In the discharge of a lead-acid battery, what is the anode reaction?

• A. Pb(s) + SO4^{2-} → PbSO4(s) + 2 e−
• B. PbO2(s) + 4 H+ + SO4^{2-} + 2 e− → PbSO4(s) + 2 H2O
• C. Pb(s) + H2SO4 → PbSO4(s) + 2 H+ + 2 e−
• D. PbSO4(s) + 2 e− → Pb(s) + SO4^{2-}

Answer: (A)

In discharge, the anode is where oxidation happens: solid lead is converted to lead(II) sulfate by reacting with sulfate ions from the electrolyte, releasing electrons into the external circuit. The correct half-reaction is Pb(s) + SO4^{2−} → PbSO4(s) + 2 e−. This shows lead being oxidized to PbSO4 and electrons leaving the electrode. The other forms either describe the cathode process (involving PbO2 and forming water) or depict the reverse process that would occur during charging, and they either place the wrong species in the oxidant role or include H+, which isn’t part of the anode discharge step.