Chapter 6:
Electrochemistry
6.1: Types of Electrical Conductivity
6.2: Products of Electrolysis
Objectives:
❏ Define electrolysis as the breakdown of an ionic compound when molten
or in aqueous solution by the passage of electricity
❏ 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
❏ Describe the transfer of charge during electrolysis to include:
(a) the movement of electrons in the external circuit
(b) the loss or gain of electrons at the electrodes
(c) the movement of ions in the electrolyte
2
What is the difference between conductors and
insulators?
3
What is the difference between conductors and
insulators?
In solids, electrical
conductors are
substances that
conducts electricity but
is not chemically
changed in the process.
Electrical insulators are
substances that does
not conduct electricity.
4
However, in liquids, electrical conductivity is
tested differently.
Why can NaCl
dissolved in a
solution conduct
electricity while
C12H22O11
(sugar) cannot?
5
However, in liquids, electrical conductivity is
tested differently.
Why can NaCl dissolved in
a solution conduct
electricity while C12H22O11
(sugar) cannot?
NaCl solution can conduct
electricity because it is an
ionic compound and it
contains ions while
C12H22O11 (sugar) solution
is a covalent compound
and does not contain ions.
6
However, in liquids, electrical conductivity is
tested differently.
Liquids that can
conduct electricity by
movement of ions are
called electrolytes
and liquids that do not
conduct electricity
through movement of
ions is called
non-electrolytes.
7
When electrolytes conduct electricity, chemical
change happens and the ionic compound is
dissociated or split up.
Electrolysis
Electrical, electricity
Splitting, breaking down
8
What is electrolysis?
Electrolysis
Splitting, breaking down
Electrical, electricity
Electrolysis is the process by which ________ compounds are _________
into simpler substances when an _________ is passed through them.
Electric current
ionic
broken down
9
What is electrolysis?
Electrolysis
Electrical, electricity
Splitting, breaking down
broken down
ionic compounds are _____________
Electrolysis is the process by which ________
electric current_is passed through them.
into simpler substances when an _____________
10
Solid sodium chloride
Sodium chloride solution
1)
What type of bonding is shown in these diagrams? How do you know?
2)
Which can conduct electricity? How?
3)
Which could be broken down by electrolysis? Why?
11
Electrolysis of copper (II)
chromate (VI)
ELECTRODES
● Electrodes are the rods that carries the
current into the solution. It should be
made-up of inert/unreactive
materials.
● Anode - is the positive electrode in
electrolysis
● Cathode - is the negative electrode in
electrolysis
copper (II) chromate (VI)
solution
12
Electrolysis of copper (II)
chromate (VI)
ANODE
CATHODE
Which ions goes to the anode and
which ions proceeds to the
cathode?
copper (II) chromate (VI)
solution
13
Electrolysis of copper (II)
chromate (VI)
Which ions goes to the anode and which
ions proceeds to the cathode?
FIRST, identify the ions present in the
solution by creating the dissociation
equation of the compound.
copper (II) chromate (VI)
CuCrO4
CuCrO4 → Cu2+ (aq)+ CrO42- (aq)
14
Electrolysis of copper (II) chromate (VI)
CuCrO4 → Cu2+ (aq)+ CrO42- (aq)
ANODE
CrO42- (aq) will go the positive
electrode
CATHODE
Cu2+ (aq) ions will go the negative
electrode
copper (II) chromate (VI)
solution
15
Electrolytic Cell
❏ An apparatus in which electrolysis is carried
out.
16
Electrolytic Cell
A power source or
battery is the one
that supplies direct
current.
Graphite rods
carries the electric
current to the liquid
electrolyte solution
17
Electrolytic Cell
What is the difference between metallic conductivity and
electrolytic conductivity?
A
B
18
Electrolytic Cell
Conductivity in Metals
Electrons flow
Electrolytic Conductivity
Ions flow
A property to describe elements
A property to describe ionic
(metals and carbon as graphite) and compounds
alloys
Takes place in solids and liquids
Takes place in liquids and solutions
(not in solids)
No chemical change happens/takes
place
Chemical decomposition takes place.
19
TASK:
Answer the following questions about electrical conductivity:
1.a. Which of the following will conduct electricity?
i. a strip of copper metal
ii. a solution of sugar in water
iii. aqueous sodium chloride solution
iv. mercury
v. dilute hydrochloric acid
vi. melted sugar
1.b. Which among i.-vi. are electrolytes?
2.a. Why is solid potassium bromide a non-conductor of electricity?
2.b. Suggest two things that could be done to potassium bromide that
would make it conduct electricity.
3. Explain the major difference between electrical conductivity in metals
and solutions in terms of the particles that are carrying the current.
20
TASK:
1.a. Which of the following will conduct
electricity?
i. a strip of copper metal ✓
ii. a solution of sugar in water
iii. aqueous sodium chloride solution ✓
iv. mercury ✓
v. dilute hydrochloric acid ✓
vi. melted sugar
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TASK:
1.b. Which among i.-vi. are electrolytes?
i. a strip of copper metal
ii. a solution of sugar in water
iii. aqueous sodium chloride solution ✓
iv. mercury
v. dilute hydrochloric acid ✓
vi. melted sugar
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TASK:
2.a. Why is solid potassium bromide a
non-conductor of electricity?
➔ The ionic solid does not conduct as the
ions present cannot move about (they
can only vibrate at fixed positions).
23
TASK:
2.b. Suggest two things that could be
done to potassium bromide that would
make it conduct electricity.
➔ Melt it.
➔ Dissolve it in water.
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TASK:
3. Explain the major difference between electrical
conductivity in metals and solutions in terms of the
particles that are carrying the current.
➔ When a metal conducts electricity it is the
delocalised electrons present in the structure
that move through the metal to carry the charge /
in aqueous solutions of ionic compounds it is the
ions present that move to carry the charge.
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MWB
Pb2+
Br-
1. Which of the above are anions?
2. Which of the above are cations?
3. Describe what happens to Pb2+ at the
electrode in terms of electrons
4. Describe what happens to Br- at the
electrode in terms of electrons
5. What needs to happen to solid lead
bromide before it can be electrolysed?
Explain how electrolysis is used to separate solid sodium
chloride
• What do you need to do the solid sodium chloride to allow the ions to
move?
• What do you have to switch on to start electrolysis?
• Which ions move to the negative cathode?
• What happens in term of electrons at the cathode?
• What is produced at the cathode?
Challenge: Write a half equation showing this
• Which ions move to the positive anode?
• What happens in term of electrons at the anode?
• What is produced at the anode?
Challenge: Write a half equation showing this
Explain how electrolysis is used to separate solid lead
bromide (6 marks).
• What do you need to do the solid lead bromide to allow the ions to
move?
• What do you have to switch on to start electrolysis?
• Which ions move to the negative cathode?
• What happens in term of electrons at the cathode?
• What is produced at the cathode?
Challenge: Write a half equation showing this
• Which ions move to the positive anode?
• What happens in term of electrons at the anode?
• What is produced at the anode?
Challenge: Write a half equation showing this
Explain how electrolysis is used to separate solid lead bromide
• Melt the solid lead
bromide to allow
ions to move
• Turn on power
supply and run a
direct current (DC)
through circuit
Explain how electrolysis is used to separate solid lead bromide
• Pb2+ would move
towards the negative
cathode
• Pb2+ ion would gain 2
electrons to become a
neutral atom (Pb) - it is
reduced
• Solid Pb metal produced
at cathode
• Pb2+ + 2e- → Pb
Explain how electrolysis is used to separate solid lead bromide
• Br- would move
towards positive anode
• Br- ion would lose 1
electron to become a
neutral molecule (Br2)
- it is oxidised
• Br2 gas produced at
anode
• 2Br- → Br2 + 2e-
Writing half equations for electrolysis
WATCH: https://www.youtube.com/watch?v=87K8QsMl8nc
The video shows the electrolysis of lead bromide. Lead bromide is split using
electricity to make pure solid lead and bromine gas. The electrolysis of lead
bromide can be shown in an equation.
However, different reactions happen at each
electrode as each ion must lose or gain
electrons to become a neutral atom again.
We can describe these reactions as oxidation or
reduction and can show them as half equations.
For each molten electrolyte state which
substance is produced at the anode and which
moves to the cathode
Electrolyte
Lead bromide
Sodium chloride
Copper sulphate
Potassium fluoride
Aluminium oxide
Products at the cathode (negative
electrode)
Products at the anode (positive
electrode)
Electrolyte
Products at the
cathode (negative
electrode)
Products at the anode
(positive electrode)
lead bromide,
PbBr2
lead (Pb)
bromine (Br2)
sodium chloride,
NaCl
sodium (Na)
chlorine (Cl2)
potassium iodide,
KI
potassium (K)
iodine (I2)
copper (II) bromide,
CuBr2
copper (Cu)
bromine (Br2)
34
2Cl- → Cl2 + 2e● The chlorine ions (Cl-) will move
to the anode.
● Each chlorine ion will lose one
electron to become chlorine.
Cl- → Cl + e-
● Two chlorine atoms will bond
to make a chlorine molecule.
Cl + Cl → Cl2
2Cl- → Cl2 + 2e-
35
We can remember whether a reaction is
oxidation or reduction by using OILRIG
Oxidation
Reduction
Is
Is
Loss of Electrons
Gain of Electrons
ReduCtion takes place at the Cathode.
OxidAtion takes place at the Anode.
Copy down OILRIG
mnemonic to help you
remember the
difference between
oxidation and
reduction and which
electrode they occur at
Oxidation & Reduction
We can represent these reactions as half equations. For
example, the electrolysis of Zinc Chloride (ZnCl2) into Zinc (Zn)
and Chlorine (Cl).
Cathode Reaction - Zn2+ ions are attracted to the negative
cathode, where they gain electrons and become neutral zinc
atoms.
Reduction reaction
Zn2+ + 2e → Zn (Reduction Is Gain)
Anode Reaction - Cl- ions are attracted to the positive anode,
where they lose electrons and become neutral chlorine atoms.
2Cl- → + Cl2 + 2e
Oxidation reaction
(Oxidation Is Loss)
Electrolyte
Decomposition
Products
lead bromide, PbBr2
lead (Pb) and
bromine (Br2)
sodium chloride,
NaCl
sodium (Na) and
chlorine (Cl2)
potassium iodide, KI
potassium (K) and
iodine (I2)
copper (II) bromide,
CuBr2
copper (Cu) and
bromine (Br2)
Cathode Reactions
Anode Reactions
Pb2+ + 2e- → Pb
2Br- → Br + 2e-
Na+ + e- → Na
2Cl- → Cl2 + 2e-
K+ + e - → K
2I- → I2 + 2e-
Cu2+ + 2e- → Cu
2Br → Br2 + 2e-
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1) Complete worksheet
2) Write electrolysis symbol equations and
half equations at the cathode and anode
for the electrolysis of molten:
a) Lead Bromide
b) Calcium Oxide
c) Magnesium Chloride
d) Aluminium Oxide