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Construction: It is consist of PVC container. Anode is pure lead and the cathode is lead plate impregnated with PbO2. The electrolyte used is 20% H2SO4 of 1.75 sp.gr. When it is connected in the circuit the operation start.
Working:At L.H.E oxidation occures, $$ Pb(s) + { SO }_{ 4 }^{ -2 } \longrightarrow Pb{ SO }_{ 4 }(s) + 2{ e }^{ - } $$ At R.H.E reduction occurs, $$ Pb{ O }_{ 2 }(s) + 4{ H }^{ + } + { SO }_{ 4 }^{ -2 } + 2{ e }^{ - } \longrightarrow Pb{ SO }_{ 4 }(s) + 2{ H }_{ 2 }O $$ The Net Cell Reaction is combination of above two reactions, $$ \boxed { Pb(s) + Pb{ O }_{ 2 }(s) + { H }_{ 2 }{ SO }_{ 4 } \longrightarrow 2Pb{ SO }_{ 4 }(s) + 2{ H }_{ 2 }O } $$ This reaction is while discharging, so for charging it will be reverse.
On applying the law of mass action to above cell reaction, $$ K = \frac { { a }_{ Pb{ SO }_{ 4 }(s) }^{ 2 } \times { a }_{ { H }_{ 2 }O }^{ 2 } }{ { a }_{ Pb(s) } \times { a }_{ Pb{ O }_{ 2 }(s) } \times { a }_{ { H }_{ 2 }{ SO }_{ 4 } }^{ 2 } } = \frac { 1 }{ { a }_{ { H }_{ 2 }{ SO }_{ 4 } }^{ 2 } } $$ $$ As \ { a }_{ Pb{ SO }_{ 4 }(s) } = 1, { a }_{ Pb(s) } = 1, { a }_{ Pb{ O }_{ 2 }(s) } = 1 \ and \ { a }_{ { H }_{ 2 }O } = 1, $$ On applying the Nernst expression for emf of the cell, $$ { E }_{ cell } = { E }_{ cell }^{ 0 } - \frac { RT }{ nF } \ln { K } $$ $$ { E }_{ cell } = { E }_{ cell }^{ 0 } - \frac { RT }{ 2F } \ln { \left( \frac { 1 }{ { a }_{ { H }_{ 2 }{ SO }_{ 4 } }^{ 2 } } \right) } $$ $$ { E }_{ cell } = { E }_{ cell }^{ 0 } + 2\frac { RT }{ 2F } \ln { \left( { a }_{ { H }_{ 2 }{ SO }_{ 4 } } \right) } $$ $$ \boxed { { E }_{ cell } = { E }_{ cell }^{ 0 } + \frac { RT }{ F } \ln { \left( { a }_{ { H }_{ 2 }{ SO }_{ 4 } } \right) } } $$ It is clear from the above equation that the emf of the cell depends on the activity of \({ H }_{ 2 }{ SO }_{ 4 }\). The emf value for the cell is 2.0 to 2.1 volt.
Representation: $$\circleddash Pb,\ Pb{ SO }_{ 4 }(s)\ |\ \underset { 20\% }{ { H }_{ 2 }{ SO }_{ 4 } } \ |\ Pb{ SO }_{ 4 }(s),\ Pb{ O }_{ 2 }\ Pb\oplus $$
Uses:
(1) Used in automobiles, and Laboratory.
Demerits:
(1) It is costly.
(2) It is bulky,
(3) It is heavy,
(4) Care must be taken as H2SO4 is used.
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