Electrolysis questions and answers (O level Chemistry)

Electrolysis questions and answers for O level Chemistry students. In this post we will be looking at some questions from different O level past exam papers. Each question is followed by an expected solution. Important explanations and background information are given in italics.

Question 1

Electrolysis is often used in the extraction and purification of elements.

Magnesium is manufactured by the electrolysis of molten magnesium chloride. Write equations for the two electrode reactions that occur during this electrolysis.
……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….[2]
Copper can be purified using the electrolysis of aqueous copper(II) sulphate.
(i) What is used as the anode (positive electrode)?
………………………………………………………………………………………………………………………[1]
(ii) What is used as the cathode (negative electrode)?
………………………………………………………………………………………………………………………[1]

Solution 1

(Since molten magnesium chloride is an electrolyte, it dissociates to produce Mg²⁺ ions and Cl^– ions. The cations, Mg²⁺, move to the cathode where they gain electrons to form Mg atoms. The anions, Cl^–, move to the anode where they loses electrons to form Cl atoms. The Cl atoms then pair up to make Cl₂ since chlorine naturally exist in diatomic form.)

at the cathode: Mg²⁺(l) + 2e^– → Mg(s)
at the anode: 2Cl^–(l) – 2e^– → Cl₂(g)
During the purification of copper by electrolysis:
(i) The anode is impure copper
(ii) The cathode is pure copper

Question 2

Only liquids that contain moving ions can be electrolysed. These liquids are called electrolytes.

Complete the following table which shows the products formed when some liquids are electrolysed using inert graphite electrodes.

electrolyte	ions present in electrolyte	product formed at the positive electrode	product formed at the negative electrode
aqueous copper(II) sulphate	Cu²⁺, H⁺, OH^– and SO₄^2–	…………………………..	…………………………..
concentrated aqueous sodium chloride	H⁺, Na⁺, Cl^– and OH^–	chlorine	hydrogen
molten lead(II) bromide	Pb²⁺ and Br^–	…………………………..	…………………………..

[3]

When concentrated aqueous sodium chloride is electrolysed, chlorine is formed at the positive electrode (anode) and hydrogen at the negative electrode (cathode).
(i) Construct the ionic equation to show the formation of chlorine at the positive electrode.
………………………………………………………………………………………………………………………[1]
(ii) Explain why hydrogen is formed at the negative electrode rather than sodium.
………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….[1]
Name a metal manufactured by the electrolysis of a molten ionic compound.
……………………………………………………………………………………………………………………………..[1]

Solution 2

Complete the following table which shows the products formed when some liquids are electrolysed using inert graphite electrodes.

electrolyte	ions present in electrolyte	product formed at the positive electrode	product formed at the negative electrode
aqueous copper(II) sulphate	Cu²⁺, H⁺, OH^– and SO₄^2–	oxygen	copper
concentrated aqueous sodium chloride	H⁺, Na⁺, Cl^– and OH^–	chlorine	hydrogen
molten lead(II) bromide	Pb²⁺ and Br^–	bromine	lead

[3]

(i) at the positive electrode: 2Cl^–(aq) – 2e^– → Cl₂(g)
(ii) Hydrogen is lower than sodium on the electrochemical series, which means hydrogen is discharged before sodium.
(Ions of a less reactive element are more likely to be discharged before ions of a more reactive element.)
magnesium, aluminium, or sodium
(Reactive metals are extracted by electrolysis)

Question 3

Aluminium is manufactured by the electrolysis of aluminium oxide dissolved in molten cryolite.

Give the equations for the reactions that occur at the electrodes during this electrolysis.
positive electrode
……………………………………………………………………………………………………….
negative electrode
………………………………………………………………………………………………… [2]
Copper is purified using electrolysis. For this electrolysis name
the electrolyte used,
……………………………………………………………………………………………
the material used for the anode,
…………………………………………………………………………..
the material used for the cathode.
………………………………………………………………….. [3]
Complete the following table about electrolysis using inert graphite electrodes.
electrolyte product at cathode product at anode
molten lead(II) bromide
aqueous copper(II) sulphate copper
dilute sulphuric acid oxygen
[3]
Describe one commercial use of electrolysis.
use
…………………………………………………………………………………………………………………..
electrolyte used
………………………………………………………………………………………………….
ionic equation for reaction at the cathode
………………………………………………………………[3]

Solution 3

(Aluminium is extracted by electrolysis from its oxide, Al₂O₃, also known as bauxite. During electrolysis to extract aluminium, bauxite is mixed with sodium hexafluoroaluminate, Na₃AlF₆, better known as cryolite. Cryolite’s purpose is to lowers the melting point of bauxite to within economically viable temperatures.)

positive electrode: 2O^2-(l) – 4e → O₂(g)
negative electrode: Al³⁺(l) + 3e → Al(l)
Electrolyte: Copper (II) sulphate
Anode: Impure copper
Cathode: Pure copper
electrolyte product at cathode product at anode
molten lead(II) bromide lead bromine
aqueous copper(II) sulphate copper oxygen
dilute sulphuric acid hydrogen oxygen
[3]
use: Extraction of magnesium
electrolyte used: Molten magnesium chloride
ionic equation for reaction at the cathode: Mg²⁺(l) + 2e^– → Mg(l)

Question 4

Electrolysis involves the decomposition of a compound by the passage of an electric current.

(i) Complete the table, which relates to the electrolysis of different solutions using inert electrodes.

electrolyte	ions in electrolyte	product at anode	product at cathode
dilute aqueous potassium nitrate	K⁺, H⁺, OH^– and NO₃^–	oxygen	hydrogen
concentrated aqueous sodium chloride	Na⁺, H⁺, OH^– and Cl^–	chlorine	hydrogen
dilute aqueous copper(II) sulphate	Cu²⁺, SO₄^2–, H⁺ and OH^–	……………………	……………………
dilute sulphuric acid	………………..	oxygen	hydrogen

[3]

The electrolysis of dilute aqueous sodium chloride liberates oxygen at the anode.
Suggest why the electrolysis of concentrated aqueous sodium chloride liberates chlorine rather than oxygen.
………………………………………………………………………………………………………………………………………………………………………………………………………………………………….[1]
Aqueous copper(II) sulphate was electrolysed using copper electrodes. The copper anode lost mass as copper(II) ions were formed and the copper cathode gained mass as copper atoms were formed.
(i) State one industrial application of this electrolysis.
………………………………………………………………………………………………………………[1]
(ii) The results of an experiment involving the electrolysis of aqueous copper(II) sulphate are shown below.
temperature of electrolyte / °C current used / amps time of electrolysis / s mass of copper formed at the cathode / g
20 1.0 1000 0.329
20 2.0 1000 0.658
20 2.0 2000 1.320
25 2.0 2000 1.320
30 1.0 1000 0.329
Use the information in the table to describe how each of the variables affects the mass of copper formed at the cathode.
temperature ……………………………………………………………………………………………………………………………………………………………………………………………………………………
current …………………………………………………………………………………………………………………………………………………………………………………………………………………………..
time …………………………………………………………………………………………………………………………………………………………………………………………………………………………..[3]

Solution 4

electrolyte	ions in electrolyte	product at anode	product at cathode
dilute aqueous potassium nitrate	K⁺, H⁺, OH^– and NO₃^–	oxygen	hydrogen
concentrated aqueous sodium chloride	Na⁺, H⁺, OH^– and Cl^–	chlorine	hydrogen
dilute aqueous copper(II) sulphate	Cu²⁺, SO₄^2–, H⁺ and OH^–	oxygen	copper
dilute sulphuric acid	SO₄^2–, H⁺ and OH^–	oxygen	hydrogen

Concentrated sodium chloride has a higher concentration of chlorine ions than hydroxyl ions. Chlorine is discharged due to the higher concentration of chlorine ions.
(i) Copper plating
(ii) Temperature has no effect on the mass of copper formed during electrolysis.
Current is directly proportional to the mass of copper formed during electrolysis, which means doubling the current doubles the mass.
Time is directly proportional to the mass of copper formed during electrolysis, which means doubling the time doubles the mass.

Question 5

Concentrated aqueous sodium chloride contains H⁺ and OH^– ions.
(i) Give the formulae of two other ions present in concentrated aqueous sodium chloride.
………………………………………………………………………………………………………………[1]
(ii) Concentrated aqueous sodium chloride is electrolysed using inert graphite electrodes.
Name the product formed at each electrode.
product at anode …………………………………………………………………………………………
product at cathode …………………………………………………………………………………..[2]
Impure copper can be purified by electrolysis.
Draw a labelled diagram of the electrolytic cell that can be used to purify copper. [3]
Aluminium is extracted commercially from an aluminium ore by electrolysis.
(i) Name an ore containing aluminium.
………………………………………………………………………………………………………………[1]
(ii) Name the element used as the anode in this process.
………………………………………………………………………………………………………………[1]

Solution 5

(i) Na⁺ and Cl^–

(ii) Product at anode: chlorine

Product at cathode: hydrogen

(i) Bauxite
(ii) Carbon

Question 6

Electrolysis is the decomposition of a liquid by the passage of an electrical current.

Aqueous copper(II) sulphate contains the following ions, Cu²⁺, H⁺, OH^– and SO₄^2–.
Aqueous copper(II) sulphate can be electrolysed using inert electrodes. The electrode reactions are represented below.
cathode Cu²⁺ + 2e^– → Cu
anode 4OH^– → O₂ + 2H₂O + 4e^–
(i) Explain why copper, not hydrogen, is formed at the cathode.
………………………………………………………………………………………………………………………………………………………………………………………………………………………………………
(ii) Explain why the formation of oxygen at the anode is an example of oxidation.
……………………………………………………………………………………………………………………………………………………………………………………………………………………………………….
(iii) The electrolysis of aqueous copper(II) sulphate using copper electrodes has a different anode reaction.
Give the equation for the electrode reaction at the anode.
…………………………………………………………………………………………………………………..
[3]
Molten lead(II) bromide decomposes when an electric current is passed through it.
(i) Explain why solid lead(II) bromide will not conduct electricity but molten lead(II) bromide will.
…………………………………………………………………………………………………………………..
…………………………………………………………………………………………………………………..
…………………………………………………………………………………………………………………..
(ii) Construct the equations for the two electrode reactions.
cathode ………………………………………………………………………………………………………
anode …………………………………………………………………………………………………………
[4]

Solution 6

(i) Copper is lower than hydrogen on the electrochemical series, which means copper is discharged before hydrogen.
(ii) Oxidation is the loss of electrons. Therefore the formation of oxygen at the anode is oxidation because it involves the loss of electrons by the hydroxyl ion.
(iii) (For the electrolysis of aqueous copper(II) sulphate using copper electrodes, the copper anode dissolves to form copper ions.)
The electrode reaction at the anode: Cu → Cu²⁺ + 2e
i) During electrolysis an electric current is carried by moving ions. The ions in solid lead (II) bromide are not mobile, whereas the ions in molten lead (II) are mobile.
ii) at the cathode: Pb²⁺(l) + 2e^– → Pb(s)
at the anode: 2Br^–(l) – 2e^– → Br₂(g)

Electrolysis questions and answers (O level Chemistry)

Published by Sydney Chako on April 14, 2021