Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 231
sending the ions, the metal rod accumulates electrons and as a result, metal rod becomes
negatively charged. On the contrary, the solution has a tendency to send M +n ions to the metal
rod due to “osmotic pressure”. When “solution pressure” equalises “osmotic pressure” there is
no net effect. But when one exceeds the other the metals
get either positively or negatively charged. As for
example, in the case of Zn/Zn +2 , the solution pressure
exceeds osmotic pressure, Zn rod sends Zn +2 ions into
the solution and gets negatively charged and attracts
positive charges towards the rod and, as a result, an
electrical double layer is set up and a potential is
established between the metal rod (Zn) and its ions
(Zn +2 ). And so is the case with Cu/Cu +2 , but in this case,
osmotic pressure exceeds solution pressure and Cu rod
gets positively charged. Here also a electrical double
+
+
+
+
Zn
Cu
– – +
– – +
– + +
– – +
– + +
– – +
– + +
ZnSO 4
CuSO 4
layer is set up and electrode potential is established. This established potential is called
electrode potential.
The electrode potential is also established in the case of every electrode where electrical
double layer is set up by exchange of electrons. For example, the system
(Pt) Fe +2 (Pt) Fe +3 + e
sets up an electrical double layer where an inert metal platinum is dipped into a mixture
containing Fe +2 and Fe +3 and an electrode potential is established.
So, an electric cell is composed of two electrodes. To consider the electrodes
separately, we call each of them as single electrode. And each electrode has a potential
which is known as single electrode potential (∈). E.M.F. of the cell is algebraic sum of the
two electrode potentials.
E = ∈ 1
+ ∈ 2
Single Electrode Potential (∈)
The electrode potential for an electrode reaction:
M +n + ne
is given by Nernst equation ∈ = ∈° + RT ln a
nF
metal.
n ⇒ Valency of metal
R ⇒ Gas constant
T ⇒ Absolute temperature
F ⇒ Faraday constant
a ⇒ Activity of M +n .
At 25°C the equation comes to :
M (metal)
M n
2. 303 × 8.
341 × 298
∈ = ∈° + log
n × 96500
10
a
+ where ∈° is a constant dependent upon the
M + n
Fig. 10.2 Electrode potential.
–
–
–
= ∈° + 0. 059
n
log 10
a
M + n