Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 259
Highlight:
The potential of reference electrodes is determined as referred to the standard
hydrogen electrode.
Q. 15. Define the electrochemical series?
Ans. When the standard reduction potentials of the electrodes are arranged in an
increasing order, the series so obtained is known as electrochemical series.
Q. 16. What is Nernst equation?
Ans. Nernst proposed the fundamental electrode relation between the formal (or
observed) potential and the standard electrode potential for the redox reaction:
2.303 RT [Reduced form]
∈ Red
= ∈° Red
– log
nF
[Oxidised form]
where, R ⇒ molar gas constant.
T ⇒ absolute temperature.
n ⇒ number of electrons involved in the redox reaction.
∈° = standard reduction electrode potential.
∈ Red
= observed reduction electrode potential.
F = Faraday (96500 Coulombs)
At 25°C the value of the term 2.303 RT/F comes out as equal to 0.059.
So at 25°C
∈ = ∈° – 0. 059 [Reduced form]
log
n [Oxidised form]
Highlight:
The concentration of metal is taken as unity.
Q. 17. What is lead storage battery?
Ans. It consists of six voltaic cells connected in series.
Reactions:
Anode: Pb + SO
2–
4
⎯⎯→ PbSO 4
+ 2e –
Cathode: PbO 2
+ SO
2–
4
+ 4H + + 2e – ⎯⎯→ PbSO 4
+ 2H 2
O
Net reaction: Pb + PbO 2
+ 4H + + 2SO 4
2–
⎯⎯→ 2PbSO 4
+ 2H 2
O.
Reaction during recharging the battery
PbSO 4
(s) + 2H 2
O(l) ⇔ Pb(s) + PbO 2
(s) + 4H + + 2SO
2–
4
Q. 18. What is the reaction in nickel-cadmium battery?
Ans. Anode:
Cd(s) + 2OH – (aq) ⎯⎯→ Cd(OH) 2
(s) + 2e–
Cathode:
NiO 2
+ 2H 2
O + 2e – ⎯⎯→ Ni(OH) 2
+ 2OH – (aq)