Electrolytic Cell
An
electrolytic cell is an arrangement in which electricity is conducted through a
solution or a molten salt by the movement of ions. The process of chemical
decomposition of the electrolyte by the passage of electricity through its
molten or dissolved state is called electrolysis. In electrolysis we supply
electrical energy to initiate non–spontaneous processes or we can also say that
we convert electrical energy into chemical energy.
In
order to pass the current through an electrolytic conductor (aqueous solution
or fused electrolyte), two rods or plates (metallic conductors) are always
needed which are connected with the terminals of a battery. These rods or
plates are known as electrodes. The electrode through which the current enters
the electrolytic solution is called the anode (positive electrode) with the
electrode through which the current leaves the electrolytic solution is known
as cathode (negative electrode).
In
the above arrangement the battery supplies electrons to the system. The
polarities of the electrodes in this case is opposite of what it was in
Galvanic Cell. That is, anode is the positive electrode and the cathode is the
negative one. In molten NaCl there are Na+ and Cl– ions.
Na+ ions move towards the cathode and Cl– ions move towards the anode due to
the opposite polarities. Then reduction of cations takes place at the cathode
and oxidation of the anions takes place at the anode.
Anode: 2Cl–
→ Cl2 (g) + 2e– Eº = – 1.36 V
Cathode: 2Na+ + 2e– → 2Na(l) Eº = – 2.71 V
Thus,
electrolysis of molten NaCl leads to the formation of sodium metal and chlorine
gas. The above two processes have no natural tendency to occur but we have made
it possible in this case by the supply of external energy. This movement of ions gives
rise to what is known as the electrolytic conduction.
Let
us now take a situation where more than one type of cation is present. The
ability of cation to move towards the negative electrode and get reduced
depends upon the size, mass, positive charge, negative charge etc. It is therefore not possible to predict, qualitatively, the order
of reduction of cations, as one factor might enhance it while another factor
might hamper it. The only way we can predict this is by giving a
quantitative value based on the cumulative effect of all the factors
responsible for a cation ability to get reduced. This quantitative value is
called the standard reduction potential (SRP). A cation with a higher value of
SRP would get reduced in preference to a cation with a lower value of SRP.
For
anions the ability to get oxidized is given by the standard oxidation potential
which is the reverse of the standard reduction potential of a molecule to form
the anion.
For
example, consider the following half reactions:
A2+ + 2e– →A E = –2 V
B2+ + 2e– → B E =
–1.5 V
C2+ + 2e– →C E = – 1 V
Amongst
the above reactions reduction of A2+ is the most non–spontaneous process and
will require the maximum amount of energy to make it happen. Reduction of C2+
ions is the easiest. When these three ions are present discharge of C2+
ions will take place in preference to the other two ions. This phenomenon is
called “Preferential discharge”.
Electrolysis of Sodium Chloride Solution
The
solution of sodium chloride besides Na+ and Cl- ions possesses H+ and OH-ions due to ionization of water. However, the number
is small as water is a weak electrolyte. When potential difference is
established across the two electrodes, Na+ and H+ ions move towards cathode and Cl- and OH- ions move
towards anode. At cathode H+ ions are
discharged in preference to Na+ ions as the
discharge potential of H+ ions is
lower than Na+ ions. Similarly at anode,
Cl- ions are discharged in preference to OH- ions.
At cathode
|
At Anode
|
H+ + e- → H
2H→ H2
|
Cl- → Cl + e-
2Cl → Cl2
|
NaCl
Na+ +
Cl-

H2O
H+ + OH-

Thus,
Na+ and OH- ions remain in solution and the
solution when evaporated yields crystals of sodium hydroxide.
Electrolysis of molten lead bromide.
The
reactions occurring at the two electrodes may be shown as follows:
At cathode
Pb2+
(l ) + 2e– → Pb (l ) Eº =
– 0.13 V
(Reduction, primary change)
At anode:
2Br– → Br2 + 2e– Eº = – 1.09 V
Overall reaction:
PbBr2 (l
)
→ Pb (l )
+ Br2 (g)
At cathode At anode
Electrolysis of Copper Sulphate Solution using Platinum Electrodes
CuSO4
Cu2+ + SO42-

Copper
is discharged at cathode as Cu2+ ions have
lower discharge potential than H+ ions. OH- ions are discharged at anode as these have
lower discharge potential than ions. Thus, copper is deposited at cathode and
oxygen gas is evolved at anode
At cathode
|
At Anode
|
Cu2+ + 2e- → Cu
|
2OH- → H2O + O + 2e-
O +
O → O2
|