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Electrochemistry
Part One:
Introduction to Electrolysis
and the Electrolysis of
Molten Salts
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Electrochemistry
What do I need to
know about
electrochemistry?
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Electrochemistry
Learning Outcomes:
Candidates should be able to:
a) Describe electrolysis as the conduction of electricity by
an ionic compound (an electrolyte), when molten or
dissolved in water, leading to the decomposition of the
electrolyte.
b) Describe electrolysis as evidence for the existence of
ions which are held in a lattice when solid but which are
free to move when molten or in solution.
c) Describe, in terms of the mobility of ions present and the
electrode products, the electrolysis of molten sodium
chloride, using inert electrodes.
• Singapore Examinations and Assessment
• Board University of Cambridge International Examinations
• Ministry of Education Singapore
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Electrochemistry
d) Predict the likely products of the electrolysis of a molten
binary compound.
e) Apply the idea of selective discharge based on:
i) Cations (linked to the reactivity series).
ii) Anions: halides, hydroxides and sulfates (e.g.
aqueous copper(II) sulfate and dilute sodium chloride
solution (as essentially the electrolysis of water)).
iii) Concentration effects (as in the electrolysis of
concentrated and dilute aqueous sodium chloride).
In all cases above, inert electrodes are used.
f) Predict the likely products of the electrolysis of an
aqueous electrolyte, given relevant information.
• Singapore Examinations and Assessment
• Board University of Cambridge International Examinations
• Ministry of Education Singapore
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Electrochemistry
g) Construct ionic equations for the reactions occurring at
the electrodes during the electrolysis, given relevant
information.
h) Describe the electrolysis of aqueous copper(II) sulfate
with copper electrodes as a means of purifying copper (no
technical details are required).
i) Describe the electroplating of metals, e.g. copper plating,
and state one use of electroplating.
j) Describe the production of electrical energy from simple
cells (i.e. two electrodes in an electrolyte) linked to the
reactivity series and redox reactions (in terms of electron
transfer).
• Singapore Examinations and Assessment
• Board University of Cambridge International Examinations
• Ministry of Education Singapore
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Electrochemistry
• Essential questions in a nutshell…
→ Electricity IN
Why does electricity cause some
chemicals to decompose?
Electricity OUT →
Why do some chemical reactions
produce electricity?
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Electrochemistry
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Electrochemistry
• What are the various uses for aluminium?
• Now imagine a world without aluminium.
→ Which items would no longer exist?
→ What would you no longer be able to do?
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Electrochemistry
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Electrochemistry
• What are the various uses for batteries?
• Now imagine a world without batteries.
→ Which items would no longer exist?
→ What would you no longer be able to do?
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Electrochemistry
Both the extraction
of aluminium, and
how batteries
produce electrical
energy, are based in
electrochemistry.
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ElectrochemistryElectrolysis – Introduction
What is
electrolysis?
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ElectrochemistryElectrolysis – Introduction
• Electrolysis is the process by which a
chemical that contains mobile ions (an
electrolyte) is decomposed (broken down)
into more simple chemicals when
electricity is passed through it.
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Which substances
can be electrolysed?
Which substances
cannot be
electrolysed?
ElectrochemistryElectrolysis – Introduction
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• Aqueous solutions of ionic compounds can be
electrolysed.
• Acids (in aqueous solution) can be electrolysed.
• Molten ionic compounds can be electrolysed.
→ Molten ionic compounds contain mobile ions that are free to move towards
the electrode of opposite charge. Anions are oxidised at the anode while cations
are reduced at the cathode.
→ Aqueous solutions of ionic compounds contain mobile ions that are free to
move towards the electrode of opposite charge. Anions are oxidised at the
anode while cations are reduced at the cathode.
→ Acids contain mobile ions that are free to move towards the electrode of
opposite charge. Anions are oxidised at the anode while cations are reduced at
the cathode.
ElectrochemistryElectrolysis – Introduction
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• Solid ionic compounds cannot be electrolysed.
• Metals cannot be electrolysed.
• Pure covalent elements and compounds cannot be
electrolysed.
→ Solid ionic compounds do not contain mobile ions that
are free to move towards the electrode of opposite charge.
→ The movement of electrons through a metal allows it to
conduct electricity without decomposing.
→ Pure covalent elements and compounds do not contain
mobile ions in any state (solid, liquid or aqueous solution).
ElectrochemistryElectrolysis – Introduction
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ElectrochemistryElectrolysis – Introduction
• Enduring understanding in a nutshell…
→ A chemical that contains mobile ions
can be electrolysed, i.e. it will decompose
when electricity is passed through it.
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So what would happen
if I placed electrodes in:
a) Solid candle wax?
b) Molten candle wax?
ElectrochemistryElectrolysis – Introduction
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• Simple covalent
molecules, such as
the long chain
hydrocarbons in
candle wax (e.g.
C30H62), do not
contain any mobile
charge particles
(electrons or ions) in
either the solid state
or the liquid state.
ElectrochemistryElectrolysis – Introduction
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• As a consequence,
simple covalent
molecules do not
conduct electricity in
either the solid state
or molten state
because they do not
contain mobile
charged particles that
can move freely
towards the positive
(anode) and negative
(cathode) electrodes.
ElectrochemistryElectrolysis – Introduction
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• Simple covalent
compounds do not
conduct electricity
and are not
decomposed by
electricity.
ElectrochemistryElectrolysis – Introduction
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Why can metals
conduct electricity
without
decomposing?
ElectrochemistryElectrolysis – Introduction
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• When the
circuit is closed,
the mobile or
delocalised sea
of electrons
within the metal
is repelled away
from the negative
electrode
(cathode) and
attracted towards
the positive
electrode
(anode).
• Because
metals contains a
mobile or
delocalised sea
of electrons, they
are able to
conduct
electricity in both
their solid and
molten states.
ElectrochemistryElectrolysis – Introduction
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• Because
metals conduct
electricity due to
the movement of
electrons, and
because the
overall number of
electrons within
the metal never
changes, metals
do not
decompose
when they
conduct
electricity.
• The battery
forces electrons
to move through
the metal.
• Electrons flow
into the metal at
the cathode and
flow out of the
metal at the
anode.
ElectrochemistryElectrolysis – Introduction
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Why are aqueous
and molten ionic
compounds
decomposed by
electricity?
ElectrochemistryElectrolysis – Introduction
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Oxidation Is Loss
(of electrons)
Reduction Is Gain
(of electrons)
ElectrochemistryElectrolysis – Introduction
Remember:
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ElectrochemistryElectrolysis – Introduction
“Red Cat”
Reduction takes
place at the
cathode (–).
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“An Ox” Anode (+) is the site of oxidation.
ElectrochemistryElectrolysis – Introduction
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ElectrochemistryElectrolysis – Introduction
• Enduring understanding in a nutshell…
→ Positively charged ions (cations) are
reduced at the cathode (negative
electrode).
→ Negatively charged ions (anions) are
oxidised at the anode (positive electrode).
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1 2 Group 3 4 5 6 7 0
1 H He
2 Li Be B C N O F Ne
3 Na Mg Al Si P S Cl Ar
4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
7 Fr Ra Ac
Key:
Blue = Metal
Yellow = Non-metal
• Remember, simple ionic compounds are formed when a
metal reacts with a non-metal.
Period
ElectrochemistryElectrolysis – Introduction
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• Due to the very
strong electrostatic
force of attraction that
holds the anions and
cations together, ionic
compounds are all
solids at room
temperature. All ionic
compounds have very
high melting points
and boiling points.
ElectrochemistryElectrolysis – Introduction
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• According to kinetic
particle theory, the
anions and cations in
a solid ionic
compound are closely
packed together,
vibrating about a fixed
position in the crystal
lattice. Apart from
vibrating, the ions are
not free to move in
any other way.
ElectrochemistryElectrolysis – Introduction
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• When molten, the
electrostatic force of
attraction between the
anions and cations is
weakened. The ions are
now able to travel short
distances by slipping and
sliding over each other.
An ionic compound will
conduct electricity in the
molten state as the ions
are free to move towards
the electrode of opposite
charge.
ElectrochemistryElectrolysis – Introduction
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• Ionic compounds
are soluble in polar
solvents such as
water.
ElectrochemistryElectrolysis – Introduction
• Ionic compounds
are insoluble in
non-polar solvents
such as oil and
hexane.
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Water molecules are
described as being
polar. This means
that there is a small
distribution of charge
over the water
molecule. As a result,
water molecules are
attracted towards
positive ions (anions)
and negative ions
(cations).
ElectrochemistryElectrolysis – Introduction
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• Due to their
polarity, water
molecules can
bond to the anions
and cations in an
ionic compound
and remove them
into solution.
ElectrochemistryElectrolysis – Introduction
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• The mobile ions
are now free to
move towards the
electrode of
opposite charge
and hence conduct
electricity.
ElectrochemistryElectrolysis – Introduction
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• Ionic compounds
are electrolytes. They
do not conduct
electricity in the solid
form, but do conduct
electricity when
molten or when
dissolved in water.
ElectrochemistryElectrolysis – Introduction
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• Kinetic particle
theory states that in a
solid ionic compound,
the positive and
negative ions vibrate
about a fixed
position. They are
unable to move
towards the electrode
of opposite charge.
ElectrochemistryElectrolysis – Introduction
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• When the ionic
compound is molten
or when it is
dissolved in water,
the positive and
negative ions
become mobile and
are free to move
towards the electrode
of opposite charge,
thus conducting
electricity.
ElectrochemistryElectrolysis – Introduction
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ElectrochemistryElectrolysis – Introduction
• The negatively
charged anions move
towards the positively
charged anode where
they lose electrons (i.e.
oxidised) to form neutral
atoms.
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ElectrochemistryElectrolysis – Introduction
• The negatively
charged anions move
towards the positively
charged anode where
they lose electrons (i.e.
oxidised) to form neutral
atoms.
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ElectrochemistryElectrolysis – Introduction
• The negatively
charged anions move
towards the positively
charged anode where
they lose electrons (i.e.
oxidised) to form neutral
atoms.
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ElectrochemistryElectrolysis – Introduction
• The battery
“pumps” electrons
that were removed
from the anions at
the anode through
the external circuit to
the cathode.
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ElectrochemistryElectrolysis – Introduction
• The positively charged
cations move towards
the negatively charged
cathode where they gain
electrons (i.e. reduced)
to form neutral atoms.
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ElectrochemistryElectrolysis – Introduction
• The positively charged
cations move towards
the negatively charged
cathode where they gain
electrons (i.e. reduced)
to form neutral atoms.
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• The positively charged
cations move towards
the negatively charged
cathode where they gain
electrons (i.e. reduced)
to form neutral atoms.
ElectrochemistryElectrolysis – Introduction
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• Electrolysis is evidence
that ionic compounds are
made up of positive ions
(cations) and negative
ions (anions) that are not
mobile in the solid state,
but are free to move
when in the aqueous or
molten states when they
are attracted towards the
electrode of opposite
charge and are either
oxidised (anions) or
reduced (cations).
ElectrochemistryElectrolysis – Introduction
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What happens during
the electrolysis of
molten sodium
chloride using inert
electrodes?
ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
• Sodium chloride
does not conduct
electricity in the
solid form, but does
conduct electricity
when molten.
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ElectrochemistryElectrolysis of Molten Salts
• The negatively
charged chloride
ions (anions) are
attracted towards
the positively
charged anode.
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ElectrochemistryElectrolysis of Molten Salts
• At the anode, the
negatively charged
chloride ions lose
electrons (i.e. they
are oxidised) to form
neutral chlorine
atoms:
2Cl–(l) → Cl2(g) + 2e–
The chlorine atoms
bond together in
pairs to form diatomic
molecules.
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ElectrochemistryElectrolysis of Molten Salts
• At the anode, the
negatively charged
chloride ions lose
electrons (i.e. they
are oxidised) to form
neutral chlorine
atoms:
2Cl–(l) → Cl2(g) + 2e–
The chlorine atoms
bond together in
pairs to form diatomic
molecules.
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ElectrochemistryElectrolysis of Molten Salts
• The battery
“pumps” electrons
that were removed
from the chloride
ions at the anode
through the
external circuit to
the cathode.
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ElectrochemistryElectrolysis of Molten Salts
• The positively
charged sodium
ions (cations) are
attracted towards
the negatively
charged cathode.
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ElectrochemistryElectrolysis of Molten Salts
• At the cathode,
the positively
charged sodium
ions gain electrons
(i.e. they are
reduced) to form
neutral sodium
atoms.
Na+(l) + e– → Na(l)
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ElectrochemistryElectrolysis of Molten Salts
• At the cathode,
the positively
charged sodium
ions gain electrons
(i.e. they are
reduced) to form
neutral sodium
atoms.
Na+(l) + e– → Na(l)
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ElectrochemistryElectrolysis of Molten Salts
• Note: Electrolysis
is an endothermic
process.
• Electrical energy
is required to
convert the
compound sodium
chloride into the
two chemical
elements sodium
and chlorine.
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
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ElectrochemistryElectrolysis of Molten Salts
Energy
Endothermic
H is Positive
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ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
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ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 68: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/68.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 69: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/69.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 70: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/70.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 71: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/71.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 72: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/72.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 73: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/73.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 74: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/74.jpg)
ElectrochemistryElectrolysis of Molten Salts
• Macroconcept of Systems: Systems are composed of sub-systems
that interact. Using interactions in a transport system as an analogy
to explain the interactions that occur in an electrochemical system.
![Page 75: Electrochemistrynygh.sg/electrochemistry/molten_salts.pdf · Electrochemistry g) Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given](https://reader030.vdocument.in/reader030/viewer/2022040320/5e47f09bdea331129435e779/html5/thumbnails/75.jpg)
• What products are formed at a) the anode and b) the cathode
when molten lead(II) bromide is electrolysed using inert electrodes?
ElectrochemistryElectrolysis of Molten Salts
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a) At the anode (+ve):
Negative bromide
ions are attracted
towards the positive
anode where they are
oxidised to form
molecular bromine.
2Br–(l) → Br2(g) + 2e–
b) At the cathode (–ve):
Positive lead(II) ions
are attracted towards
the negative cathode
where they are reduced
to form elemental lead.
Pb2+(l) + 2e– → Pb(l)
ElectrochemistryElectrolysis of Molten Salts
• What products are formed at a) the anode and b) the cathode
when molten lead(II) bromide is electrolysed using inert electrodes?
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• What products are formed at a) the anode and b) the cathode
when molten calcium oxide is electrolysed using inert electrodes?
ElectrochemistryElectrolysis of Molten Salts
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a) At the anode (+ve):
Negative oxide ions
are attracted towards
the positive anode
where they are
oxidised to form
molecular oxygen.
2O2–(l) → O2(g) + 4e–
b) At the cathode (–ve):
Positive calcium ions
are attracted towards
the negative cathode
where they are reduced
to form elemental
calcium.
Ca2+(l) + 2e– → Ca(l)
ElectrochemistryElectrolysis of Molten Salts
• What products are formed at a) the anode and b) the cathode
when molten calcium oxide is electrolysed using inert electrodes?
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• What products are formed at a) the anode and b) the cathode
when molten potassium iodide is electrolysed using inert electrodes?
ElectrochemistryElectrolysis of Molten Salts
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a) At the anode (+ve):
Negative iodide ions
are attracted towards
the positive anode
where they are
oxidised to form
molecular iodine.
2I–(l) → I2(g) + 2e–
b) At the cathode (–ve):
Positive potassium ions
are attracted towards
the negative cathode
where they are reduced
to form elemental
potassium.
K+(l) + e– → K(l)
ElectrochemistryElectrolysis of Molten Salts
• What products are formed at a) the anode and b) the cathode
when molten potassium iodide is electrolysed using inert electrodes?
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ElectrochemistryChoice of Electrodes
Why does
electrolysis tend
to use graphite
electrodes?
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ElectrochemistryChoice of Electrodes
• Graphite is a good conductor of electricity. Because it
contains a mobile (delocalised) “sea” of electrons, it will
conduct electricity without decomposing.
• Graphite has a very high melting point (3730 C). It will not
melt at the high temperatures required for electrolysis of
molten salts.
• In general, graphite is considered to be chemically inert. It
will not react with the chemicals that are being discharged at
the anode or cathode. Note: Graphite can occasionally react
with oxygen at high temperatures to produce carbon dioxide.
• Graphite is relatively cheap. The alternative to an inert
graphite electrode is a platinum electrode!
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ElectrochemistryPresentation on
Introduction to Electrolysis and the
Electrolysis of Molten Salts
by Dr. Chris Slatter
Nanyang Girls’ High School
2 Linden Drive
Singapore
288683
8th February 2016