Question

(a) What is a fuel cell? What is its main advantage?

(b) What are the reactions occurring at the cathode and anode of a Leclanche cell?

(c) In the button cell widely used for watches and other devices, the following reaction takes place-

Zn() +Ag2O() + H2O(l) → Zn2+(aq) + 2Ag(*) + 2OH–(aq)

Give the cell representation and determine the value of Kc for the above reaction using the following data-

Ag2O() + H2O(l) + 2e– → 2Ag() + 2OH−(aq) (E0 = 0.344V)

Zn2+(aq) + 2e– → Zn(*) (E0 = –0.76V)

Answer 1

(a) Galvanic cells, that are designed to convert the energy of combustion of fuels like H2, CH4, CH3OH etc. directly into electrical energy are called fuel cells. They produce electricity with an efficiency of about 70% and are pollution free.

(b) Reactions taking place in Leclanche cell Anode : Zn(*) → Zn2+ (aq) + 2e–

Cathode : 2MnO2(*) + 2NH+4(aq) + 2e– ⎯→ 2MnO(OH) + 2NH3

(c) Zn()Zn2+(aq) | | Ag2O() | Ag(*) | OH–(aq)

E0cell = E0(Ag2O/Ag) –E0Zn2+/Zn

E0(Ag2O/Ag) = 0.344V – (–0.76V) = 1.104V

Zn() + Ag2O() + H2O(l) → Zn2 +(aq)+2Ag(*) + 2OH–(aq)