A look at the factors that affect

1. Factors
affecting
electrolysis
Prepared by Janadi
Gonzalez-Lord

2. Electrolysis summarized
A brief overview of what electrolysis is and how it
works

3. What is electrolysis?
 Electrolysis is the chemical separation of
an ionic compound using a direct
electrical current.

4. Key points in
Electrolysis
Ions (instead
of electrons)
conduct
electrical
current.
Ionic
compound in
molten or
aqueous state
Needs power
source
Key points in
electrolysis

5. The electrolytic cell
The cathode is
negatively charged. It
attracts positively
charged ions (cations)
The anode is positively
charged. It attracts
negatively charged
ions (anions)
The power source
provides electrical
current that gives
the electrodes
their respective
charges
These are known as
electrodes. They are usually
a metallic (or in the case of
graphite – a non-metallic)
conductor
The electrolyte is the
ionic compound in its
molten or aqueous state.
It provides mobile
electrons that allow
electrical conduction

6. How electrolysis works
Anions move towards the
positive electrode
(anode).
In doing so, they lose
electrons to become a
neutral element.
nXn- - ne- -> Xn
The cations move
towards this negatively
charged electrode
(cathode).
In doing so, they gain
electrons to become
an electrically neutral
element.
Yn+ + ne- -> Y
The electrons from
the anions then
move along the
circuit through the
power source to
the negative
electrode
These electrons then provide
the negative charge for the
negative electrode
(cathode)

7. Factors affecting
electrolysis
What conditions affect what products are formed
during and after electrolysis?

8. Key factors affecting products
of electrolysis
 Type of electrolyte
 The electrochemical series
 Molarity/Concentration of Solution
 Type of Electrodes

9. Type of electrolyte
A look at the role electrolytes play in determining
the products of electrolysis

10. Electrolytes can be either
Molten
 Pure
 Ionic compound
 Liquid form
Solution
 Impure
 Mixture of ionic
compounds
or

11. Comparison of molten vs
solution electrolytes
Products limited to
cation and anion
present in binary ionic
compound only
High energy
consumption due to
high melting points of
pure ionic
compounds
Multiple products
possible based on
cations/anions
present in solution
Lower energy
consumption due to
impurities
Molten
Solution

12. Electrolysis of
molten lead (II)
bromide
The only ions here are Pb2+ and
Br-
This why lead is deposited and
bromine gas is released
So at the anode,
bromide ions lose
electrons to form
bromine gas:
2Br- - 2e- -> Br2 (g)
While at the cathode,
lead ions gain
electrons to form solid
lead metal:
Pb2+ + 2e- -> Pb (s)

13. Electrolysis of
aqueous lead (II)
bromide
Water is slightly ionised:
H2O ↔ H+ + OH-
This means that in aqueous
solutions, there are two set of
cations and two sets of anions.
However, only one set gets off
at the electrodes. The ions that
do so are said to be displaced
from solution. Which ions get
displaced is discussed in later
parts of this presentation
At the cathode,
hydrogen is
displaced
(strangely
enough!):
2H+ + 2e- -> H 2 (g)
At the anode, hydroxide
ions lose electrons to form
water and oxygen gas:
2OH- - 4e- -> 2H2O (l) + O2 (g)

14. The electrochemical
series
A look at how the electrochemical series affects the
products of electrolysis

15. What is the
electrochemical
series?
This is a list of elements in order of
their ability to be reduced.
For cations, the higher the
element in the series, the less
likely it is that this will gain
electrons (that is be reduced).
For anions, the higher it is on the
series the less likely will it lose
electrons (that is be oxidized)

16. Reduction potential
Why some ions have a greater ability to be reduced
than others

17. What affects the ability to be
reduced?
Group to which
ion belongs
Ionic
radii
Whether a metal
or non-metal
Bond
energies

18. Reduction potential -
cations
An overview of the factors affecting the ability of
cations to be reduced.

19. For Group 1 elements
Group 1
elements
only need to
lose 1
electron in
order to
achieve a
stable octet
of electrons
This means
that they
would not
easily gain
back the
electrons lost
The ionic
radii is usually
very small
and
therefore
Group 1 ions
tend to bond
very strongly
with their
anion
counterparts
The ionic
bond has a
low bond
energy
because it is
very strong
and would
be
energetically
preferred
that the
metal alone.
This is why they tend to be at the
top of the electrochemical series

20. For Group 2 elements
Group 2
elements
need to lose 2
electron in
order to
achieve a
stable octet
of electrons
However, it is
more difficult
to lose the 2nd
electron than
the 1st
electron as
you will be
trying to
remove an
electron from
an already
positive ion
The ionic radii
is usually very
small but
larger than
that of Group
1 elements in
the same
period.
Therefore
Group 2 ions
tend to bond
very strongly
with their
anion
counterparts
but not as
strongly when
compared to
Group 1 ions
The bond
energy would
be lower than
that of the
metal alone
but higher
than that of
Group 1
elements
They would
be more likely
to gain
electrons
than Group 1
metals
This is why they tend to be at
the top of the electrochemical
series but under Group 1
elements generally

21. For Group 3 elements
Group 3
elements
need to lose 3
electron3 in
order to
achieve a
stable octet
of electrons
However, it is
more difficult
to lose the 3rd
electron than
the 2nd and 1st
electrons as
you will be
trying to
remove an
electron from
an already
positive ion
The ionic radii
is usually small
but larger
than that of
Group 1& 2
elements in
the same
period.
Therefore
Group 3 ions
tend to bond
strongly with
their anion
counterparts
but not as
strongly when
compared to
Group 1 &
Group 2 ions
The bond
energy would
be lower than
that of the
metal alone
but higher
than that of
Group 1 &
Group 2
elements
They will be
more likely to
gain electrons
than Group 1
or 2 metals
This is why they tend to be at
the top of the electrochemical
series but under Group 2
elements generally

22. For Transition elements
Transition
elements
need to lose
a varying
number of
electrons in
order to
achieve a
stable octet
as they are in
the 4th
period.
The ionic radii
is usually
larger than
that of other
elements in
the same
period.
Therefore
transition
element ions
tend to not
bond strongly
with their
anion
counterparts
The bond
energy would
be higher
than that of
metallic
elements
They will be
more likely to
gain
electrons
than other
metals.
This is why they tend to be at
the bottom of the
electrochemical series for
cations

23. Reduction potential -
anions
An overview of the factors affecting the ability of
anions to be reduced.

24. For polyatomic anions
The larger the
anion, the less
strongly the
electrons are held
by the nucleus
and the more
likely electrons will
be easily lost.
The ionic radii for
polyatomic anions
is usually larger
than that of other
anions. Therefore
polyatomic anion
tend to not bond
strongly with their
cation
counterparts
The bond energy
would be higher
than that of
monatomic
anions
They will be more
likely to lose
electrons than
other anions.
This is why they tend to be at
the top of the electrochemical
series for anions

25. For halide anions
The larger the
anion, the less
strongly the
electrons are held
by the nucleus
and the more
likely electrons will
be easily lost.
Ionic radii for
Group 7 anions
increases as you
go down the
group. Therefore
elements lower
down in the group
tend to not bond
strongly with their
cation
counterparts
The bond energy
would be
increasingly
higher as you go
down the group
Those elements
lower down the
group will be
more likely to lose
electrons than
other anions. That
is,in order of
oxidizing power,
Cl- > Br-> I-
This is why chloride ions (Cl-) is
higher in the series than
bromide ions (Br-) which is
higher than iodide ions (I-)

26. Displacement
How an ion’s position in the electrochemical series
affects its displacement from solution

27. For cations,
The lower the ion is in the electrochemical
series, the more likely it will want to gain
electrons (that is be reduced)
The cathode
supplies electrons to
cations in solution
Ions lower in the series would therefore
be more strongly attracted to the
cathode than other cations in solution.
CATIONS LOWER
IN THE SERIES WILL
BE DISPLACED
MORE READILY
THAN OTHER
CATIONS AT THE
CATHODE

28. For cations,
The lower the anion is in the
electrochemical series, the more likely it will
want to lose electrons (that is be oxidised)
The anode accepts
electrons from
anions in solution
Ions lower in the series would therefore
be more strongly attracted to the
anode than other anions in solution.
ANIONS LOWER IN
THE SERIES WILL BE
DISPLACED MORE
READILY THAN
OTHER ANIONS AT
THE ANODE

29. Electrolysis of
copper (II)
sulphate solution
At the cathode, pink copper
metal is deposited.
At the anode, oxygen gas is
produced.
In solution, the cations are Cu2+
and H+.
Since Cu2+ is lower in the series
than H+, it would preferentially
be displaced at the cathode
In solution, the anions are
So42- and OH-. Since OH- is
lower in the electrochemical
series than SO42-, it will be
preferentially displaced
At the cathode, Cu2+ ions then
gain 2 electrons to form solid
copper:
Cu2+ + 2e- -> Cu(s)
At the anode, hydroxide
ions lose electrons to form
water and oxygen gas:
2OH- - 4e- -> 2H2O (l) + O2 (g)

30. Solution concentration
How a solution’s molarity can affect the products of
electrolysis

31. Concentration defined
In chemistry,
concentration
deals with the
amount of
solute in a
given volume
of solvent
 Higher concentration means
that the given solute: solvent
ratio is high, that is, there is a
high level of solute to a given
volume of solvent
 Low concentration means that
the given solute: solvent ratio is
low, that is, there is a low level
of solute to a given volume of
solvent

32. What happens during
electrolysis of concentrated
solutions?
 Although the electrochemical series can
still predict which ions will be displaced, in
concentrated solution, sheer numbers
means that other ions slightly higher in the
series can be preferentially displaced
 However, if the ions are very far apart in
the electrochemical series, then the lower
ion will be discharged preferentially

33. Electrolysis of
brine
Brine is concentrated sodium
chloride solution.
At the end of electrolysis,
hydrogen is produced at the
cathode while chlorine is
produced at the anode.
The cations in solution are H+
and Na+. Although the
concentration of Na+ ions is
high, Na+ is very high on the
electrochemical series as
compared to H+. This great
difference between the
reduction potentials of the two
cations means that it requires a
great deal less energy to
displace H+ ions compared to
Na+ ions. This is why H+ ions are
preferentially displaced at the
cathode
The anions in solution are
OH- and Cl-. Since the
concentration of Cl- ions is
high and there is Cl- is not
much higher on the
electrochemical series as
compared to OH-, it
requires a almost the same
amount of energy to
displace Cl- ions compared
to OH- ions. This is why Cl-
ions are preferentially
displaced at the anode
Reduction half-equation at cathode:
2H+ + 2e- -> H2(g)
Oxidation half-equation at anode:
2Cl- - 2e- -> Cl2(g)

34. Types of electrodes
How inert and active electrodes affect the products
of electrolysis

35. Types of electrodes
Inert
Active
Inert electrodes do not actually
participate in electrolysis but just
provide electrical current
Active electrodes actually participate
in electrolysis while providing
electrical current

36. Example of inert electrodes
Graphite Platinum Mercury

37. How inert electrodes work
The word “inert”
means
“unreactive”
This means that
inert electrodes do
not actively
participate in the
reaction
They just behave
as electrodes
should behave –
accepting and
providing electrons
to the ions in the
electrolyte

38. Active electrodes
 Usually made of the metal that
corresponds to the metallic ion in the
electrolyte
 Actively donates electrons and ions to the
electrolytic circuit

39. How active electrodes work
The cathode is
negatively charged in
order to attract
cations.
With an inert cathode,
the electrons are given
to the cations,
reducing the cation to
the corresponding
metal.
With active electrodes, it
really depends on how
close the ions in solution
are to the metal in the
electrode in the
electrochemical series
If the metal of the electrode is
very close or the same that of
the ions in solution, it is easier to
lose or gain electrons from the
electrode itself compared to
waiting for electron loss and/or
gain from ions in solution.
It is really like getting an ice-cream cone
from across the street versus one right on
the corner from where you are.
It is easier to use the store closest to you,
just like it is easier to use the region (in this
case the electrode) closest to the circuit.

40. Let’s compare electrolysis of
Copper (II) sulphate
using copper electrodes
Copper (II) sulphate solution
using graphite electrode

41. Electrolysis using
inert electrodes
Copper(II)sulphatesolutionusing
graphiteelectrode
Once the power supply is
turned on, the Cu2+ ions
gravitate towards the
cathode as Cu2+ ions are
lower in the
electrochemical series
than H+ ions.
The OH- ions go towards the
anode as OH- is much lower in
the electrochemical series than
SO4
2-.
At the cathode, the Cu2+ ions gain
2e- to form solid Cu metal:
Cu2+
(aq) + 2e- -> Cu(s)
At the anode, the OH- ions lose 4e-
to form oxygen gas and water:
2OH-
(aq) - 4e- -> 2H2O(l) + O2(g)

42. Electrolysis using
active electrodes
Copper(II)sulphatesolutionusing
copperelectrode
Once the power supply is turned
on, the Cu2+ ions gravitate towards
the cathode as Cu2+ ions are lower
in the electrochemical series than
H+ ions.
The OH- ions go towards the
anode as OH- is much lower in
the electrochemical series than
SO4
2-.
At the cathode, the Cu2+ ions gain
2e- to form solid Cu metal:
Cu2+
(aq) + 2e- -> Cu(s)
At the anode, the OH- ions lose 4e-
to form oxygen gas and water:
2OH-
(aq) - 4e- -> 2H2O(l) + O2(g)