Engineering Chemistry S Datta

240 ENGINEERING CHEMISTRY
Ag | AgNO 3
(C 1
) || AgNO 3
(C 2
) | Ag
Ag AgNO 3
(C 1
) Salt bridge AgNO 3
(C 2
) Ag
(Dilute) of satd. [Concentrated]
NH 4
NO 3
or Ag | AgNO 3
(C 1
M) || AgNO 3
(C 2
M) | Ag (C 2
> C 1
)
Voltmeter
e –
NH4NO3
Salt bridge
– +
Electron
flow
Ag
electrode
Ag
electrode
– + –
Ag Ag + + e
Ag + e Ag
CMAg ions
1
+
C > C 2 1
CMAg ions
2
+
Fig. 10.5 Concentration cell.
Imagine that current passes through the external wire as shown by the arrow. We shall
get silver deposited on the right hand electrode and dissolution of silver from the left hand
electrode as if silver being transported from concentration C 2
to concentration C – 1
.
So, the cell reaction is
Half cell reaction (left) oxidation Ag – e Ag + (C 1
)
Half cell reaction (right) Ag + (C 2
) + e Ag°
Total: Ag + (C 2
) = Ag + (C 1
)
∈° R
= ∈° L
and n = 1
E = 2.303 RT C
log
nF
C
2
1
2. 303 × 8. 138 × ( 298) C
= log
n×
96500 C
2
1
So, E = 0.059
1
C2
log [Q n = 1]
C
1
Therefore, e.m.f. of such cell depends on the relative concentration and not on the absolute
values.
If C C2
1
= 10, the observed e.m.f. is 59 mV.
The stronger solution acts as positive electrode.