When a lead storage battery is charged — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Write the Electrochemical Reactions During Discharging A lead storage battery is a rechargeable secondary cell. When the battery is discharging (acting as a galvanic cell to provide electrical energy), the following half-cell reactions occur: * **At the Anode (Oxidation):** $$\ce{Pb(s) + SO4^{2-}(aq) -> PbSO4(s) + 2e^-}$$ * **At the Cathode (Reduction):** $$\ce{PbO2(s) + SO4^{2-}(aq) + 4H^+(aq) + 2e^- -> PbSO4(s) + 2H2O(l)}$$ Combining these half-reactions gives the net discharging reaction: $$\ce{Pb(s) + PbO2(s) + 2H2SO4(aq) -> 2PbSO4(s) + 2H2O(l)}$$ During discharge, both electrodes become coated with insoluble lead sulfate ($\ce{PbSO4}$), and sulfuric acid ($\ce{H2SO4}$) is consumed, which decreases the density and concentration of the electrolyte. Step 2 - Write the Electrochemical Reactions During Charging When the battery is connected to an external direct current (DC) source for charging, it behaves as an electrolytic cell. The non-spontaneous reverse reactions are forced to occur at the electrodes: * **At the Cathode (Reduction):** Lead ions in solid lead sulfate gain electrons to reform metallic lead: $$\ce{PbSO4(s) + 2e^- -> Pb(s) + SO4^{2-}(aq)}$$ * **At the Anode (Oxidation):** Lead sulfate is oxidized back to lead dioxide: $$\ce{PbSO4(s) + 2H2O(l) -> PbO2(s) + SO4^{2-}(aq) + 4H^+(aq) + 2e^-}$$ Combining these charging half-reactions gives the overall net charging reaction: $$\ce{2PbSO4(s) + 2H2O(l) -> Pb(s) + PbO2(s) + 2H2SO4(aq)}$$ Step 3 - Analyze the Physical and Chemical Changes During Charging By examining the overall balanced chemical equation for the charging process: 1. Solid lead sulfate ($\ce{2PbSO4}$) on both electrodes is consumed. 2. Pure metallic lead ($\ce{Pb}$) is regenerated at the cathode. 3. Solid lead dioxide ($\ce{PbO2}$) is regenerated at the anode. 4. Sulfuric acid ($\ce{H2SO4}$) is produced as a key product, meaning **sulfuric acid is regenerated**, which increases both its concentration and density (specific gravity) in the electrolytic solution. Step 4 - Evaluate the Given Options * **Option (A) is incorrect:** During charging, lead dioxide ($\ce{PbO2}$) is formed (deposited) at the anode rather than being dissolved. It only dissolves during the discharging process. * **Option (B) is incorrect:** The lead electrode becomes coated with solid lead sulfate ($\ce{PbSO4}$) during *discharging*. During charging, this lead sulfate coating is consumed and converted back to active metallic lead. * **Option (C) is correct:** As demonstrated by the net charging reaction, sulfuric acid is actively regenerated, raising the concentration and density of the electrolyte. * **Option (D) is incorrect:** The amount of sulfuric acid increases during charging, whereas it decreases during discharging. $$\text{Correct Option: } \boxed{\text{C}}$$