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Q.
When an acid cell is charged then :
AFMCAFMC 2005
Solution:
The discharged acid cell (lead storage battery or cell) can be recharged by passing a direct current through it in opposite direction. During recharging of cell, the electrode material are restored in their original forms. Due to formation of $H ^{+}$and $SO _{4}^{2-}$ ions the specific gravity of sulphuric acid increases and it attains the original value $\left(1.3\, g / cm ^{3}\right)$ and also its EMF (voltage) increases.
During charging
At negative electrode :
$PbSO _{4}(s)+2 e^{-} \xrightarrow[\text{current}]{\text { direct }}Pb (s)+ SO _{4}^{2-}(a q)$
At positive electrode :
$PbSO _{4}(s)+2 H _{2} O \xrightarrow[{\text{direct}}]{\text { current }}$
$PbO _{2}(s)+4 H ^{+}(a q)+ SO _{4}^{2-}(a q)+2 e^{-}$ overall charging reaction :
$\underbrace{2 PbSO _{4}(s)+2 H _{2} O } \xrightarrow{\text{Charging}}\underbrace{ Pb (s)+ PbO _{2}(s)}_{\text { electrode materials recovered }}$
$+\underbrace{4 H ^{+}(a q)+ SO _{4}^{2-}(a q)}_{ H _{2} SO _{4} \text { reformed }}$
overall reaction during charging :
$Pb (s)+ PbO _{2}(s)+4 H ^{+}(a q)+2 SO _{4}^{2-}(a q) \longrightarrow 2 PbSO _{4}(s)+2 H _{2} O$