lead-acid battery

Meaning: A lead-acid battery is a rechargeable battery of galvanic cells with a Pb anode, a PbO₂ cathode and aqueous H₂SO₄. On discharge, the anode reaction is Pb + SO₄²⁻ → PbSO₄ + 2e⁻ and the cathode reaction is PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O. Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. Each cell gives about 2 V; six in series give about 12 V. Acid is consumed and water is formed, so the electrolyte becomes less dense. Recharging uses an external voltage to run the reaction backward.

Equation: ΔG = −nFE, where ΔG is the free energy change (J/mol), n is moles of electrons transferred per mole of reaction (2 here), F is 96,485 C/mol, and E is the cell potential (V).

On the MCAT: Passages give half-reactions and ask you to name the anode and cathode, write the overall reaction, or predict changes during discharge: PbSO₄ builds up on both electrodes, H₂SO₄ concentration falls and pH rises. Cells in series add their potentials.

Common confusion: Students call the Pb electrode the anode in every mode. During charging the cell is electrolytic and PbSO₄ is reduced to Pb at that electrode, so it becomes the cathode. Oxidation occurs at the anode, and that rule decides the label.