IGCSE
Cambridge (CIE)
Chemistry
2 hours
15 questions
Theory Questions
Redox
Oxidation & Reduction / Redox & Electron Transfer
Easy (5 questions)
/31
Medium (5 questions)
/36
Hard (5 questions)
/50
Total Marks
/117
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1
Easy Questions
1 (a) This question is about the reactions of metals with oxygen.
Complete the sentences.
In terms of oxygen, ____________________ is the loss of oxygen.
In terms of oxygen, ____________________ is the gain of oxygen.
(2 marks)
(b) Iron can react with oxygen to form iron(II) oxide.
2Fe + O2 → 2FeO
Iron can also react with oxygen to form iron(III) oxide
4Fe + 3O2 → 2Fe2O3
Nickel can react with oxygen to form nickel(II) oxide or nickel(III) oxide.
Write two balanced symbol equations to show both reactions of nickel with oxygen.
(2 marks)
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(c) Nickel is below carbon in the reactivity series.
This means that carbon can be used to extract nickel from nickel(II) oxide.
nickel(II) oxide + carbon → nickel + carbon dioxide
The reaction of nickel(II) oxide with carbon is a redox reaction.
Is carbon oxidised or reduced? Explain your answer.
Carbon is ____________________
Reason: ____________________
(2 marks)
(d)
Extended Only
Nickel can also be extracted from nickel(III) oxide using carbon.
Nickel(III) gains electrons to form nickel.
Balance the half-equation to show how nickel gains electrons:
Ni3+
+
.... e– →
.........
(2 marks)
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2 (a)
Extended Only
During metal displacement reactions, the metal that is the least reactive metal is
reduced.
Which of these statements correctly describes what reduction is?
Tick one box.
The gain of oxygen
The gain of electrons
The simulatanous gain and loss of electrons
An increase in oxidation number
(1 mark)
(b) Which of the following reactions shows iron being reduced? Tick one box.
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copper nitrate + iron → iron nitrate + copper
iron + oxygen → iron(II) oxide
iron(II) oxide + carbon → iron + carbon dioxide
hydrochloric acid + iron → iron chloride + hydrogen
(1 mark)
(c)
Separate: Chemistry and Extended Only
When iron reacts with sulfuric acid, iron(II) sulfate is produced:
Fe + H2SO4 → FeSO4 + H2
Give the oxidation number of iron in the following species:
Fe ............................
FeSO4 ............................
(2 marks)
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(d)
Separate: Chemistry and Extended Only
Explain how the oxidation numbers in part (c) show that iron has been oxidised.
(1 mark)
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3 (a)
Separate: Chemistry and Extended Only
Redox reactions can be identified by colour changes.
Potassium manganate(VII), KMnO4, is an oxidising agent which is used to test for the
presence of reducing agents.
What is meant by the term oxidising agent?
(2 marks)
(b) Calculate the relative formula mass of potassium manganate(VII), KMnO4. Use your
Periodic Table to help you.
relative formula mass = ...................................
(2 marks)
(c)
Separate: Chemistry and Extended Only
When potassium manganate(VII) is added to a reducing agent, a colour change is seen.
Which of the following correctly shows the colour change seen? Tick one box.
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colourless to blue
red to blue
yellow to red
purple to colourless
(1 mark)
(d)
Separate: Chemistry and Extended Only
Potassium iodide is a reducing agent that can be used to test for the presence of
oxidising agents. Complete the half-equation to show how iodide ions act as a reducing
agent.
... I– → I2 + .... e–
(1 mark)
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4 (a) The reaction of zinc oxide with carbon is an example of a redox reaction.
What is meant by the term redox reaction?
(1 mark)
(b) When zinc oxide reacts with carbon, carbon displaces zinc from its oxide.
ZnO + C → Zn + CO
How does this equation show that zinc oxide is reduced?
(1 mark)
(c) Iron oxide can also be reduced by carbon. Iron oxide can exist as iron(II) oxide or iron(III)
oxide.
What do the Roman numerals indicate in the names of the compound?
(1 mark)
(d) Complete the balanced equation to show the reduction of iron(III) oxide with carbon.
2Fe2O3 + .... C → .... Fe + .... CO2
(2 marks)
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5 (a)
Separate: Chemistry and Extended Only
This question is about redox reactions.
Match the key term on the left with the definition on the right.
(4 marks)
(b)
Separate: Chemistry and Extended Only
Chromium is an element that can have different oxidation numbers when it is combined
with other elements. State the charge on the chromium ion in the compound
chromium(II) oxide.
(1 mark)
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(c)
Extended Only
Chromium(II) oxide can be formed from the reaction between chromium and oxygen.
Complete the half-equation for the reaction.
......... → Cr2+ + ..... e–
(1 mark)
(d)
Extended Only
The other half-equation for the reaction is shown below.
O2 + 4e– → 2O2–
Explain, in terms of electrons, how this shows that oxygen has been reduced.
(1 mark)
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Medium Questions
1 (a)
Separate: Chemistry and Extended Only
The following reactivity series shows both familiar and unfamiliar elements in order of
decreasing reactivity. Each element is represented by a redox equation.
Rb ⇌ Rb+ + e–
Mg ⇌ Mg2+ + 2e–
Mn ⇌ Mn2+ + 2e–
Zn ⇌ Zn2+ + 2e–
H2 ⇌ 2H+ + 2e–
Cu ⇌ Cu2+ + 2e–
Hg ⇌ Hg2+ + 2e–
Two of the uses of the series are to predict the thermal stability of compounds of the
metals and to explain their redox reactions.
i) Define in terms of electron transfer the term oxidation.
[1]
ii) Explain why the positive ions in the above equations are oxidising agents.
[1]
(2 marks)
(b) i) Which metals in the series above do not react with dilute acids to form hydrogen?
[1]
ii) Describe an experiment which would confirm the prediction made in (b)(i).
[1]
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(2 marks)
(c)
Separate: Chemistry and Extended Only
i) Which metal in the series above can form a negative ion which gives a pink / purple
solution in water?
[1]
ii) Describe what you would observe when zinc, a reducing agent, is added to this pink /
purple solution.
[1]
(2 marks)
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2 (a) The table below shows the elements in the second period of the Periodic Table and some
of their oxidation states in their most common compounds.
i) What does it mean when the only oxidation state of an element is zero?
[1]
ii) Explain why some elements have positive oxidation states but others have negative
ones.
[2]
iii) Select two elements in the table which exist as diatomic molecules of the type X2.
[2]
(5 marks)
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(b)
Separate: Chemistry and Extended Only
Beryllium hydroxide, a white solid, is an amphoteric hydroxide.
i) Name another metal which has an amphoteric hydroxide.
[1]
ii) Suggest what you would observe when an excess of aqueous sodium hydroxide is
added gradually to aqueous beryllium sulfate.
[2]
(3 marks)
(c) i) Give the formulae of lithium fluoride and nitrogen fluoride.
Lithium fluoride ................................................................................................... Nitrogen
fluoride ..................................................................................................
[2]
ii) Predict two differences in their properties.
[2]
iii) Explain why these two fluorides have different properties.
[2]
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(6 marks)
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3 (a) When the oxide, Co3O4 is heated in hydrogen, cobalt metal is formed.
Co3O4 + 4H2 → 3Co + 4H2O
Explain how this equation shows that Co3O4 is reduced.
(1 mark)
(b)
Separate: Chemistry and Extended Only
When copper(II) oxide is heated at 800 °C it undergoes the reaction shown by the
equation.
4 CuO → 2 Cu 2 O +O2
Identify the changes in oxidation numbers of copper and oxygen in this reaction.
Explain in terms of changes in oxidation numbers why this is a redox reaction.
change in oxidation number of copper: from …………… to …………… change in oxidation
number of oxygen: from …………… to ……………
explanation ...
(3 marks)
(c) The reaction of iron with steam is shown.
3Fe + 4H2O → Fe3O4 + 4H2
How does this equation show that iron gets oxidised?
(1 mark)
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4 (a) Ammonia is manufactured by the Haber process.
Ammonia, NH3, is used to produce nitric acid, HNO3. This happens in a three-stage
process.
Stage 1 is a redox reaction.
4NH3 + 5O2 → 4NO + 6H2O
Identify what is oxidised in stage 1.
Give a reason for your answer.
substance oxidised .....................................
reason
.....................................
(2 marks)
(b) Zinc and copper are elements next to each other in the Periodic Table.
Aqueous potassium iodide reacts with aqueous copper(II) sulfate to produce iodine.
Balance the chemical equation for this reaction.
KI + CuSO 4 → CuI + I2 + K2 SO 4
(2 marks)
(c)
Extended Only
Deduce the charge on the copper ion in CuI.
(1 mark)
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(d)
Extended Only
In terms of electron transfer, explain why copper is reduced in this reaction.
(1 mark)
(e)
Separate: Chemistry and Extended Only
Identify the reducing agent.
(1 mark)
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5 (a)
Separate: Chemistry and Extended Only
Sulfur dioxide is a reducing agent.
Give the colour change that occurs when excess sulfur dioxide is bubbled into acidified
aqueous potassium manganate(VII).
starting colour of the solution ..............................................
final colour of the solution
..............................................
(1 mark)
(b)
Separate: Chemistry and Extended Only
Displacement reactions occur between metals and metal ions. Displacement reactions
can be used to determine the order of reactivity of metals such as lead (Pb), nickel (Ni),
and silver (Ag). The ionic equation for a displacement reaction is shown.
Ni ( s ) + Pb 2+ ( aq ) → Pb ( s ) + Ni2 + ( aq )
The ionic half-equations for this reaction are shown.
Ni ( s ) →Ni2 + ( aq ) +2e −
Pb 2 + ( aq ) +2e − → Pb ( s )
The ionic half-equations show that electrons are donated by nickel atoms and accepted
by lead ions. Identify the reducing agent in the displacement reaction. Give a reason for
your answer. reducing agent......................................... reason
..........................................
(2 marks)
(c)
Extended Only
What is the general term given to the type of reaction in which electrons are transferred
from one species to another?
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(1 mark)
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Hard Questions
1 (a) Vanadium is a transition element. It has more than one oxidation state.
The element and its compounds are often used as catalysts.
Predict three physical properties of vanadium which are typical of transition elements.
(2 marks)
(b)
Separate: Chemistry and Extended Only
Vanadium(V) oxide is used to catalyse the exothermic reaction between sulfur dioxide
and oxygen in the Contact Process.
2SO2 + O2 ⇌ 2SO3
The rate of this reaction can be increased either by using a catalyst or by increasing the
temperature. Explain why a catalyst is used and not a higher temperature.
(2 marks)
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(c)
Separate: Chemistry and Extended Only
The oxidation states of vanadium in its compounds are V(+5), V(+4), V(+3) and V(+2).
The vanadium(III) ion can behave as a reducing agent or an oxidising agent.
i) Indicate on the following equation which reactant is the oxidising agent.
2V3+ + Zn → 2V2+ + Zn2+
[1]
ii) Which change in the following equation is oxidation? Explain your choice.
V3+ + Fe3+ → V4+ + Fe2+
[2]
(3 marks)
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2 (a) The following are examples of redox reactions.
Bromine water was added to aqueous sodium sulfide. The ionic equation is:
Br2 (aq) + S2– (aq) → 2Br– (aq) + S(s)
i) Describe what you would observe when this reaction occurs.
[2]
ii) Write a full symbol equation for this reaction.
[1]
iii) Explain, in terms of electron transfer, why bromine is the oxidant (oxidising agent) in
this reaction.
[2]
(5 marks)
(b)
Extended Only
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Iron and steel in the presence of water and oxygen form rust.
The reactions involved are:
reaction 1
Fe → Fe2+ + 2e–
The electrons move through the iron onto the surface where a colourless gas forms.
reaction 2
Fe2+ + 2OH– → Fe(OH)2
reaction 3
..........Fe(OH)2 + O2 + ..........H2O → ..........Fe(OH)3
The water evaporates to leave rust.
i) What type of reaction is reaction 1?
[1]
ii) Deduce the name of the colourless gas mentioned in reaction 1.
[1]
iii) What is the name of the iron compound formed in reaction 2?
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[1]
iv) Balance the equation for reaction 3. ..........Fe(OH)2 + O2 + ..........H2O → ..........Fe(OH)3
[1]
v) Explain why the change Fe(OH)2 to Fe(OH)3 is oxidation.
[1]
vi) Explain why iron in electrical contact with a piece of zinc does not rust.
[3]
(8 marks)
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3 (a) The distinctive smell of the seaside was thought to be caused by ozone, O3. Ozone is a
form of the element oxygen.
A mixture of oxygen and ozone is formed by passing electric sparks through oxygen.
3O2 ⇌ 2O3
Suggest a technique that might separate this mixture. Explain why this method separates
the two forms of oxygen.
(2 marks)
(b)
Extended Only
Ozone is an oxidant. It can oxidise an iodide ion to iodine.
2I– + O3 + 2H+ → I2 + O2 + H2O
i) What would you see when ozone is bubbled through aqueous acidified potassium
iodide?
[2]
ii) Explain in terms of electron transfer why the change from iodide ions to iodine
molecules is oxidation.
[1]
iii) Explain, using your answer to b(ii), why ozone is the oxidising agent in this reaction.
[1]
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(4 marks)
(c)
Extended Only
It is now known that the smell of the seaside is due to the chemical dimethyl sulfide,
(CH3)2S.
i) Draw a diagram that shows the arrangement of the outer electrons in one molecule of
this covalent compound. Use x to represent an electron from a carbon atom. Use ● to
represent an electron from a hydrogen atom. Use o to represent an electron from a
sulfur atom.
[3]
ii) Name the three compounds formed when dimethyl sulfide is burnt in excess oxygen.
[2]
(5 marks)
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4 (a) An ore of tungsten conatins WO3. Tungsten can be obtained from WO3 by reacting it with
hydrogen. The reaction is a redox reaction, water is also produced.
Write the balanced symbol equation for the reaction.
(2 marks)
(b)
Extended Only
Explain reduction and oxidation in terms of electrons and oxidation numbers.
(2 marks)
(c)
Separate: Chemistry and Extended Only
The oxidation number of oxygen is -2 and this remains unchanged during the reaction.
Identify, using oxidation numbers, what has been oxidised and what has been reduced in
this reaction.
(3 marks)
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(d)
Extended Only
50 tonnes of the tungsten ore were processed. The ore was found to contain 3.4% of
WO3 by mass. Calculate the maximum mass, in tonnes, of tungsten that could be
obtained from this ore.
Give your answer to three significant figures.
[Ar (W) = 184;
Mr (WO3) = 232]
maximum mass of tungsten = ................................................. tonnes
(5 marks)
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5 (a)
Separate: Chemistry and Extended Only
What is meant by the term oxidising agent?
(2 marks)
(b)
Separate: Chemistry and Extended Only
The dichromate ion, Cr2O72-, reacts with sulfite ions, SO32-, according to the following
equation
Cr2O72- + 8H+ + 3SO32- → 2Cr3+ + 4H2O + 3SO42Are the sulfite ions, SO32-, acting as an oxidising or reducing agent. Justify your answer.
(2 marks)
(c)
Separate: Chemistry and Extended Only
Identify which species is acting as the oxidising agent in the following reaction. Justify
your answer.
Cl2 + 2Br– → 2Cl– + Br2
(2 marks)
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(d)
Separate: Chemistry and Extended Only
Redox titrations can be carried out between reducing agents and oxidising agents in a
similar way to the method used for titrations between acids and alkalis.
Suggest why an indicator is not needed when potassium manganate(VII) is added to a
reducing agent.
(1 mark)
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