CIE IGCSE Chemistry

Topic Questions

IGCSEChemistryCIETopic Questions4. Electrochemistry4.1 Electrolysis

4.1 Electrolysis

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17 marks

The diagram shows a cell. This is a device which produces electrical energy. The reaction in a cell is a redox reaction and involves electron transfer.

cie-igcse-sq-4-1-set-up-q3

i)
Complete the sentence.
A cell will change ................................................ energy into electrical energy.

[1]

ii)
Draw an arrow on the diagram to show the direction of the electron flow.

[1]

iii)
In the left-hand beaker, the colour changes from brown to colourless. Complete the equation for the reaction.

Br2 + ............... → ...............

[2]

iv)
Is the change in (iii) oxidation or reduction? Give a reason for your choice.

[1]

v)
Complete the following description of the reaction in the right-hand beaker.
Fe2+ changes into ..........................

[1]

vi)
When a solution of bromine is replaced by a solution of chlorine, the voltage increases. When a solution of bromine is replaced by a solution of iodine, the voltage decreases.
Suggest an explanation for this difference.

[1]

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2a2 marks

Hydrogen can be manufactured from methane by steam reforming.

  
CH4 (g) + H2O (g) rightwards harpoon over leftwards harpoon CO (g) + 3H2 (g)

The reaction is carried out using a nickel catalyst at temperatures between 700 °C and 1100 °C and using a pressure of one atmosphere.
The forward reaction is endothermic.

What is meant by the term catalyst?

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2b2 marks

Separate: Chemistry and Extended Only

Suggest two reasons why a temperature lower than 700 °C is not used.

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2c1 mark

Suggest one advantage of using a pressure greater than one atmosphere.

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2d1 mark

Separate: Chemistry and Extended Only

Suggest one disadvantage of using a pressure greater than one atmosphere.

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2e7 marks

Hydrogen can also be manufactured by electrolysis. The electrolyte is concentrated aqueous sodium chloride. The electrodes are inert.

The products of electrolysis are hydrogen, chlorine and sodium hydroxide.

i)
Define electrolysis.
[2]
ii)
Name a substance that can be used as the inert electrodes.

[1]
iii)
Write an ionic half-equation for the reaction in which hydrogen is produced.

[1]
iv)
Where is hydrogen produced in the electrolytic cell?

[1]
v)
Describe a test for chlorine.

test ......................................................................................................................................
result ...................................................................................................................................

[2]

How did you do?

2f2 marks

Extended Only

The electrolysis of concentrated aqueous sodium chloride can be represented by the following word equation.

sodium chloride + water → sodium hydroxide + hydrogen + chlorine

Construct a chemical equation to represent this reaction. Do not include state symbols.

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2g3 marks

State one use of:

chlorine, .....................................................................................................................................
sodium hydroxide, .....................................................................................................................
hydrogen. ...................................................................................................................................

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3a5 marks

Separate: Chemistry and Extended Only

Chemical reactions are always accompanied by an energy change.

Aluminium is extracted by the electrolysis of a molten mixture which contains aluminium oxide, Al2O3. This decomposes to form aluminium at the negative electrode and oxygen at the positive electrode.

i)
Write an ionic equation for the reaction at the negative electrode.
[2]

ii)
Complete the ionic equation for the reaction at the positive electrode.

2O2– → ....... + .......

[2]

iii)
Is the reaction exothermic or endothermic? Explain your answer.
[1]

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3b2 marks

The combustion of propane, C3H8, is exothermic.

Give an equation for the complete combustion of propane.

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3c2 marks

Separate: Chemistry and Extended Only

Photosynthesis is an unusual endothermic reaction.

i)
Where does the energy for photosynthesis come from?
[1]
ii)
Give the word equation for photosynthesis.


[1]

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4a2 marks

Molten magnesium bromide is electrolysed.
The incomplete apparatus is shown.

q5_0620-s20-qp-32_cie-igcse-chemistry

Complete the diagram by:

  • labelling the anode and cathode
  • adding the power supply and connecting wires.

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4b1 mark

Name a suitable element that can be used as an electrode.

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4c2 marks

Predict the products of this electrolysis at the:

positive electrode

negative electrode

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4d1 mark

The electrodes must be able to conduct electricity.

Give one other property that the electrodes must have.

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5a2 marks

Sulfuric acid, H2SO4, undergoes electrolysis to form two gases. 

Complete the following table

Electrode  Product 
Anode  
Cathode  

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5b4 marks

Describe the test and positive result for the gases produced during the electrolysis of sulfuric acid, H2SO4.

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5c2 marks

Extended Only

A new electrolysis cell was set up containing copper sulfate solution. Copper is formed in this experiment.

Complete the table giving the observations at each electrode

Electrode Observation 
Anode  
Cathode  

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5d1 mark

Extended Only 

Complete the ionic half equation for the formation of copper

Cu2+ (aq) +  ........... → ............. (s) 

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1a2 marks

Extended Only

The diagram shows a simple cell.

7-2-electrochemical-cell-m6a-

Write an equation for the overall reaction occurring in the cell.

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1b3 marks

Extended Only

Explain why all cell reactions are exothermic and redox.

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1c2 marks

Extended Only

Which electrode, zinc or iron, is the negative electrode? Give a reason for your choice.

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2a6 marks

Extended Only

Zinc alloys have been used for over 2500 years.

i)
Explain the phrase zinc alloy.

[1]

ii)
Describe the bonding in a typical metal, such as zinc, and then explain why it is malleable. You may use a diagram to illustrate your answer.

[3]

iii)
Suggest why the introduction of a different atom into the structure makes the alloy less malleable than the pure metal.

[2]

How did you do?

2b4 marks

Extended Only

Describe how zinc metal can be obtained from zinc sulfate solution by electrolysis.

A labelled diagram is acceptable. Include all the products of this electrolysis. The electrolysis is similar to that of copper(II) sulfate solution with inert electrodes.

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3a4 marks

The ore of aluminium is bauxite which is impure aluminium oxide. Alumina, pure aluminium oxide, is obtained from bauxite.

Aluminium is formed at the cathode when a molten mixture of alumina and cryolite, Na3AlF is electrolysed.

i)
Name two products formed at the anode in this electrolysis.

[2]

ii)
All the aluminium formed comes from the alumina not the cryolite.

Suggest two reasons why the electrolyte must contain cryolite.

[2]

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3b7 marks

The purification of bauxite uses large amounts of sodium hydroxide.

i)
Describe the chemistry of how sodium hydroxide is made from concentrated aqueous sodium chloride. The description must include at least one ionic equation.

[5]

ii)
Making sodium hydroxide from sodium chloride produces two other chemicals.

Name these two chemicals and state one use of each chemical.

chemical ....................................................................................................................
use .............................................................................................................................
chemical ....................................................................................................................
use ....................................................................................................................... 

[2]

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4a2 marks

Zinc is an important metal. Its uses include making alloys and the construction of dry cells (batteries).

Name an alloy which contains zinc. What is the other metal in this alloy?

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4b3 marks

The main ore of zinc is zinc blende, ZnS.

i)
The ore is heated in the presence of air to form zinc oxide and sulfur dioxide.
Write the equation for this reaction.

[2]

ii)
Give a major use of sulfur dioxide.

[1]

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4c4 marks

Extended Only

Zinc can be obtained from zinc oxide in a two step process. Aqueous zinc sulfate is made from zinc oxide and then this solution is electrolysed with inert electrodes. The electrolysis is similar to that of copper(II) sulfate with inert electrodes.

i)
Name the reagent which will react with zinc oxide to form zinc sulfate.

 [1]

ii)
Complete the following for the electrolysis of aqueous zinc sulfate.

Write the equation for the reaction at the negative electrode.

Name the product at the positive electrode.

The electrolyte changes from zinc sulfate to .................................................................... .


[3]

How did you do?

4d3 marks

Extended Only

A dry cell (battery) has a central rod, usually made of graphite. This is the positive electrode which is surrounded by the electrolyte, typically a paste of ammonium chloride and manganese(IV) oxide, all of which are in a zinc container which is the negative electrode.

i)
Suggest why the electrolyte is a paste.


 [1]

ii)
The following changes occur in a dry cell.

For each change, decide if it is oxidation or reduction and give a reason for your choice.

Zn to Zn2+
.............................................................................................................................................

manganese(IV) oxide to manganese(III) oxide
.............................................................................................................................................
[2]

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5a5 marks

Lithium bromide is an ionic compound. It can be electrolysed when it is molten or in aqueous solution. It cannot be electrolysed as a solid.

Solid lithium bromide is a poor conductor of electricity. The ions cannot move to the electrodes, they are held in an ionic lattice by strong forces.

i)
Describe the motion of the ions in the solid state.

[1]

ii)
Define the term ionic bonding.

[2]

iii)
What is meant by the term ionic lattice?

[2]

How did you do?

5b6 marks

Extended Only

The diagram shows the electrolysis of molten lithium bromide.

electrolysis-of-molten-lithium-bromide

i)
Mark on the diagram the direction of the electron flow. 


[1]

ii)
Write an ionic equation for the reaction at the negative electrode (cathode).

[1]

iii)
Write an ionic equation for the reaction at the positive electrode (anode).

[2]

iv)
Which ion is oxidised? Explain your answer.

[2]

How did you do?

5c3 marks

Extended Only

When aqueous lithium bromide is electrolysed, a colourless gas is formed at the negative elctrode and the solution becomes alkaline.

Explain these observations and include an equation in your explanation.

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1a1 mark

Aluminium is obtained from purified alumina, Al2O3, by electrolysis.

Alumina is obtained from the main ore of aluminium.

State the name of this ore.

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1b6 marks

Separate: Chemistry and Extended Only

Describe the extraction of aluminium from alumina. Include the electrolyte, the electrodes and the reactions at the electrodes.

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1c5 marks

Extended Only

Aluminium is resistant to corrosion. It is protected by an oxide layer on its surface.
The thickness of this oxide layer can be increased by anodising.

i)
State a use of aluminium due to its resistance to corrosion.

[1]

ii)
Anodising is an electrolytic process. Dilute sulfuric acid is electrolysed with an aluminium object as the anode. The thickness of the oxide layer is increased.

Complete the equations for the reactions at the aluminium anode.

.......OH– → O2 + 2H2O + ......e–

....Al + ......... → ......... Al2O3

[4]

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2a4 marks

Extended Only

During electrolysis, ions move in the electrolyte and electrons move in the external circuit.
Reactions occur at the electrodes.

The diagram shows the electrolysis of molten lithium iodide.

electrolysis-of-molten-lithium-iodide

 

i)
Draw an arrow on the diagram to show the direction of the electron flow in the external circuit.

[1]


 

ii)
Electrons are supplied to the external circuit. How and where is this done?

[2]



iii)
Explain why solid lithium iodide does not conduct electricity but when molten it is a good conductor.

[1]

How did you do?

2b4 marks

Extended Only

The results of experiments on electrolysis are shown in the following table.

Complete the table. The first line has been done as an example.

electrolyte electrodes product at cathode product at anode change to electrolyte
molten lithium iodide carbon lithium iodine used up
aqueous copper (II) sulfate platinum   oxygen  
concentrated aqueous potassium chloride carbon   chlorine  

[4]

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2c4 marks

The diagram below shows the electrolysis of dilute sulfuric acid. Hydrogen is formed at the negative electrode (cathode) and oxygen at the positive electrode (anode) and the concentration of sulfuric acid increases.

electrolysis-of-sulfuric-acid

The ions present in the dilute acid are H+ (aq), OH– (aq) and SO42– (aq).
 

i)
Write an equation for the reaction at the negative electrode (cathode).

[2]

ii)
Complete the equation for the reaction at the positive electrode (anode).

4OH– (aq) → O2 (g) + .....H2O (l) + ....... 

[1]

iii)
Suggest an explanation of why the concentration of the sulfuric acid increases.

[1]

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3a6 marks

The electrolysis of concentrated aqueous sodium chloride, between inert electrodes, is used to make four important chemicals.

hydrogen
chlorine
sodium hydroxide
sodium chlorate(I)

The ions present in the electrolyte are Na+, H+, Cl– and OH–

i)
Hydrogen ions are discharged at the negative electrode (cathode).

Write an equation for this reaction.


[2]

ii)
The hydrogen ions are from the water.

H2O  rightwards harpoon over leftwards harpoon   H+ + OH–

   Suggest an explanation why the concentration of hydroxide ions increases.

[2]

iii)
When a dilute solution of sodium chloride is used, chlorine is not formed at the positive electrode (anode), a different gas is produced. Name this gas.


[1]

iv)
State an example of an inert electrode.

[1]

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3b2 marks
i)
State a use of hydrogen. 
[1]
ii)
Why is chlorine used to treat the water supply?

[1]

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48 marks

Extended Only

The results of experiments on electrolysis using inert electrodes are given in the table.

Complete the table; the first line has been completed as an example.

electrolyte change at negative electrode change at positive electrode change to electrolyte
molten lead(II) bromide lead formed bromine formed used up 
  potassium formed iodine formed used up 
dilute aqueous sodium chloride      
aqueous copper(II) sulfate      
  hydrogen formed bromine formed potassium hydroxide formed 

[8]

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5a3 marks

Copper has the structure of a typical metal. It has a lattice of positive ions and a “sea” of mobile electrons. The lattice can accommodate ions of a different metal.

Give a different use of copper that depends on each of the following.

i)
The ability of the ions in the lattice to move past each other
[1]

ii)
The presence of mobile electrons
[1]

 

iii)
The ability to accommodate ions of a different metal in the lattice
[1]

How did you do?

5b3 marks

Aqueous copper(II) sulphate solution can be electrolysed using carbon electrodes. The ions present in the solution are as follows.

Cu2+      SO42–      H+     OH–

i)
Write an ionic equation for the reaction at the negative electrode (cathode).
[1]
ii)
A colourless gas was given off at the positive electrode (anode) and the solution changes from blue to colourless.
Explain these observations.
[2]

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5c3 marks

Aqueous copper(II) sulphate can be electrolysed using copper electrodes. The reaction at the negative electrode is the same but the positive electrode becomes smaller and the solution remains blue.

i)
Write a word equation for the reaction at the positive electrode.
[1]

ii)
Explain why the colour of the solution does not change.
[1]

iii)
What is the large scale use of this electrolysis?
[1]

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6a4 marks

Separate: Chemistry and Extended Only

Zinc is extracted from zinc blende, ZnS.

Zinc blende is heated in air to give zinc oxide and sulfur dioxide. Most of the sulfur dioxide is used to make sulfur trioxide. This is used to manufacture sulfuric acid. Some of the acid is used in the plant, but most of it is used to make fertilisers.

i)
Describe how sulfur dioxide is converted into sulfur trioxide.

[3]

ii)
Name a fertiliser made from sulphuric acid.

[1]

How did you do?

6b4 marks

Extended Only

The remaining zinc oxide reacts with sulfuric acid to give aqueous zinc sulfate. This is electrolysed with inert electrodes (the electrolysis is the same as that of copper(II) sulfate with inert electrodes).

Ions present: Zn2+ (aq)    SO42- (aq)     H+ (aq)     OH- (aq)

i)
Zinc forms at the negative electrode (cathode). Write the equation for this reaction.

[1]

ii)
Write the equation for the reaction at the positive electrode (anode).

[2]

iii)
The electrolyte changes from aqueous zinc sulfate to .............................. . 

[1]

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6c2 marks

Extended Only

Give two uses of zinc.

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7a2 marks

Molten potassium bromide can be electrolysed.
Predict the products of this electrolysis at:

the anode
the cathode

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7b2 marks

Electrolysis of concentrated aqueous sodium chloride using inert electrodes forms chlorine, hydrogen and sodium hydroxide.

What is meant by the term electrolysis?

How did you do?

7c1 mark

Name a substance that can be used as the inert electrodes.

How did you do?

7d1 mark

Extended Only

Write an ionic half-equation for the formation of hydrogen during this electrolysis.

How did you do?

7e2 marks

Extended Only

Give the formulae of the four ions present in concentrated aqueous sodium chloride.

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7f2 marks

Extended Only

Explain how sodium hydroxide is formed during this electrolysis.

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8a2 marks
When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

q5_0620-s20-qp-31_cie-igcse-chemistry

Complete the diagram by:

  • labelling the anode and cathode
  • showing how the gases are collected.

How did you do?

8b2 marks

Extended Only

When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.q5_0620-s20-qp-31_cie-igcse-chemistry

Predict the products of this electrolysis at the:

positive electrode:

negative electrode:

How did you do?

8c2 marks

When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

q5_0620-s20-qp-31_cie-igcse-chemistry

Graphite (carbon) electrodes are used in this electrolysis.

Suggest one other element that can be used as an electrode and give a reason, other than that it can conduct electricity.

element:

reason:

How did you do?

8d2 marks

Extended Only

When concentrated hydrochloric acid is electrolysed, gases are produced at the electrodes.

The incomplete apparatus is shown.

q5_0620-s20-qp-31_cie-igcse-chemistry

Hydrogen chloride is produced when chlorine reacts with hydrogen.

Complete the chemical equation for this reaction.

Cl2 + .......... → .....HCl

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8e2 marks

Predict the products of the electrolysis of molten aluminium oxide at:

the positive electrode ........................................

the negative electrode.......................................

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