CIE IGCSE Chemistry

Topic Questions

IGCSEChemistryCIETopic Questions6. Chemical Reactions6.3 Redox

6.3 Redox

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1a2 marks

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.

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1b2 marks

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.

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1c2 marks

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: ____________________

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1d2 marks

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–     →    .........  

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2a1 mark

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  

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2b1 mark

Which of the following reactions shows iron being reduced? 
Tick one box.

copper nitrate + iron → iron nitrate + copper  
   
iron + oxygen → iron(II) oxide  
   
iron(II) oxide + carbon → iron + carbon dioxide  
   
hydrochloric acid + iron → iron chloride + hydrogen  

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2c2 marks

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 ............................

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2d1 mark

Separate: Chemistry and Extended Only

Explain how the oxidation numbers in part (c) show that iron has been oxidised.

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3a2 marks

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?

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3b
Sme Calculator2 marks

Calculate the relative formula mass of potassium manganate(VII), KMnO4.
Use your Periodic Table to help you.



relative formula mass = ...................................

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3c1 mark

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.

colourless to blue  
   
red to blue  
   
yellow to red  
   
purple to colourless  



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3d1 mark

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–

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4a1 mark

The reaction of zinc oxide with carbon is an example of a redox reaction.

What is meant by the term redox reaction?

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4b1 mark

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?

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4c1 mark

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?

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4d2 marks

Complete the balanced equation to show the reduction of iron(III) oxide with carbon.

2Fe2O3 + .... C → .... Fe + .... CO2

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5a4 marks

Separate: Chemistry and Extended Only

This question is about redox reactions.

Match the key term on the left with the definition on the right.

redox-key-terms

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5b1 mark

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.

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5c1 mark

Extended Only

Chromium(II) oxide can be formed from the reaction between chromium and oxygen.

Complete the half-equation for the reaction.

.........  →  Cr2+  +  ..... e–

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5d1 mark

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.

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1a2 marks

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.

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1b2 marks

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 begin mathsize 12px style rightwards harpoon over leftwards harpoon end style2SO3

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.

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1c3 marks

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]

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2a5 marks

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]

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2b8 marks

Extended Only

Iron and steel in the presence of water and oxygen form rust.

cie-igcse-sq-6-3-rusting-diagram-

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?

[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]

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3a2 marks

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 rightwards harpoon over leftwards harpoon2O3

Suggest a technique that might separate this mixture. Explain why this method separates the two forms of oxygen.

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3b4 marks

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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3c5 marks

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]

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4a2 marks

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.

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4b2 marks

Extended Only

Explain reduction and oxidation in terms of electrons and oxidation numbers.

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4c3 marks

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.

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4d
Sme Calculator5 marks

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

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5a2 marks

Separate: Chemistry and Extended Only

What is meant by the term oxidising agent?

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5b2 marks

Separate: Chemistry and Extended Only

The dichromate ion, Cr2O72-, reacts with sulfite ions, SO32-, according to the following equation 

Cr2O72- + 8H+ + 3SO32-  →  2Cr3+ + 4H2O + 3SO42- 

Are the sulfite ions, SO32-, acting as an oxidising or reducing agent. Justify your answer.

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5c2 marks

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 

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5d1 mark

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.

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1a2 marks

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 begin mathsize 12px style rightwards harpoon over leftwards harpoon end style Rb+ + e–
      Mg begin mathsize 12px style rightwards harpoon over leftwards harpoon end style Mg2+ + 2e–
      Mn begin mathsize 12px style rightwards harpoon over leftwards harpoon end style Mn2+ + 2e–
      Zn begin mathsize 12px style rightwards harpoon over leftwards harpoon end style Zn2+ + 2e–
      H2 begin mathsize 12px style rightwards harpoon over leftwards harpoon end style 2H+ + 2e–
      Cu begin mathsize 12px style rightwards harpoon over leftwards harpoon end style Cu2+ + 2e–
      Hg begin mathsize 12px style rightwards harpoon over leftwards harpoon end style 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]

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1b2 marks
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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1c1 mark

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]

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2a5 marks

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.

cie-igcse-sq-6-3-q4a
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]

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2b3 marks

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]

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2c6 marks
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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3a1 mark

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.

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3b3 marks

Separate: Chemistry and Extended Only

When copper(II) oxide is heated at 800 °C it undergoes the reaction shown by the  equation.


4 CuO space rightwards arrow 2 Cu subscript 2 straight O space plus straight O subscript 2


 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 ...

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3c1 mark

The reaction of iron with steam is shown.

3Fe + 4H2O → Fe3O4 + 4H2

How does this equation show that iron gets oxidised?

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4a2 marks

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    .....................................

How did you do?

4b2 marks

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 space plus space CuSO subscript 4 space rightwards arrow space CuI space plus space straight I subscript 2 space plus space straight K subscript 2 SO subscript 4

How did you do?

4c1 mark

Extended Only

Deduce the charge on the copper ion in CuI.

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4d1 mark

Extended Only

In terms of electron transfer, explain why copper is reduced in this reaction.

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4e1 mark

Separate: Chemistry and Extended Only

Identify the reducing agent.

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5a1 mark

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       ..............................................

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5b2 marks

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 space open parentheses straight s close parentheses space plus Pb to the power of 2 plus end exponent space open parentheses aq close parentheses space rightwards arrow space Pb space open parentheses straight s close parentheses space plus space Ni to the power of 2 plus space end exponent open parentheses aq close parentheses


The ionic half-equations for this reaction are shown.

Ni space open parentheses straight s close parentheses space rightwards arrow Ni to the power of 2 plus space end exponent open parentheses aq close parentheses space plus 2 straight e to the power of minus
Pb to the power of 2 plus space end exponent open parentheses aq close parentheses space plus 2 straight e to the power of minus space rightwards arrow Pb space open parentheses straight s close parentheses


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              ..........................................

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5c1 mark

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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