4
Electrochemistry
4.1 Electrolysis
Core
1
Define electrolysis as the decomposition of an
ionic compound, when molten or in aqueous
solution, by the passage of an electric current
2
Identify in simple electrolytic cells:
(a) the anode as the positive electrode
(b) the cathode as the negative electrode
(c) the electrolyte as the molten or aqueous
substance that undergoes electrolysis
www.cambridgeinternational.org/igcse
4.1 Electrolysis continued
Supplement
Core
8 Describe
transfer
charge during
3 the
Identify
theofproducts
formedelectrolysis
at the electrodes
to include:and describe the observations made during the
electrolysis
(a) the movement
of of:
electrons in the external
circuit(a) molten lead(II) bromide
(b)orconcentrated
aqueous
chloride
(b) the loss
gain of electrons
at thesodium
electrodes
(c) diluteofsulfuric
(c) the movement
ions inacid
the electrolyte
using inert electrodes made of platinum or
carbon / graphite
4 State that metals or hydrogen are formed at
the cathode and that non-metals (other than
hydrogen) are formed at the anode
5 Predict the identity of the products at each
Back to contents page
electrode for the electrolysis of a binary
compound in the molten state
Supple
9
Id
an
el
in
co
10 Pr
el
co
so
11 Co
at
ca
6
State that metal objects are electroplated to
improve their appearance and resistance to
corrosion
7
Describe how metals are electroplated
1
4.2 Hydrogen–oxygen fuel cells
11 corrosion
Construct ionic half-equations for reactions
at the anode
showare
oxidation)
and at the
7 Describe
how(to
metals
electroplated
cathode (to show reduction)
6 State
that
metalfor
objects
are electroplated
to content
Supplement
mbridge IGCSE
Chemistry
0620
syllabus
2023, 2024
and 2025. Subject
s
e
theirof
appearance
andelectrolysis
resistance to
8 Supplement
Describeimprove
the transfer
charge during
corrosion
include:
9 toIdentify
the products formed at the electrodes
7thedescribe
Describe
how
metals
areinelectroplated
the
observations
made
during the
(a)and
movement
of
electrons
the
external
circuit
electrolysis
of aqueous copper(II) sulfate using
4.2 Hydrogen–oxygen fuel cells
/ graphite
electrodes
when using
(b)inert
the carbon
loss or gain
of electrons
at theand
electrodes
electrodes
4.2
Hydrogen–oxygen
fuelelectrolyte
cells
(c)copper
the movement
of ions in the
Supplement
Core the products formed at the electrodes
9 Identify
State the
thatobservations
a hydrogen–oxygen
fuel cell
and1describe
made during
the uses
hydrogen
and oxygen
toIIproduce
with
electrolysis
of aqueous
copper(
) sulfate electricity
using
water/ graphite
as the only
chemicaland
product
inert carbon
electrodes
when using
copper electrodes
Core
Supplem
1
2
State that a hydrogen–oxygen fuel cell uses
hydrogen and oxygen to produce electricity with
water as the only chemical product
Des
usin
wit
Supplement
2
Describe the advantages and disadvantages of
using hydrogen–oxygen fuel cells in comparison
with gasoline / petrol engines in vehicles
Back to contents page
10 Predict the identity of the products at each
electrode for the electrolysis of a halide
compound in dilute or concentrated aqueous
solution
11 Construct ionic half-equations for reactions
at thethe
anode
(to show
at the
10 Predict
identity
of theoxidation)
products atand
each
cathodefor
(tothe
show
reduction)
electrode
electrolysis
of a halide
compound in dilute or concentrated aqueous
solution
11 Construct ionic half-equations for reactions
at the anode (to show oxidation) and at the
cathode (to show reduction)
Back to contents page
2
CONDUCTORS
A material that allow electricity to pass through
Batteries and electric current
positive
terminal
negative
terminal
+
battery
carbon rod
electron
flow
bulb
3
Testing substances to see if they conduct
You can test any substance to see if it conducts, by connecting it into a
circuit like the one on page 102. For example:
Bunsen
burner
Tin. A strip of tin is connected
into the circuit, in place of the
graphite rod. The bulb lights, so
tin must be a conductor.
Ethanol. The liquid is connected
into the circuit by placing graphite
rods in it. The bulb does not light,
so ethanol is a non-conductor.
Lead bromide. It does not
conduct when solid. But if you
melt it, it conducts, and gives off
a choking brown vapour.
4
The results
These are the results from a range of tests:
1 The only solids that conduct are the metals and graphite.
These conduct because of their free electrons (pages 61 and 62).
The electrons get pumped out of one end of the solid by the battery,
while more electrons flow in the other end.
For the same reason, molten metals conduct. (It is hard to test molten
graphite, because at room pressure graphite goes from solid to gas.)
2 Molecular substances are non-conductors.
This is because they contain no free electrons, or other charged
particles, that can flow through them.
Ethanol (above) is made of molecules. So is petrol, paraffin, sulfur,
sugar, and plastic. These never conduct, whether solid or molten.
3 Ionic substances do not conduct when solid. But they do conduct
when melted or dissolved in water – and they decompose at the
same time.
An ionic substance contains no free electrons. But it does contain ions,
which have a charge. The ions become free to move when the substance
is melted or dissolved, and it is they that conduct the electricity.
Lead bromide is ionic. It does not conduct when solid, but conducts
when it melts. The brown vapour that forms is bromine. Electricity
has caused the lead bromide to decompose.
Decomposition brought about by electricity is called electrolysis.
A liquid that contains ions, and therefore conducts electricity,
is called an electrolyte.
So molten lead bromide is an electrolyte. Ethanol is a non-electrolyte.
Metals conduct, thanks to their free
electrons, which form a current.
solid
heat
liquid
An ionic solid conducts when it
melts, because the ions become free
to move.
5
Electricity: a form of energy
Electricity is a current of electrons. Like heat, it is a form of energy.
When you burn a fuel, chemical energy is converted to heat.
But a reaction can also give out energy as electricity.
Electricity from a redox reaction
bulb
strip of
magnesium
bulb
strip of
copper
Connect a strip of magnesium,
a strip of copper, and a light bulb,
like this. (Note: no battery!)
Nothing happens.
bulb
dilute
solution of
sodium chloride
Now stand the strips in a dilute
solution of sodium chloride.
Something amazing happens: the
bulb lights! A current is flowing.
magnesium
dissolving
bubbles of
hydrogen
At the same time bubbles of
hydrogen start to form on the
copper strip, and the magnesium
strip begins dissolving.
6
What is electrolysis?
It is the process of using
electricity to
break down a compound.
Where does electrolysis take place?
Electrolysis takes place in an electrolytic cell.
It contains
a battery,
electrodes and
an electrolyte.
What is an electrolyte?
A molten ionic compound
or an aqueous solution
that conducts electricity.
Examples are
dilute sulphuric acid,
molten sodium chloride
and copper(II) sulphate
solution.
How does an electrolyte conduct
electricity?
The electrolyte has
mobile ions that allow
electricity to flow.
What are electrodes?
Solids that conduct
electricity and connect
the battery to the
electrolyte.
Examples: carbon,
platinum, copper
What are anode and cathode?
Anode:
positive
terminal
Cathode: negative
terminal
Elements and Compounds Conduct
Electricity Differently
Element
Compound
By mobile electrons
By mobile ions
Element remains
unchanged as
electricity passes
Decomposes and
forms new
substances
Why Can’t Solid Ionic Compounds
Conduct Electricity?
The ions in solid ionic compounds are held
in fixed positions in a lattice.
They are unable to move to conduct
electricity.
What happens during electrolysis?
In the external circuit:
During electrolysis,
electrons flow from the
negative terminal to the
positive terminal of the
battery.
What happens during electrolysis?
Within the electrolyte:
During electrolysis,
• cations move towards the
cathode.
• anions move towards the
anode.
What happens during electrolysis?
At the cathode:
• cations receive electrons
from the cathode.
• reduction occurs at the
cathode.
Cathode
What happens during electrolysis?
At the anode:
• anions give up electrons
at the anode.
• oxidation occurs here.
Anode
What happens to the ions at the
electrodes?
Cations (positive ions) receive electrons from the
cathode.
Anions (negative ions) give up electrons to the
anode.
The process of gaining or losing electrons at
the electrodes is called discharge.
When ions are discharged at the electrodes, they
form atoms or molecules.
Examples :
Electrolysis of Molten
Lead Bromide
20
Electrolysis of Molten Lead Bromide
Sodium chloride is a binary compound.
A binary compound is a compound
containing only two elements.
When solid lead bromie is heated strongly,
it melts at 801 °C.
It forms mobile Pb+2 ions and Br – ions.
Electrolysis of Molten Sodium Chloride
At the anode:
Reason 4:
Bromide (Br–) ions, being
negatively charged, are
attracted to the anode.
Each bromide ion gives up
one electron to form a
bromide atom. It is oxidised.
Br–(l) ! Br(l) + e–
Electrolysis of Molten Lead Bromide
At the anode:
Reason 4:
Two chlorine atoms then
combine to form a chlorine
molecule.
2Br−(l) ! Br2(g)
Electrolysis of Molten Sodium Chloride
Reason 4:
At the cathode:
Lead ions (Pb+2), being
positively charged, are
attracted to the cathode.
Each Pb+2 ion gains one
electron to form a lead atom.
It is reduced.
Pb+2(l) + e– ! Pb(l)
Electrolysis of Molten Lead Bromide
Reason 4:
Overall reaction:
2PbBr2(l) ! 2Pb(l) + Cl2(g)
Why are carbon rods used in the
electrolysis of molten lead bromide?
Carbon rods are inert electrodes.
Inert electrodes such as carbon
electrodes are used to prevent reactions
from occurring between bromine and the
electrode.
Platinum electrodes are also inert
electrodes.
The electrolysis of molten lead bromide
The diagram on the right shows the apparatus.
The graphite rods are called electrodes.
The electrode attached to the positive terminal of the
battery is also positive. It is called the anode.
The negative electrode is called the cathode.
+
5 Electrons flow from the
anode to the positive
terminal of the battery.
battery
1 Electrons flow from the
negative terminal of the
battery to the cathode.
switch
closed
cathode
anode
+
2 In the liquid, the ions move to
the electrode of opposite charge.
4 At the anode (!), the Br " ions
give up electrons. Red-brown
bromine vapour bubbles off.
heat
3 At the cathode ("), the Pb 2 ! ions
accept electrons. Lead begins to
appear below the cathode.
The result is that the lead bromide has decomposed:
lead bromide
PbBr2 (l)
Note that:
lead ! bromine
Pb (l) ! Br2 ( g)
27
What happens to ions in the molten lead bromide?
In molten lead bromide, the ions are free to move. This shows what
happens to them, when the switch in the circuit is closed:
–
+
Br
Br –
Pb
Br
e–
Br –
2+
Br
Pb 2+
–
+
–
Br –
First, the ions move.
Opposite charges attract.
So the positive lead ions (Pb2 !)
move to the cathode (").
The negative bromide ions (Br ")
move to the anode (!).
The moving ions carry the current.
The free ions move.
Remember OILRIG:
Oxidation Is Loss of electrons,
Reduction Is Gain of electrons.
Pb 2+
–
Br
–
+
Br –
Br
–
Pb 2+ e
–
Br –
–
e–
Pb
–
Br
At the cathode (!):
the lead ions each receive two
electrons and become lead atoms.
The half-equation is:
Pb (l)
Pb2 ! (l) ! 2e "
Lead collects on the electrode and
eventually drops off it.
Ions gain electrons: reduction.
Pb 2+
e – Br –
–
At the anode ("):
the bromide ions each give up an
electron, and become atoms. These
then pair up to form molecules.
The half-equation is:
2Br " (l)
Br2 (g) ! 2e "
The bromine gas bubbles off.
Ions lose electrons: oxidation.
Overall, electrolysis is a redox reaction.
Reduction takes place at the cathode
and oxidation at the anode.
28
Electrolysis of Aqueous Solutions
of Sodium Chloride
29
Electrolysis of Aqueous Solutions
of Compounds
An aqueous solution of a compound is really a
mixture of two electrolytes.
E.g. a solution of copper(II) sulphate solution
contains copper(II) sulphate and water.
The solution has copper(II) ions (Cu2+) and
sulphate ions (SO42–), and hydrogen ions (H+)
and hydroxide ions (OH–).
Electrolysis of Dilute Sodium
Chloride Solution
An aqueous solution of sodium chloride
contains four different types of ions:
Na+(aq), Cl–(aq), H+(aq) and OH–(aq)
The Na+ and H+ ions are attracted to the
cathode.
The Cl– and OH– ions are attracted to the
anode.
Electrolysis of Dilute Sodium Chloride
Solution
At the cathode:
The H+ and Na+ ions are attracted to
the platinum cathode.
H+ ions are reduced as they gain
electrons from the cathode to form
hydrogen gas.
2H+(aq) + 2e– ! H2(g)
Na+ ions remain in solution.
Electrolysis of Dilute Sodium Chloride
Solution
At the anode:
OH– and Cl– ions are attracted to
the platinum anode.
OH– ions are oxidised as they give
up electrons to the anode to form
water and oxygen gas.
4OH–(aq) ! 2H2O(l) + O2(g) + 4e–
Cl– ions remain in solution.
Electrolysis of Dilute Sodium Chloride
Solution
Summary:
Overall reaction is
2H2O(l) ! 2H2(g) + O2(g)
Electrolysis of dilute sodium
chloride is essentially the
electrolysis of water.
Since water is being removed, the
concentration of sodium chloride
solution increases gradually.
Compare the electrolysis of dilute
sodium chloride solution with molten
sodium chloride
Molten sodium chloride
Cathode: Na+ ions discharged
Anode: Cl– discharged
Dilute sodium chloride solution
Cathode: H+ ions discharged
Anode: OH– discharged
In the electrolysis of dilute sodium chloride
solution, H+ ions are discharged in preference to
Na+ ions.
!
Order of reactivity
This shows the order of reactivity of
some metals compared to hydrogen:
potassium
sodium
calcium
magnesium
aluminium
zinc
increasing
reactivity
!
!
!
T
N
A
T
R
O
IMP
iron
hydrogen
copper
silver
36
Electrolysis of Aqueous Copper(II)
Sulphate Using Inert Electrodes
37
Electrolysis of Aqueous Copper(II)
Sulphate Using Inert Electrodes
Rule 1:
An aqueous solution of
copper(II) sulphate contains
Cu2+ and SO42–, H+ and OH–
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Electrolysis of Aqueous Copper(II)
Sulphate Using Inert Electrodes
Rule 2: At the anode
OH– ions and SO42– ions are
attracted to the anode.
OH– ions are preferentially
discharged to give oxygen
gas.
4OH–(aq) ! 2H2O(l) + O2(g) + 4e–
The SO42– ions remain in solution.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Electrolysis of Aqueous Copper(II)
Sulphate Using Inert Electrodes
Rule 3: At the cathode
H+ ions and Cu2+ ions are
attracted to the cathode.
Cu2+ ions are preferentially
discharged as copper metal
(atoms).
Cu2+(aq) + 2e– ! Cu(s)
The H+ ions remain in solution.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Electrolysis of Aqueous Copper(II)
Sulphate Using Inert Electrodes
Rule 4: Summary
The overall reaction is
2CuSO4(aq) + 2H2O(l)
! 2Cu(s) + O2(g) + 2H2SO4(aq)
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Electrolysis of Water
42
Electrolysis of Water
Pure water is a poor
conductor of electricity
because it consists almost
entirely of molecules and has
very few ions in it.
It is difficult to electrolyse
pure water.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Electrolysis of Water
To electrolyse water, a small
amount of an ionic compound
or dilute sulphuric acid is
added to water.
The products of the
electrolysis of water:
2 volumes of hydrogen at
the cathode and
1 volume of oxygen at the
anode.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
Examples :
Concentrated Sodium Chloride
45
8.4 The electrolysis of brine
What is brine?
Brine is a concentrated solution of sodium chloride, or common salt.
It can be obtained by pumping water into salt mines to dissolve the salt,
or by evaporating seawater.
The ions present are Na ! and Cl " from the salt, and H ! and OH " from the
water. The reactions at the electrodes are exactly as shown at the bottom
of page 106. (Look back at them.)
Brine might not sound very exciting – but from it, we get chemicals
needed for thousands of products we use every day. When it undergoes
electrolysis, the overall reaction is:
At the cathode Hydrogen is discharged in preference to sodium:
2H ! (aq) ! 2e "
H2 ( g)
2NaCl (aq) ! 2H2O (l)
brine
electrolysis
2NaOH (aq) ! Cl2 ( g) ! H2 ( g)
sodium hydroxide chlorine hydrogen
The electrolysis
The diagram below shows one type of cell used for this electrolysis.
The anode is made of titanium, and the cathode of steel. Now look at the
diaphragm down the middle of the cell. Its function is to let ions through,
but keep the gases apart. (So the cell is called a diaphragm cell.)
chlorine
out
As usual at the cathode, this is a reduction.
At the anode Chlorine is discharged in preference to oxygen:
2Cl " (aq)
Cl2 ( g) ! 2e "
As usual at the anode, this is an oxidation.
The two gases bubble off. Na ! and OH " ions are left behind, giving a
solution of sodium hydroxide. Some of the solution is evaporated to a give
a more concentrated solution, and some is evaporated to dryness, giving
solid sodium hydroxide.
hydrogen
out
brine in
hydrogen
gas
chlorine
gas
nickel
cathode
titanium
anode
Na+
+
membrane
sodium hydroxide
solution out
46
What the products are used for
The electrolysis of brine is an important process,
because the products are so useful. Look at these:
Chlorine, a poisonous yellow-green gas
Used for making ...
the plastic PVC (nearly 1/3 of it used for this)
solvents for degreasing and drycleaning
medical drugs (a large % of these involve chlorine)
Sodium hydroxide solution, alkaline and corrosive
Used in making ...
soaps
detergents
viscose (rayon) and other textiles
paper (like the paper in this book)
ceramics (tiles, furnace bricks, and so on)
dyes
medical drugs
weedkillers and pesticides (most of these involve chlorine)
paints and dyestuffs
bleaches
hydrogen chloride and hydrochloric acid
It is also used as a sterilising agent, to kill bacteria in water
supplies and swimming pools.
Hydrogen, a colourless flammable gas
Used ...
in making nylon
to make hydrogen peroxide
to ‘harden’ vegetable oils to make margarine
as a fuel in hydrogen fuel cells
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