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CAIE A-Level Chemistry AS 6.1 Redox Electron Transfer and Oxidation Number Questions

Practise identifying oxidation and reduction through electron transfer or oxidation-number change and balancing redox equations.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • assign oxidation numbers using charge-sum rules and identify which element changes
  • define oxidation as electron loss or oxidation-number increase and reduction as gain or decrease
  • identify oxidising and reducing agents and combine half-equations with equal electron transfer

Covered syllabus points

6.1.1—Oxidation numbers of elements in compounds6.1.2—Changes in oxidation numbers to help balance6.1.3 Redox Terminology6.1.4—And use the terms oxidising agent and reducing6.1.5 Oxidation Numbers in Compound Names

Question 1

[Maximum number: 1]

Sodium chromate(VI), Na2CrO4\mathrm{Na}_{2} \mathrm{CrO}_{4}, is manufactured by heating chromite, FeCr2O4\mathrm{FeCr}_{2} \mathrm{O}_{4}, with sodium carbonate in an oxidising atmosphere. Chromite contains Cr2O42−\mathrm{Cr}_{2} \mathrm{O}_{4}{ }^{2-} ions.

2FeCr2O4+4Na2CO3+312O2→4Na2CrO4+Fe2O3+4CO22 \mathrm{FeCr}_{2} \mathrm{O}_{4}+4 \mathrm{Na}_{2} \mathrm{CO}_{3}+3 \frac{1}{2} \mathrm{O}_{2} \rightarrow 4 \mathrm{Na}_{2} \mathrm{CrO}_{4}+\mathrm{Fe}_{2} \mathrm{O}_{3}+4 \mathrm{CO}_{2}

What happens in this reaction?

A

Chromium and iron are the only elements oxidised.

B

Chromium, iron and carbon are oxidised.

C

Only chromium is oxidised.

D

Only iron is oxidised.

Question 2

[Maximum number: 1]

Oxygen can be prepared by the reaction of potassium manganate(VII), KMnO4\mathrm{KMnO}_{4}, hydrogen peroxide, H2O2\mathrm{H}_{2} \mathrm{O}_{2}, and sulfuric acid, H2SO4\mathrm{H}_{2} \mathrm{SO}_{4}. Each H2O2\mathrm{H}_{2} \mathrm{O}_{2} molecule loses two electrons in this reaction. The other products of the reaction are potassium sulfate, manganese(II) sulfate and water.

How many moles of oxygen gas are produced when 1.0 mol of KMnO4\mathrm{KMnO}_{4} reacts with an excess of H2O2\mathrm{H}_{2} \mathrm{O}_{2} in acidic conditions?

A

2.0 mol

B

2.5 mol

C

4.5 mol

D

5.0 mol

Question 3

[Maximum number: 1]

Nitrogen dioxide reacts with water.

2NO2+H2O→HNO2+HNO32 \mathrm{NO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{HNO}_{2}+\mathrm{HNO}_{3}

Which statement about this reaction is correct?

A

Both products are formed because oxygen atoms gain electrons.

B

Nitrogen atoms undergo disproportionation.

C

The oxidation number of hydrogen is increased.

D

Water acts as an oxidising agent.

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