DSE Chem notes (Electrolysis)
Chapter 32 – Electrolysis
32.1
What is electrolysis?
Definition
Electrolysis is the decomposition of an electrolyte by passing electricity
through it. In the process, electrical energy is converted to chemical energy.
-
An electrolyte is a compound that produces __________ ______ when
dissolved in water or when in molten state (e.g. _______________________
______________________ )
Set-up for electrolysis
-
A _________________ power supply provides the electrical energy.
An __________ is used to measure the electric current passing through the
circuit.
A ___________ is included in the circuit to regulate the current by changing
the resistance.
Inert ___________ such as ___________ or ____________ are usually used,
except in electroplating.
Differences between a chemical cell and an electrolytic cell
Chemical cell
Electrolytic cell
It is a source of electrical energy
It requires an electrical energy source
Chemical energy
Electrical energy
Electrical energy
Chemical energy
A spontaneous redox reaction takes place
A redox reaction is forced to occur
Anode ( - ) pole (oxidation)
Anode ( + ) pole (oxidation)
Cathode ( + ) pole (reduction)
Cathode ( - ) pole (reduction)
32-1
DSE Chem notes (Electrolysis)
32.2
Electrolysis of molten salts
Electrolysis of lead(II) bromide
At cathode (reduction):
At anode (oxidation):
Overall equation:
Pb2+ + 2e- 
 Pb (Silvery solid)
2Br 
 Br2 + 2e- (Reddish brown fumes)
PbBr2 
 Pb + Br2
Lead(II) bromide is decomposed into ________ (deposited at cathode) and
____________ (given out at anode).
Electrolysis of sodium chloride
At cathode (reduction):
At anode (oxidation):
Overall equation:
Sodium chloride is decomposed into ____________ (deposited at cathode) and
_____________ (given out at anode).
32-2
DSE Chem notes (Electrolysis)
In electrolysis, electric current is conducted by the flow of electrons in the
external circuit.
However, there is no electron flow in the electrolyte. Electric current passes
through the electrolyte by the movement of ions.
32.3
-
Electrolysis of aqueous solutions
Water molecules can ionize very slightly:
H2O(l)
H+(aq) + OH-(aq)
-
Hence, all aqueous solutions contain a very small amount of H+(aq) and
OH-(aq) in addition to the constituent ions of the solute dissolved.
-
During electrolysis, H+(aq) will move towards the cathode and compete for
reduction while OH-(aq) will move towards _________ and compete for
______________.
Factors affecting preferential discharge of ions at electrodes
1.
Position of ion in the electrochemical series
At cathode: The _________ the position of the cation in the ecs, the more
easily it will be discharged.
Ease of discharge: Ag+ > Cu2+ > H+ > Pb2+ ….……..
At anode:
The _________ the position of the anion in the ecs, the more
easily it will be discharged.
Ease of discharge: OH- > [I- > Br- > Cl-]* > NO3- > SO42-
2.
Concentration of ions
I-, Br- and Cl- will be discharged in preference to OH- if they are present in
higher concentrations. This is known as ______________ effect.
3.
Material of electrodes
Nature of electrode may also affect preferential discharge of some ions.
32-3
DSE Chem notes (Electrolysis)
32.4
Electrolysis of dilute H2SO4
Hofmann voltameter
At cathode (reduction):
2H+ + 2e- 
 H2
At anode (oxidation):
4OH- 
 2H2O + O2 + 4eThough both OH- and SO42- move towards anode, ______ is discharged
preferentially because it is higher in position than SO42- in the ecs.
Overall equation:
2H2O(l) 
 2H2(g) + O2(g)
(Volume of H2 to O2 = 2:1)
Water is decomposed into hydrogen and oxygen, hence concentration of the acid
_______________ in the electrolyte.
32.5
Electrolysis of sodium chloride solution
A. Very dilute sodium chloride solution
32-4
DSE Chem notes (Electrolysis)
At cathode (reduction):
2H+ + 2e- 
 H2
Both ______ and ______ are present, but H+ is preferentially discharged because
it is __________ in position than Na+ in the ecs.
At anode (oxidation):
4OH- 
 2H2O + O2 + 4eBoth ______ and ______ are present, but OH- is discharged preferentially because
it is __________ in position than Cl- in the ecs.
Overall equation:
2H2O(l) 
 2H2(g) + O2(g)
Water is decomposed into hydrogen and oxygen, while concentration of the
sodium chloride ____________ in the electrolyte.
B. Concentrated sodium chloride solution with graphite
electrodes
 H2
At cathode (reduction):
2H+ + 2e- 
Both ______ and ______ are present, but H+ is preferentially discharged because
it is __________ in position than Na+ in the ecs.
 Cl2 + 2eAt anode (oxidation):
2Cl- 
(A greenish yellow gas with a choking smell is given out)
Both ______ and ______ are present, but Cl- is preferentially discharged because
of ________________ effect.
The actual volume of Cl2(g) collected is ___________ than that of H2(g) because
some Cl2(g) ___________ in water.
32-5
DSE Chem notes (Electrolysis)
After H+ and Cl- are discharged, _______ and _______ are left behind in the
solution. Hence, the electrolyte changes gradually from sodium chloride to
____________________ solution.
Overall equation: 2NaCl(aq) + 2H2O(l) 
 2NaOH(aq) + H2(g) + Cl2(g)
N.B. Graphite electrode should be used if chlorine is given out because chlorine
attacks platinum.
C. Concentrated sodium chloride solution with mercury cathode
At Hg cathode (reduction):
Na+ + e- 
 Na
Na + Hg 
 Na/Hg (sodium amalgam)
Na+ is preferentially discharged when mercury is used as the cathode. The sodium
metal formed dissolves immediately in the mercury forming sodium amalgam.
At anode (oxidation):
2Cl- 
 Cl2 + 2eBoth ______ and ______ are present, but Cl- is preferentially discharged because
of ________________ effect.
Overall equation:
 2Na/Hg(l) + Cl2(g)
2Hg(l) + 2Na+(aq) + 2Cl-(aq) 
As more and more Na+ and Cl- are discharged, the sodium chloride solution
becomes more and more _____________.
N.B. When Na/Hg is passed into water, the following reaction occurs,
regenerating the mercury cathode.
 2NaOH(aq) + H2(g) + 2Hg(l)
2Na/Hg(l) + 2H2O(l) 
32-6
DSE Chem notes (Electrolysis)
32.6
Electrolysis of copper(II) sulphate solution
A. With graphite electrodes
At cathode (reduction):
Cu2+ + 2e- 
 Cu
2+
Cu is preferentially discharged because it is __________ in position than H+ in
the ecs.
At anode (oxidation):
4OH- 
 2H2O + O2 + 4eOH- is preferentially discharged because it is ____________ in position than SO42in the ecs.
As more and more Cu2+ and OH-(aq) are discharged, _______ and _______ are
left behind in the solution. Hence, the electrolyte changes gradually from
copper(II) sulphate solution to __________________.
Observable changes:
1. ____________________ solid is deposited on the cathode
2. ___________________ are given out at the anode.
3. The blue colour of the solution gradually __________ away.
B. With copper electrodes
32-7
DSE Chem notes (Electrolysis)
At cathode (reduction):
Cu2+ + 2e- 
 Cu
Cu2+ is preferentially discharged because it is __________ in position than H+ in
the ecs.
At anode (oxidation):
Cu 
 Cu2+ + 2e-
Copper _________ electrons more readily than both OH- and SO42-, since its
position in the ecs is ____________ than those of OH- and SO42The number of Cu2+ discharged at the cathode is ______________ as the number
of Cu2+ produced at the anode. Hence, the concentration and colour of the
electrolyte remains unchanged.
Observable changes:
1. The copper cathode becomes ____________.
2. The copper anode becomes ___________.
3. The blue colour of the electrolyte remains _____________.
Class discussion
In the above example, if the copper anode contains impurities, will the gain in
mass of the copper cathode the same as the loss in mass of the copper anode after
electrolysis?
32.7 Electrolysis of other aqueous solutions with inert
electrodes
A. Sodium sulphate solution
At cathode (reduction):
_____ is preferentially discharged because it is _________ in position than
_____ in the ecs.
At anode (oxidation):
_____ is preferentially discharged because it is _________ in position than
_____ in the ecs.
As more and more ______ and ______ are discharged, the electrolyte
becomes more _____________.
32-8
DSE Chem notes (Electrolysis)
Observable changes:
___________________ are given out at both cathode and anode. The volumes
of gases collected at the cathode and the anode are in the ratio of__________.
B. Potassium nitrate solution
At cathode (reduction):
_____ is preferentially discharged because it is _________ in position than
_____ in the ecs.
At anode (oxidation):
_____ is preferentially discharged because it is _________ in position than
_____ in the ecs.
As more and more ______ and ______ are discharged, the electrolyte
becomes more ________________.
Observable changes:
___________________ are given out at both cathode and anode.
The volumes of gases collected at the cathode and the anode are in the ratio
of________.
C. Concentrated potassium iodide solution
At cathode (reduction):
_____ is preferentially discharged because it is _________ in position than
_____ in the ecs.
At anode (oxidation):
Even though OH- is higher in position than I- in the ecs, _____ is
preferentially discharged because of ______________ effect.
After ______ and _____ are discharged, _______ and _______ are left behind
in the solution. Hence, the electrolyte changes gradually from potassium
iodide to ____________________ solution.
Observable changes:
1.
___________________ are given out at the cathode.
2.
The solution turns ___________ around the anode due to the formation
of ___________.
32-9
DSE Chem notes (Electrolysis)
32.8
Summary of products of electrolysis
Electrodes
Products
Change in
electrolyte
Substance
Cathode
Anode
at cathode
at anode
PbBr2(l)
C
C
Gradually
disappears
NaCl(l)
C
C
Gradually
disappears
Dilute
H2SO4(aq)
C
C
Becomes more
concentrated
Very dilute
NaCl(aq)
C
C
Conc
NaCl(aq)
C
C
Conc
NaCl(aq)
Hg
C
CuSO4(aq)
C
C
CuSO4(aq)
Cu
Cu
AgNO3(aq)
C
C
Na2SO4(aq)
C
C
AgNO3(aq)
Ag
Ag
Na/Hg(l)
Ag(s)
32-10
O2(g)
DSE Chem notes (Electrolysis)
32.9
Factors affecting rate of electrolysis
The following changes can speed up electrolysis:
1.
Increase voltage of power supply (must be a d.c. source)
2.
Increase contact surface area of electrodes with electrolyte (e.g. using
larger electrodes or pushing the electrodes further into the electrolyte)
3.
Move the electrodes closer together
4.
Increase concentration of electrolyte solution
32.10 Industrial application of electrolysis
1.
Electroplating
Electroplating is used to _________ a metal object from ___________
-
or to make the object look more attractive.
The object to be plated acts as ___________ while the anode is made
of the plating metal.
e.g. Electroplating an iron cup with silver
-
2.
Commonly used plating metals include copper, silver, gold, tin and
chromium.
Plastic objects can also be electroplated by first spraying it with a layer
of powdered graphite or metal to make it conduct electricity.
Purification of metals (electrolytic refining)
A block of the impure metal is used as the anode while a pure metal
rod is used as the cathode.
32-11
DSE Chem notes (Electrolysis)
e.g. Electrolytic refining of copper
3.
Anode gradually dissolves and only pure copper is deposited on
cathode.
Impurities from the anode remain in the solution.
Loss in mass of anode is usually greater than gain in mass of cathode.
Extraction of metals
Reactive metals like K, Na, Ca, Mg and Al are extracted by electrolysis of
their molten ores.
e.g. Electrolysis of molten aluminium oxide
The graphite anodes have to be replaced from time to time because oxygen
formed at anode oxidizes the carbon to _____________________ .
4.
Production of useful chemicals
Electrolysis of concentrated sodium chloride solution (brine) produces
_______________, ______________ and ________________________
which are all very useful chemicals.
Hydrogen and chlorine can further combine to form hydrochloric acid.
-
Chlorine and sodium hydroxide react to form sodium hypochlorite
solution which is a common bleaching solution.
32-12
DSE Chem notes (Electrolysis)
5.
Anodizing aluminium
In the electrolysis, the aluminium object acts as the __________ where
____________ is formed.
The aluminium will be oxidized to form a thicker layer of ___________
-
__________, making the object more corrosion-resistant.
The aluminium oxide layer can also be dyed to make the object more
______________.
32.11 Environmental impact of electroplating industry
Water pollution
-
Electroplating factories produce liquid wastes containing acids, alkalis, metal
salts, toxic chemicals (e.g. potassium cyanide), organic solvents etc.
-
Harmful effects of industrial effluents include:
1. Corrosion of drainage pipes and ships
2. Poisoning of aquatic life leading to death
3. Poisoning of humans caused by consumption of contaminated seafood
Control of chemical waste disposal
-
Control measures are implemented by the government to reduce pollution of
waters.
-
Industrial effluents can be treated before disposal
1.
2.
Neutralize acidic effluents with _________________
Neutralize alkaline effluents with __________________
3.
Remove heavy metal ions (e.g. chromium) by precipitating them as
_____________ or _____________ by adding NaOH or Na2CO3
32-13
DSE Chem notes (Electrolysis)
Electrolysis
Molten
state (l)
Cathode
(Red)
Aqueous
solution (aq)
Anode
(Ox)
Anode
(Ox)
Cathode
(Red)
(Dilute
solutions)
Metal
Non-metal
Application: Extraction of reactive
metals (e.g. K, Na, Ca, Mg, Al)
1. Ag+ + eCu2+ +2e-
Ag
Cu
(Ni2+ or Cr3+ is also
discharged in
electroplating
experiment)
2. 2H+ +2eH2
(Position in ecs)
(Conc.
solution)
1. M
Mn+ + ne(If anode is a metal)
(Effect of electrode)
1. Mn+
2. 4OH3. 2I2Br2Cl-
2. I2
Br2
Cl2
(Conc
effect)
2H2O+O2+4eI2 + 2eBr2 + 2eCl2 + 2e-
3. O2
Exception: when using Hg
cathode in electrolysis of
NaCl(aq)
Na+ + Hg + eNa/Hg
Application: 1. Electroplating 2. Electrolytic refining 3. Making useful chemicals
32-14