Tamilnadu BoardEnglish MediumSTD 12ChemistryElectro Chemistry5 Marks
Question
Describe about lead storage battery construction and its uses.
✓
Answer
Lead storage battery:
1. Anode - Spongy lead
Cathode - Lead plate bearing $PbO _2$
Electrolyte $-38 \%$ by mass of $H _2 SO _4$ with density $1.2 g / ml$
2. Oxidation occurs at the anode
$Pb _{( s )} \rightarrow Pb _{( aq )}^{2+}+2 e ^{-} \text {. }$
The $Pb ^{2+}$ ions combine with $SO _4{ }^{2-}$ to form $PbSO _4$ precipitate
$Pb ^{2+}{ }_{( aq )}+ SO _4{ }^{2-} \rightarrow PbSO _{4( s )}$
3. Reduction occurs at the cathode
$PbO _{2( s )}+4 H ^{+}{ }_{( aq )}+2 e ^{-} \rightarrow+2 H _2 O _{(1)}$
The $Pb ^{2+}$ ions also combines with $SO _4{ }^{2-}$ ions to form sulphuric acid to form $PbSO _4$ Precipitate
$Pb ^{2+}{ }_{( aq )}+ SO ^{2-}{ }_{2+} \rightarrow PbSO$
4. The overall reaction is,
$(1)+(2)+(3)+(4)$
$Pb _{(5)}+ pbO _{2(s)}+4 H _{(s)}^{+}+2 SO ^{2-}{ }_{4(s)} \rightarrow 2 PbSO _{4(s)}+2 H _2 O _{(1)}$
5. The emf of a single cell is about $2 V$. Usually six such cells are combined in series to produce 12 volts.
6. The emf of the cell depends on the concentration of $H _2 SO _4$. As the cell reaction uses $SO _4{ }^{2-}$ ions, the concentration $H _2 SO _4$ decreases. When the cell potential falls to about $1.8 V$, the cell has to be recharged.
7. Recharge of the cell - During recharge process, the role of anode and cathode is reversed and $H _2 SO _4$ is regenerated. Oxidation occurs at cathode (now anode)
$\stackrel{+2}{ PbSO _4( s )}+2 H _2 O _{( l )} \rightarrow PbO _{2( s )}^{+4}+4 H _{( aq )}^{+}+ SO _{4( aq )}+2-$
Reduction occurs at anode (now cathode)
$PbSO _4( s )+2 e ^{-} \rightarrow Pb _{( s )}+ SO ^{2-}{ }_{4( sq )}$
Overall reaction
The above reaction is exactly the reverse of redox reaction which takes place while discharging.
8. Uses: Lead storage battery is used in automobiles, trains, inverters.
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