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CAIE A-Level Chemistry AS 6 Electrochemistry Questions

Practise oxidation numbers, electron-transfer equations and identification of oxidising or reducing agents, using redox changes to construct and interpret balanced reactions.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Covered syllabus points

6.1 Redox: electron transfer and oxidation number

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.

Question 4

[Maximum number: 1]

The vanadium salt, VOSO4\mathrm{VOSO}_{4}, is soluble in water and reacts readily with powdered manganese in dilute sulfuric acid.

The equation for the reaction is shown.

Mn( s)+2VOSO4(aq)+2H2SO4(aq)→V2(SO4)3(aq)+MnSO4(aq)+2H2O(l)\mathrm{Mn}(\mathrm{~s})+2 \mathrm{VOSO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{V}_{2}\left(\mathrm{SO}_{4}\right)_{3}(\mathrm{aq})+\mathrm{MnSO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Which statement about this reaction is correct?

A

Hydrogen is oxidised in the reaction.

B

Manganese is the reducing agent in this reaction.

C

Sulfuric acid is the oxidising agent in this reaction.

D

The oxidation state of the vanadium changes from +5 to +3 .

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