You will learn in this chapter about:
* Electrolytes and non-electrolytes.
* Electrolysis of molten NaCl.
* Electrolysis of water.
* Faraday's laws of electrolysis.
* Electrochemical equivalent.
* Uses of electrolysis.
* Electrochemical cells.
Chemical energy is associated with chemical reactions
and the electrical energy is obtained from electricity.
Thus chemical and electrical energies are the two forms
of energy which are inter convertible.
The branch of chemistry that deals with the relationship
between electricity and chemical reactions i.e. with the
conversion of electrical energy into chemical energy and
chemical energy into electrical energy is defined as
8.1 ELECTROLYTES AND NON-ELECTROLYTES
The chemical compound which conducts electricity in
molten condition or through its aqueous solution with
chemical change is called an electrolyte.
All acids, bases and salts are electrolytes (in aqueous
solutions or fused state); Such as Hydrochloric acid
(HCl), Sulphuric acid (H2SO4),
Sodium hydroxide (NaOH), Sodium chloride (NaCl), Copper
sulphate (CuSO4) etc, are electrolytes
You cannot electrolyse something unless it conducts
electricity. But all the conductors are not electrolytes
you can pass electric current through a metal for years
but it will not decompose. Metals are good conductors
but are not electrolytes. Metals are used as electrodes
through which electric current enters and leaves the
All electrolytes are ionic compounds or polar compounds
like acids and bases. Salts are ionic compounds and are
solids. These solids do not conduct "electricity because
in the solid state the ions are very tightly packed or
held together showing no movement of the ions. They
cannot move. However an ionic solid is melted or
dissolved in water, its ions become free to move. Thus
an ionic compound in molten states or in aqueous
solution, conduct electricity. This conduction is due to
the free movement of ions.
Non - Electrolytes:
The chemical compounds which do not conduct electricity
in, molten or in aqueous solutions are called
non-electrolytes; such as Sugar, Petrol, Benzene etc.
8.2 ELECTROLYSIS OR ELECTROLYTIC CONDUCTION
A chemical process or reaction which does not have the
capability to take place spontaneously can be made to
take place by passing electric current. If appropriate
quantity of electric current is passed under suitable
condition through an electrolytic solution, the
reactions occurs non-spontaneously i.e. under stress.
The cell used for the reaction to occur by passing
electric current is called electrolytic cell. Thus
electrolytic cell is a device which is used to convert
electrical energy into chemical energy (a
non-spontaneous chemical reaction takes place by the
loss and gain of electrons) for example a device for
electroplating or electrolysis of water in the presence
of an acid. The process is called electrolysis or
Electrolysis may be defined as a process in which
movements of the ions take place towards their
respective electrodes to undergo chemical changes under
the influence of an applied electric field.
Electrolysis of Molten Sodium Chloride:
Sodium chloride, (Salt) does not conduct electricity in
the solid state. To make it conduct electricity, either
fuse or melt the salt or dissolve it in water. So there
are two ways of doing electrolysis of sodium chloride.
We are doing electrolysis of molten sodium chloride.
Sodium chloride melts at 800°C. It is easy to predict
the result of electrolysing a molten electrolyte. The
salt just splits into two parts, sodium (metal) positive
ion (Na+) and chloride (non-metal) negative
ion (Cl-). We can write the ionization
NaCl(s) fused Na+ (l) + Cl-
In solid sodium chloride, the ions are held together
tightly in a regular lattice arrangements. In solid the
ions cannot move, but when sodium chloride is melted,
the ions are freed from their lattice and they can move
freely to conduct electricity.
Some fused NaCl is taken in an electrolytic cell i.e.;
in a glass vessel. Two platinum rods (electrodes) are
dipped into the fused salt (NaCl). The two electrodes
are connected to a source of electricity i.e. to a
battery outside the cell by wires. The electrode
connected to the negative terminal of the battery is
called cathode and the electrode connected to the
positive terminal of the battery is called anode.
When an electrical potential is passed through the
molten sodium chloride salt, electrolysis starts. The
positive ions i.e. cations (Na+) are
attracted towards cathode and the negative ions i.e.
anions (Cl-) are attracted towards anode. At
the two electrodes chemical reactions take place.
Sodium ions (Na+) which are positively
charged. Move towards cathode and gain electrons to get
neutralized. Thus Na+ ions are discharged at
cathode and form neutral molten sodium metal.
We can represent the happening at cathode as:
Sodium ion + electron ------à
Na+ (l) +e-
Chloride ions (Cl-) which are negatively
charged move towards anode; Cl- ions have one
electron in excess. The anode has a shortage of
electrons and is positively charged. When Cl-
ions arrive at anode, the anode attracts their electrons
and CI ions lose their extra electrons at anode, forming
neutral chlorine (Cl-) atoms but we know Cl-
atoms do not exist in the free state. They combine to
form Cl2 molecules. So Cl- ions
are discharged at anode to liberate Cl2 gas
by the loss of electrons at anode.
Chloride ion ------à
Chlorine atom + e-
Or Cl-(l) ------à
Cl (g) +e
Then Cl (g) + Cl (g)
Over all it amounts to
Cl2 (g) +2e-
The overall reaction can be described as:
2(Na+ (l) + e
Na (l)) Cathode
Cl2 (g) +2e- Anode
On adding the two reactions, the loss and gain of
electrons cancel and we get final result.
2Na+ (l) +
Cl2 (g) +
2Na+ (l) + 2C1- (l)
2Na (l) + Cl2 (g)
The electrolysis of molten sodium chloride shows that we
get sodium metal at cathode and Cl2 gas is
liberated at anode. The following fig 8.1 describes the
electrolysis of molten NaCl.
2. Electrolysis of Water:
Pure water on its own hardly conducts electricity
because it ionizes feebly into ions. But when few drops
of acid or base or very small quantity of a salt is
added in water, it makes water to conduct electricity
and then water can be electrolysed to produce hydrogen
gas at cathode and oxygen gas at anode. Consider an
electrolytic cell containing acidulated water. Two
electrodes are dipped in the acidulated water. Water in
the presence of few drops of acid ionizes as:
2H2O (l) acid H3O+
(aq) + OH-(aq)
Hydronium ion hydroxide ion
When electricity is passed from the battery, the
positive ions (H30+) move towards cathode.
They gain electrons and get neutralized to liberate H2
2H3O+ (aq) + 2e- -----
> (g) + 2H2O (1)
Hydroxide ion (OH-) moves towards anode. OH-
ions lose electrons at anode and get neutralized to
liberate O2 gases.
4OH- (aq) ----- > O2 (g) + 2H2O(1)
The over all reaction should be balanced according to
gain and loss of ' ' electrons at the two electrodes
so the cathode reaction is multiplied by 2 and then
added in the anode reaction.
2(2H3O+ + 2e- ----- > H2 (g) + 2H2O(1))
Or 4H3O+ 4e- ----- > 2H2 (g)
40H- ----- > 02(g) + 2H20(1)
+ 4e-Anode reaction
4H3O+ + 4OH- ----- > 2H2
(g) + O2 (g) + 6H2O (1)
4H3O+ and 40H- together
will be equivalent 8H2O, so
8H2O ----- > 2H2 (g) + O2
(g) + 6H2O
Or 8H2O - 6H2O ----- > 2H2
(g) + O2 (g)
Or 2H2O ----- > 2H2 (g) + O2
For simplicity of the electrolysis of water, we write
the equation as:
2H2O (1) Electric current>
2H2 (g) + O2 (g)
Cathode product anode product
It is observed that on electrolysis of water, we get two
volumes of hydrogen gas for each volume of oxygen gas.
Humphry Davy, who first did the electrolysis of water,
confirmed the formula of water is H2O (2:1
ratio of Hydrogen and Oxygen).