C6.2
Student practical
Name .....................................................................
Class .................
Date .....................
The electrolysis of aqueous solutions of ionic
compounds
Specification references


C4.3.1 The process of electrolysis
C4.3.4 Electrolysis of aqueous solutions

C4.3.5 Representation of reactions at electrodes as half equations

WS1.2
Aims
In this activity you will observe the electrolysis of aqueous solutions of ionic
compounds. After describing your observations you will answer questions
about what happens during the electrolysis.
Learning outcomes
After completing this activity, you should be able to:







describe the experimental set-up for electrolysis
state that oxygen can be produced at the anode and hydrogen at the cathode
when some solutions are electrolysed
write a word equation to describe electrolysis of a solution
describe electrolysis of solutions in terms of movement of ions
write a balanced symbol equation including state symbols for the overall
electrolysis of a solution
predict the products at each electrode for the electrolysis of aqueous ionic
compounds
explain how hydrogen ions and hydroxide ions can be present in solutions,
including a balanced symbol equation with state symbols, for the reversible
reaction in which water ionises

describe electrolysis with half equations at the electrodes

explain the classification of reactions at the electrodes as oxidation
or reduction.
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
1
C6.2
Student practical
Name .....................................................................
Class .................
Date .....................
Safety




chemical splash proof eye protection
1 mol/dm3 copper(II) chloride LOW HAZARD
1 mol/dm3 potassium bromide LOW HAZARD
1 mol/dm3 sodium sulfate LOW HAZARD
The practical should be carried out in a well-ventilated laboratory as significant
amounts of toxic chlorine and bromine can be produced, as well as highly
flammable hydrogen. Take care if you are asthmatic.
Equipment





3 beakers (100 cm3)
retort stand and clamp to hold electrodes
DC power supply (6 V)
leads and crocodile clips
low voltage lamp





1 mol/dm3 copper(II) chloride
1 mol/dm3 potassium bromide
1 mol/dm3 sodium sulfate
2 carbon electrodes (about 5 mm diameter)
blue litmus paper
Setting the scene
In aqueous solutions of ionic compounds, four different ions are present: the metal
ion and the non-metal ion from the ionic compound, plus hydrogen ions and
hydroxide ions from the water.
The elements produced at the electrodes when an aqueous solution is
electrolysed using inert electrodes depend on the relative reactivities of the
elements involved.
This activity involves the electrolysis of aqueous solutions of potassium bromide,
sodium sulfate, and copper(II) chloride.
Method
1
2
3
4
Pour about 50 cm3 of potassium bromide solution into a beaker.
Put in the carbon electrodes and set up the circuit as shown below.
Turn on the current.
After a few minutes, record what you see at the two electrodes in the results
table below.
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This resource sheet may have been changed from the original.
2
C6.2
Student practical
Name .....................................................................
5
Class .................
Date .....................
Repeat using a new beaker containing sodium sulfate solution and then repeat
with copper(II) chloride solution.
Task
1
2
Use the information from ‘Setting the scene’ (above) to work out which ions
are present in each solution. Add this into the relevant column in the results
table below. Then colour in red the ions that are attracted to the negative
electrode, and colour in green the ions that are attracted to the positive
electrode.
Use your observations to work out which element is actually produced at each
electrode. Add this into the relevant column in the results table below.
Additional information
If a gas is produced at the negative electrode, it is hydrogen.
If a gas is produced at the positive electrode that does not bleach blue litmus
paper, it is oxygen.
Solution
Ions present
Observations
at negative
electrode
Product at
negative
electrode
Observations
at positive
electrode
Product at
positive
electrode
potassium
bromide
sodium sulfate
copper(II)
chloride
Results table 1
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
3
C6.2
Student practical
Name .....................................................................
Class .................
Date .....................
Questions
1
Which of the solutions contain metals that are more reactive than hydrogen?
(1 mark)
2
Which solutions contain halide ions?
(1 mark)
3
Complete the following sentences:
a In the aqueous solution water molecules break down producing
ions and
b
ions.
(1 mark)
Write an equation, with state symbols, to show how water breaks down
into H+ ions and OH− ions.
(1 mark)
c
At the negative electrode (cathode), hydrogen is produced if the metal is
reactive than hydrogen.
d
At the positive electrode (anode),
(1 mark)
is produced unless the
solution contains halide ions. In this case the
4
is produced.
(2 marks)
State how you would test for the presence of the following gases:
a hydrogen
(1 mark)
b
oxygen
(1 mark)
c
chlorine.
(1 mark)
5
Brine is a concentrated solution of sodium chloride. When it is electrolysed,
three products are formed:
 chlorine gas is produced at the anode (positive electrode)
 hydrogen gas is produced at the cathode (negative electrode)
 the remaining solution is sodium hydroxide.
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
4
C6.2
Student practical
Name .....................................................................
a
Class .................
Date .....................
Explain why hydrogen is produced at the cathode.
(1 mark)
b
i
Explain why sodium hydroxide is left in the solution.
(2 marks)
ii
Predict what you would see if you tested the solution with universal
indicator. Explain your answer.
(2 marks)
c
Write an overall word equation for the electrolysis of brine.
(1 mark)
d
Explain why this is an important industrial process.
(1 mark)
6
Predict the products when the following aqueous solutions are electrolysed. Use
the reactivity series to remind you which metals are more reactive than hydrogen.
Aqueous solution
Product at negative electrode
Product at positive electrode
potassium chloride
copper(II) nitrate
magnesium chloride
silver nitrate
(4 marks)
7
Complete the following sentences:
a During electrolysis, at the cathode (negative electrode), positively charged ions
electrons and so the reactions are
b
.
(2 marks)
At the anode (positive electrode), negatively charged ions
electrons and so the reactions are
.
(2 marks)
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This resource sheet may have been changed from the original.
5
C6.2
Student practical
Name .....................................................................
8
Class .................
Complete the following half equations to show what happens when
a hydrogen is produced at the cathode:
2H+ 
b
→ H2
(1 mark)
oxygen is produced at the anode
4OH− → O2  2H2O 
9
Date .....................
(1 mark)
Complete the table by writing half equations for the reactions at the electrodes
for the three solutions that you observed in the practical activity.
Solution
Half equation at negative
electrode
Half equation at positive
electrode
potassium bromide
sodium sulfate
copper(II) chloride
(6 marks)
10 Electrolysis is a redox reaction.
Discuss this statement by explaining the changes that take place at the anode
and cathode during the electrolysis of aqueous sodium chloride.
Illustrate your answer by writing half equations for the reactions that occur at
each electrode.
(8 marks)
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
6