out this video to learn how to balance half equation for redox reactions.

Since we only need the mole ratio of the reactants, we can ignore the moles of other species involved such as the products CrO42- and OH-.

From the balanced equation we can determine mole ratio of O2 to Cr2O42- is 3 to 2.

Hence answer to this question is option B.

2021 p1 q25 answer

2. Compare Oxidation State


There is an increase in oxidation state of chromium from +3 in chromite Cr2O42- to +6 in chromate CrO42-.

Each Cr oxidation state increases by 3 units, ie loses 3 electrons.

Hence each Cr2O42- loses 6 electrons.


There is a decrease in oxidation state of oxygen from 0 in oxygen molecule O2 to -2 in hydroxide OH-.

Each O oxidation state decreases by 2 units, ie gains 2 electrons.

Hence each O2 gains 4 electrons.

Putting it together, since moles of electrons lost by 1 mole of chromite is equal to moles of electrons gained by x moles of oxygen:

2021 p1 q25 compare oxidation state

Hence we will get the same mole ratio of 3 O2 to 2 Cr2O42-.

Topic: Redox Reactions, Physical Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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