2021 P1 Q25 - Redox Reaction between Chromite and Oxygen

Let Chemistry Guru, Singapore's top choice for JC Chemistry tuition, guide you through 2021 A Levels H2 Chemistry Paper 1 Question 25.

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 Cr₂O₄^2- to +6 in chromate CrO₄^2-.

Hence chromite is oxidised while oxygen is reduced.

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

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.

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 CrO₄^2- and OH^-.

From the balanced equation we can determine mole ratio of O2 to Cr₂O₄^2- is 3 to 2.

Hence answer to this question is option B.

2. Compare Oxidation State

Oxidation:

There is an increase in oxidation state of chromium from +3 in chromite Cr₂O₄^2- to +6 in chromate CrO₄^2-.

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

Hence each Cr₂O₄^2- loses 6 electrons.

Reduction:

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:

Hence we will get the same mole ratio of 3 O₂ to 2 Cr₂O₄^2-.

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

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