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Stephen Carridice

ELECTROLYSIS AND ITS APPLICATIONS

ELECTRICAL CONDUCTORS
Electrical conductors allows a current to flow
through them
...

 Electrical conductors can be divided into two
groups:
 Metals and graphite
 Electrolytes- compounds that conduct an
electric current and are decomposed by it
...
Mobile
electrons are present in the solid metal and
graphite
...


ELECTROLYTES: IONS ARE MOVING


Conductivity in electrolytes is due to the
presence of mobile ions
...


Electrodes
Positive ions
Negative ions

-Negative

+Positive

ELECTROLYTES: IONS ARE MOVING
Ionic compounds do not conduct electricity in
the solid state
...
In the solid state, the ions are
not free to move
...



ELECTROLYTES: IONS ARE MOVING
Electrolytes can be strong or weak, based on
how completely they ionized
...
Ex
...
Ex weak acids ( acetic acid)
and weak alkalis (aqueous ammonia)


ELECTROLYSIS


When electrolytes conduct electricity they are
decomposed by the current
...


ELECTROLYSIS

An electrolytic Cell

ELECTROLYSIS


Electrodes are conducting rods often made of
graphite(carbon) or platinum
...
However, some are active and
these do take part in these chemical changes
...


ELECTROLYSIS
Electrodes
 The positive electrode, or anode, is connected
to the positive terminal of the power supply or
battery
...


ELECTROLYSIS


The movement of ions in the electrolysis of
molten magnesium chloride
...

However, when an aqueous solution is
electrolyzed, hydrogen ions and hydroxide ions
from partial ionization of water are also
present, in addition to the ions from the
electrolyte
...
When this
happens, one type of ion is discharges in
preference to the other
...



ELECTROLYSIS
The ion preferentially discharge is determined
mainly by its position in the electrochemical series
...

 When more than one anion is present at the
anode, the one which is pr
 deferentially discharged is the one higher in the
reactive series
...

 Cations
Anions














K+
Na+
Mg2+
Al3+
Zn2+
Fe2+
Pb2+
Cu2+
Ag++
Au+

OHBrClSO42F-

ELECTROLYSIS
Using Inert Electrodes:
 Electrolysis of dilute sulphuric acid
 Electrolysis of dilute sodium hydroxide
 Electrolysis of Aqueous Sodium Chloride
 Electrolysis of Aqueous Copper(II) Sulphate
 Using copper Electrodes:
 Electrolysis of Aqueous Copper(II) Sulphate


ELECTROLYSIS


Electrolysis of dilute sulphuric acid
O2

H2

+

Water + dilute
sulphuric acid

Platinum
electrode



OH-

H+

SO42-

H+

0H-

H+

Platinum
electrode

Ox: Anode: 4OH-(aq) 2H2O (l) +O2(g)+4e- Red: Cathode: 2H+ (aq) +2e-H2(g)

ELECTROLYSIS
The ions in solution are:
From water
H2O  H+ + OH-

From Sulphuric Acid
H2SO42-  2H+ + SO42The H+ ions move towards the cathode during electrolysis
where they are discharge as Hydrogen gas (H2)
...


ELECTROLYSIS


Electrolysis of dilute sodium hydroxide
O2

H2

+

Water + dilute
sodium hydroxide

Platinum
electrode
(anode)

OH-

H+

OH-

Na+

0H-

H+

Platinum
electrode
(cathode)

Ox: Anode: 4OH-(aq) 2H2O (l) +O2(g)+4e- Red: Cathode: 2H+ (aq) +2e-H2(g)

ELECTROLYSIS
The ions in solution are:
From water
H2O  H+ + OH-

From Sodium Hydroxide
NaOH  Na+ + OHThe H+ ions and Na+ move towards the cathode during
electrolysis where the Hydrogen is preferentially
discharge as (H2) base on the electrochemical series
...


ELECTROLYSIS


Electrolysis of aqueous sodium chloride(diluted)
O2

H2

+

dilute sodium
chloride
OH-

Platinum
electrode
(anode)

Cl0H-

H+
Na+
H+

Platinum
electrode
(cathode)

Ox: Anode: 4OH-(aq) 2H2O (l) +O2(g)+4e- Red: Cathode: 2H+ (aq) +2e-H2(g)

ELECTROLYSIS
The ions in solution are:
From water
H2O  H+ + OH-

From Sodium Chloride
NaOH  Na+ + ClThe H+ ions and Na+ move towards the cathode where the
Hydrogen is preferentially discharge as (H2) base on the
electrochemical series
...
Here very little chlorine is
evolved under these conditions
...
)
Cl2

H2

+

Concentrated
sodium Chloride
OH-

Platinum
electrode
(anode)

Cl0H-

H+
Na+
H+

Platinum
electrode
(cathode)

Ox: Anode: 4OH-(aq)  2H2O (l) +O2(g) +4e- Red: Cathode: 2H+ (aq) +2e- H2(g)

ELECTROLYSIS
What happens at the anode is determined by
the concentration of the sodium chloride
solutions
...
While very little
oxygen is obtained, hence, remaining
electrolyte is NaOH:
 2Cl- (aq) -2eCl2(g) ( some books)
 Or 2Cl- (aq)
Cl2(g) + 2e

ELECTROLYSIS


Electrolysis of aqueous Copper Sulphate(dilute)
O2

+

Aqueous Copper
Sulphate
OH-

Platinum
electrode
(anode)

SO42 -

0H-

H+

Layer of
Copper

Cu2+

H+

OX: Anode: 4OH-(aq) 2H2O (l) +O2(g) +4e-

Platinum
electrode
(cathode)
Red: Cathode: Cu2+ (aq) +2e- Cuo(s)

ELECTROLYSIS
The ions in solution are:
From water
H2O  H+ + OHFrom Copper sulphate
CuSO4  Cu2+ + SO42-

The Cu2+ ions move towards the cathode where it is
preferentially discharge than the (H2) As the copper is
deposited at the cathode, the blue color of the
solution gradually fades
...
The concentration of the
electrolyte with respect to copper decreases during
electrolysis ( i
...
the blue color fades); the electrolyte
becomes more acidic

ELECTROLYSIS


Electrolysis of aqueous Copper Sulphate(dilute)
O2

+

Aqueous Copper
Sulphate

Copper leaves the
electrode
Copper
electrode
(anode)
OX: Anode: Cuo(s) Cu2+ +2e-

OHSO42 -

0H-

H+

Layer of new
Copper

Cu2+

H+

Copper
electrode
(cathode)
Red: Cathode: Cu2+ (aq) +2e-  Cuo(s)

ELECTROLYSIS
The ions in solution are:
From water
H2O  H+ + OHFrom Copper sulphate
CuSO4  Cu2+ + SO42-

The reaction t the cathode is:
Cu2+ (aq) +2e-Cu0(s)
The copper atoms are deposited at the cathode
The reaction at the anode is:
CU (s) Cu2+ (aq) + 2e-

ELECTROLYSIS
The copper atoms leave the anode and enter the
electrolytes copper(II) ions, Cu2+
...

The concentration of the electrolyte is
unchanged, i
...
, the blue color remains the
same
...


ELECTROLYSIS


Electrolysis of aqueous molten lead bromide
-

+

Pb forms at
the cathode
and collects
beneath it

Bromine gas
bubbles from
anodes
Molten
lead(II)
bromide

Platinum
electrode
(cathode)

Heat
Ox: Anode: 2Br-(l)  Br2(g) +2e-

Platinum
electrode
(anode)
Red: Cathode: Pb2+ (l) +2e- Pbo(s)

ELECTROLYSIS-MOLES APPLIED
Michael Faraday the British scientist did work
on electrolysis
...


MαQ
 Where Q – quantity of electricity in coulombs
and m= mass of substance


ELECTROLYSIS-MOLES APPLIED
1Coulomb=1ampereX 1 second
Q= I * t
Where Q= quantity of electricity in coulombs
I= current strength in amperes
t= time in seconds
 1 mole of electron has a charge of 96,500C

ELECTROLYSIS-MOLES APPLIED
When 1 mole of a singly charge ion is discharged
at an electrode the following happens
...

Cation: M2+ + 2e-  M or
Anion: X X2- + 2eThis process requires a passage of 2 * 96500C( or
addition or removal of 2 mole of electrons)

ELECTROLYSIS-MOLES APPLIED
Similarly
Cation: Pb2+ + 2e-  Pb
Anion: Br2  Br2- + 2e-



Lets look at a work example

ELECTROLYSIS-MOLES APPLIED
What mass of magnesium is deposited at the
cathode by the passage of 2amps through
molten magnesium chloride for 30minutes?
Quantity of electricity passed:
Q=IT = 2*30*60 =3600C
From eq: Mg2+ + 2e-  Mg
2 moles of electrons are required for the
formation of 1 mole of Mg

ELECTROLYSIS-MOLES APPLIED
From eq: Mg2+ + 2e-  Mg
2 moles of electrons are required for the formation of 1 mole of
Mg, Ar=24g

1 mol of Mg has a Ar of 24g,
so if 1 mol of e = 96,500C,
then, 2 mol of e = 2* 96500C = 193,000C






If, 193,000C=24g mg
Then 3600C = 24*3600/193000= 0
...
32x104 C? What
mass of chlorine is liberated by this charge? (Ar
Cl=35
...
32x104 C
From eq: 2Cl-  Cl2+ 2e2 moles of electrons are required for the
formation of 1 mole of Cl2 molecules
...
32x104C = 4
...
224mol of Cl2
Moles of Cl2 liberated with a charge of 4
...
224 mol

ELECTROLYSIS-MOLES APPLIED
The Mass of Chlorine?
 Since it is the chlorine molecules
Cl2=35
...
224 mol Cl2= 15
...

 Answers:
 Moles Of Al atom=0
...
058g Al


ELECTROLYSIS-MOLES APPLIED
Try same question with molten NaCl, with I=8 A
t=4hrs Ar, Na=23 Ar, Cl =35
...
2mol Na
 The Mass of Na atoms = 27
...
60mol Cl2
 The Mass of Cl2 Atoms = 42
...
0 Kg of copper
in 2Hr? (Ar Cu=63

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