Galvanic cell
It
is a device in which chemical energy is converted into electrical energy. In
this type of cell, energy is extracted from a spontaneous chemical process or
reaction and it is converted to electric current. In redox reaction, there is a
transfer of electrons. If we can somehow utilize this transfer of electrons
then electric current can be generated. This is exactly what we do in a
Galvanic Cell. For example, Daniell Cell is a Galvanic Cell in which Zinc and
Copper are used for the redox reaction to take place. When we dip a Zn rod in a
solution containing Cu2+ then the following reaction takes place spontaneously:
Zn(s) + Cu2+ (aq)
--------- > Zn2+ (aq)
+ Cu(s)
Oxidation
Half: Zn(s) ------- > Zn2+ (aq)
+ 2e–
Reduction Half: Cu2+(aq) + 2e– ------------ > Cu(s)
That is the natural tendency of Zn is to get oxidized in presence of Cu2+
But
if an arrangement is made as shown in the figure then the electrons can be
transferred through a wire from one phase to the other and the energy may be
converted into electrical energy.
This is an indirect redox reaction.
The cell consists of two beakers, one of which contains a solution of Cu2+ and
a copper rod, the other a Zn2+ solution and a zinc rod. A connection is made
between the two solutions by means of a “salt bridge”, a tube containing a
solution of an electrolyte, generally NH4NO3 or KCl
When the two metallic rods are
connected through an ammeter, a deflection is observed in ammeter which is evidence
that a chemical reaction is occurring. The zinc rod starts to dissolve, and
copper is deposited on the copper rod.
The solution of Zn2+ becomes more concentrated, and the solution of Cu2+
becomes more dilute. The ammeter indicates that electrons are flowing from the
Zinc rod to the copper rod. This activity is continuous as long as the
electrical connection and the salt bridge are maintained. Now let us analyze
what happens in each beaker more carefully. We note that electrons flow from
the Zinc rod through the external circuit, and that Zinc ions are produced as
the Zinc rod dissolves. We can summarize these observations by writing,
Zn →
Zn2+ + 2e–
Also, we observe that electrons flow
to the copper rod as cupric ions leave the solution and metallic copper is
deposited. We can represent these occurrences by
Cu2+ (aq) + 2e– → Cu
Hence from the above observations it
is concluded that zinc has higher tendency to get oxidized than the copper.
Functions of the Salt Bridge
(a) A salt bridge completes the inner
cell circuit.
(b) It keeps the solutions in two
half-cells electrically neutral. In anodic half cell, positive ions pass into
the solution and there shall be accumulation of extra positive charge in the
solution around the anode which will prevent the flow of electrons from anode.
This does not happen because negative ions are provided by salt bridge.
Similarly, in cathodic half-cell negative ions will accumulate around cathode
due to deposition of positive ions by reduction. To neutralize these negative
ions, sufficient number of positive ions is provided by salt bridge. Thus, salt
bridge maintains electrical neutrality.
(c) It prevents liquid-liquid
junction-potential, i.e., the potential difference which arises between two
solutions when in contact with each other.
(d) It prevents transference or
diffusion of the solutions from one half-cell to the other.
Representation of An
electrochemical cell
A Galvanic cell is represented by using a
shorthand notation in which anode is represented on the left and cathode on the
right of salt bridge which is represented by double vertical lines “||”. The electrodes are
represented by writing the two phases separated by a single vertical line (|).
For example the Daniell Cell is represented by:
Zn(s) /Zn2+
(aq) || Cu2+ (aq) / Cu(s)
ANODE
SALT BRIDGE CATHODE