Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 247
When current flows from Cu-plate to Zn-plate, a layer of hydrogen bubbles is found to be
formed on the surface of the Cu-plate. The effect of accumulation of hydrogen is two-fold:
(i) Hydrogen layer increases the resistance offered to the flow of current by covering
the Cu-plate.
(ii) The layer of H 2
in contact with the acid solution is found to acquire a potential lower
than that of Zn. The Zn-plate, therefore, behaves as positive electrode and H 2
accumulated
as negative electrode, so, an additional e.m.f. is generated, which is called
polarisation e.m.f. or back e.m.f. and which tends to drive a current opposite to
the direction of the main current flow; and hence this back e.m.f. opposes the main
e.m.f. As the hydrogen bubble is generated more and more, the polarisation e.m.f.
increases and ultimately it comptetely neutralises original e.m.f. and the current
stops totally. This phenomenon is known as polarisation.
Polarisation effect is also seen during electrolysis: when an electric current is passed
through dil. H 2
SO 4
using Pt-electrodes, H 2
and O 2
are evolved.
H 2
SO 4
2H + + SO
–2
4
SO
–2
4
+ H 2
O H 2
SO 4
+ O –2
At anode: The reaction is,
2O –2 – 4e O 2
↑
At cathode: The reaction is,
2H + + 2e H 2
↑
If, after some time, the battery is removed and a galvanometer is fitted in the outer
circuit, a current is seen to pass between the two electrodes, but in opposite direction to that
during electrolysis. Hydrogen and oxygen gases covering the two Pt-electrodes become the
source of this e.m.f. (back e.m.f.). This phenomenon of back e.m.f. developed by products
of electrolysis is known as polarisation (Fig. 10.12).
e
Galvanometer
dil. H SO 2 4
Pt Pt
+ –
+ –
O 2
H 2
( a ) Polarisation during
( b) Polarised cell
electrolysis
Fig. 10.12 Development of polarisation by the products of electrolysis.
Decomposition Potential
If we start electrolysis of dil. H 2
SO 4
using Pt electrodes applying an e.m.f. of one volt, it
will soon come to an end due to back e.m.f. developed during electrolysis. So, to keep the
electrolysis going on, we are to apply an e.m.f. which can overcome the effect of back e.m.f. The
minimum voltage, which is just sufficient to overcome the back e.m.f. or polarisation
e.m.f. is called decomposition potential of the given electrolyte (Fig. 10.13).