1
ELECTROLYSIS
01.
Definitions:
i)
Electrolysis:
The decomposition of an ionic compound either in molten
state or in aqueous state into its constituents after passing
electric current through it is known as electrolysis.
ii)
Electrolyte:
An electrolyte is a compound in solution or a molten
compound which conducts electric current with the
decomposition at the electrodes.
iii)
Non- electrolyte:
A non-electrolyte is a solution or a molten compound which
cannot be decomposed by an electric current.
iv)
Strong electrolyte:
An electrolyte which ionizes completely.
v)
Weak electrolyte:
An electrolyte which ionizes partially.
vi)
Electrodes:
The electrodes are two poles of carbon or metal at which the
current (as a flow of electrons) enters and leaves an
electrolyte.
vii)
Anode:
The electrode which is attached to the positive end of the
battery.
viii)
Cathode:
The electrode which is attached to the negative end of the
battery.
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ix)
Anion:
Negatively charged ion. During electrolysis, it moves to the
anode.
x)
Cation:
Positively charged ion. During electrolysis, it moves to the
cathode.
xi)
Inert electrode:
Do not take part in any chemical reaction during
electrolysis.
Example: graphite (carbon) and platinum.
xii)
Reactive electrode:
Take part in chemical reaction during electrolysis.
Example: all electrodes except graphite and platinum.
xiii)
Oxidation:
Loss of electron by a substance.
During electrolysis oxidation always takes place at anode.
xiv)
Reduction:
Gain of electron by a substance.
During electrolysis reduction always takes place at cathode.
xv)
Discharge:
The process of gaining or losing electrons at the electrodes
is known as discharge. When ions are discharged at the
electrodes, they form atoms or molecules.
xvi)
Binary compound:
A compound containing only two elements. A binary
compound often contains a metal cation and a non-metal
anion.
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02.
When electricity is passed through an electrolyte:
When electricity is passed through an electrolyte, chemical
reactions take place at the electrodes and the electrolyte is
decomposed. The reactions taking place at the electrodes are
called electrolytic reactions.
03.
Flow of electrons:
Electrons the enter external circuit through cathode and leaves
the external circuit through anode.
04.
Existence of mobile ions:
Electrolytes only conduct electricity because they contain mobile
ions. Molten or aqueous ionic compounds can acts as an electrolyte,
so they must contain mobile ions. In the ionic compounds, oppositely
charged ions are held fixed in a lattice in solid state. Ions become
immobile. They are unable to move freely thus unable to conduct
electricity.
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05.
Differences between electrical conductor and electrolytic
conductor:
Metals, carbon (graphite) and ionic compounds (molten or aqueous)
are conductors of electricity. Both metals and graphite are known
as electrical conductor and ionic compounds are known as
electrolytic conductor.
06.
Electrical
Electrolytic
conduction
conduction
Electricity is
Electricity is
conducted by the
conducted by the
Method of
flow of electrons
movement of positive
conduction
from one end of the
ions and negative
conductor to the
ions across the
other end.
electrolyte.
Metals and graphite
The electrolytes
Effect of
remain unchanged
are decomposed to
conduction
chemically when an
form new substances
electric current
when they conduct
flows through them.
electricity.
Electrolysis of water:
Pure water is a poor conductor of electricity because it consists
almost entirely of molecules and has very few ions in it. To
increase the conductivity of pure water, small amount of
concentrated sulfuric acid or an ionic compound is to be added.
The products of electrolysis of water are always two volumes of
hydrogen at the cathode and one volume of oxygen at the anode.
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07.
Electrolysis of molten lead (II) bromide [PbBr2] using inert
electrodes.
Ions present: Pb2+ and BrAt the anode:
2Br-(l) → Br2 (l) + 2eAt the cathode:
Pb2+(l) + 2e- → Pb(s)
Observations:
08.
i)
Red brown vapour around anode.
ii)
Solid deposited at the bottom of the cathode.
Selective discharge of ions:
i)
Position of the metal or group in the electrochemical series:
Less reactive ion will discharge. More reactive ion will remain
in the electrolyte.
[This factor is applicable to both anion and cation.]
ii)
Concentration:
If reactive anion has greater concentration it will discharge.
[This factor is applicable to anion only.]
iii)
Nature of the electrode:
Use of reactive electrode instead of inert electrode,
influence the choice of ion discharge. In the electrolysis of
copper (II) sulfate solution using copper electrodes, the
anode dissolves. Thus, produces more copper (II) ions in the
electrolyte.
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09.
Reactivity of anions:
Left to right reactivity decreases.
SO42-, NO3-, Cl-, Br-, I-, OH-
10.
Reactivity of metals:
Left to right reactivity decreases.
PoSoCaMAl (C) ZIL (Hy) CoMSG
11.
Electrolysis of dilute sulfuric acid [H2SO4] using inert electrodes.
Ions present: H+ , SO42- , OHAt the anode:
4OH-(aq) → 2H2O(l) + O2(g) + 4eAt the cathode:
4H+(aq) + 4e- → 2H2(g)
[Ultimately electrolysis of water.]
12.
Electrolysis of copper (II) sulfate [CuSO4] solution using inert
electrodes.
Ions present: Cu2+ , SO42- , H+ and OHAt the anode:
4OH-(aq) → 2H2O(l) + O2(g) + 4eAt the cathode:
2Cu2+ (aq) + 4e- → 2Cu(s)
Observations:
i)
Blue colour of the solution turns colourless.
ii)
Cathode gets thicker.
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13.
Electrolysis of copper (II) sulfate [CuSO4] solution using copper
electrodes.
Ions present: Cu2+ , SO42- , H+ and OHAt the anode:
Cu(s) → Cu2+ (aq) + 2eAt the cathode:
Cu2+ (aq) + 2e- → Cu(s)
Observations:
iii)
Blue colour of the solution remain unchanged.
iv)
Cathode gets thicker.
∗ Important information:
 Impure copper is purified by this method. Impure copper is
made anode and pure copper is made cathode. Copper purified
by this method is 99.999% pure. Impurities include platinum,
silver and gold. These metals are deposited at the bottom of
the cathode. These metals are separated and sold out.
Deposited impurities are collectively known as sludge.
14.
Electrolysis of dilute sodium chloride [NaCl] solution using inert
electrodes.
Ions present: Na+ , Cl- , H+ and OHAt the anode:
4OH-(aq) → 2H2O(l) + O2(g) + 4eAt the cathode:
4H+(aq) + 4e- → 2H2(g)
[Ultimately electrolysis of water.]
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15.
Electrolysis of concentrated sodium chloride [NaCl] solution
(Brine) using inert electrodes.
Ions present: Na+ , Cl- , H+ and OHAt the anode:
2Cl-(aq) →Cl2(g) + 2eAt the cathode:
2H+(aq) + 2e- → H2(g)
∗ Important information:
 Ions left behind are Na+ and OH- . Sodium hydroxide is
manufactured by this method.
16.
Electroplating:
The process in which a layer of a metal is deposited on metallic or
non-metallic electrode by electrolysis in an electrolytic cell is
known as electroplating.
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a)
b)
17.
Conditions for electroplating:
i)
Object is made cathode
ii)
Anode is the electroplating metal
iii)
Electrolyte is the electroplating metal ion solution.
Uses:
i)
In making inexpensive jewelry.
ii)
For decorative purposes.
iii)
Increases the life of the metal.
iv)
Prevents corrosion.
v)
In industries.
Copper plating:
The anode is pure copper, often called plating copper. The metal
object to be copper-plated is made the cathode and the
electrolyte is copper (II) sulfate solution.
At the anode:
Cu(s) → Cu2+ (aq) + 2eAt the cathode:
Cu2+ (aq) + 2e- → Cu(s)
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18.
Simple cells:
A simple cell is a device that converts chemical energy into
electrical energy. A simple cell consists of two electrodes (two
different metals) and an electrolyte solution (acid, alkali or salt
solution).
a) How simple electric cell work:
The working principle of simple electric cell depends upon the
principle that, whenever two dissimilar metals are immersed
inside an electrolyte solution, the more reactive metal will have
a tendency to dissolve in the electrolyte as positive metal ions,
leaving electrons behind on the metal plate. This phenomenon
makes the more reactive metal plate negatively charged. The
cell voltage depends on the positions of the metals in the
reactivity series. The further apart the metals are in the
reactivity series, the bigger the voltage of the cell.
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19.
Extraction of aluminium by the electrolysis of purified aluminium
oxide [Al2O3] / alumina.
The steel container is coated with graphite and this is used as the cathode.
The aluminium ore bauxite [Al2O3, 2H2O] is purified to yield aluminium oxide,
alumina. To the molten cryolite in the steel tank, alumina is added. Blocks
of graphite are dipped into the electrolyte solution which is used as anode.
At the anode:
6O2-(l) → 3O2(g) + 12eAt the cathode:
4Al3+(l) + 12e- → 4Al(l)
Molten aluminium runs down to the bottom of the cell where it can be
trapped off as pure liquid aluminium.
The oxygen formed at the anode reacts with carbon (graphite) to form
carbon dioxide.
C(s) + O2(g) → CO2(g)
Thus, the graphite anode is eaten away by hot oxygen and needs to be
replaced frequently.
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
Important information:

The overall reaction is
2 Al2O3(l) → 4Al(l) + 3 O2(g)

Cryolite is used to reduce the melting point of alumina. Melting point
of alumina, Al2O3 is 20500 C.

a)

Cryolite:
i)
Formula:
Na3AlF6
ii)
Systemic name: Sodium hexafluoroaluminate (III)
Properties of aluminium:
i)
It is a strong, malleable and ductile metal element.
i)
It has a low density.
ii)
It is resistant to corrosion.
iii)
It is a good conductor of heat and electricity.
iv)
It can be polished to give a highly reflective surface.
v)
It is durable.
vi)
It is odorless.
vii)
It is impermeable.
viii)
It is 100% recyclable with no deterioration in quality.
ix)
It is non-toxic.
b) The apparent lack of reactivity of aluminium:
 Aluminium forms a protective oxide layer which makes it appear to be
less reactive than it really is. When this layer is removed, the
observations are more reliable.
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c)
Uses of aluminium:
Sl.
No.
01
Use of aluminium
Aircrafts
Reason
Light, strong, corrosion
resistant
02
Other transports, such as
Light, strong, corrosion
ships superstructures,
resistant
container vehicle bodies and
metro trains
03
Overhead power cables (with
Light, strong, corrosion
a steel core to strengthen
resistant, good conductor of
them)
electricity
Light, strong, corrosion
04
Cookers, cook wares, boilers
resistant, good appearance,
good conductor of heat.
Mirrors, reflectors and heat
05
High reflectivity
resistant clothing for fire
fighting.
06
Drink cans, roofing materials
Light, strong, corrosion
resistant
07
Window frames, greenhouses
Corrosion resistant,
low density
08
Food container
Light, strong, corrosion
resistant
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d) Recycling aluminium:


Advantages:
i)
Saves a lot of energy.
ii)
Mining avoided
iii)
It never wears out
iv)
Reduces the emission of greenhouse gases
v)
Reduces the use of natural resources and chemicals
vi)
Sustainability increases jobs.
Disadvantages:
i)
Need to be separated from plastic, steel and debris.
ii)
Sorting is time consuming and costs money
iii)
High energy cost in reprocessing and transporting materials
iv)
When aluminum is continually recycled it loses it quality. So, a
product made with new aluminum will be of higher quality than
the recycled product.