What is the overall discharge reaction for a lead-acid battery?

• A. Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O
• B. Pb + PbO2 + 2 H2SO4 → PbSO4 + 2 H2O
• C. PbO2 + 2 H2O → Pb(OH)2 + O2
• D. PbSO4 + 2 H2O → Pb + PbO2 + 2 H2SO4

Answer: (A)

During discharge a lead-acid battery converts chemical energy into electrical energy by converting the active materials on both plates into lead sulfate. The negative plate (lead) and the positive plate (lead dioxide) each react with the sulfuric acid electrolyte to form PbSO4 on both plates, with water produced as a byproduct. The overall balanced reaction is Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O. This reflects the simultaneous consumption of the acid to form solid lead sulfate on each electrode and the release of water, which is exactly what happens as the battery delivers current. The other forms either aren’t balanced correctly, imply a different process like oxygen evolution, or correspond to charging rather than discharging.