Batteries: primary and secondary sheet5-mark answer frameWhat is a secondary battery? Describe the lead storage battery, giving the reactions during discharge and during charging.
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define. A secondary battery can be recharged after use, by passing current through it in the opposite direction, which reverses the cell reaction.
- 2 · Describe the parts. Anode: lead. Cathode: a lead grid packed with PbO₂. Electrolyte: 38% H₂SO₄.
- 3 · Draw. A box of acid with alternate Pb and PbO₂ plates; label the anode (−), the cathode (+) and the H₂SO₄.
- 4 · Write the discharge reactions.
- Anode: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
- Cathode: PbO₂(s) + SO₄²⁻(aq) + 4H⁺(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)
- Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
- 5 · Write the charging reaction. 2PbSO₄(s) + 2H₂O(l) → Pb(s) + PbO₂(s) + 2H₂SO₄(aq): the battery now works as an electrolytic cell.
- 6 · Link to real life. Used in cars, two-wheelers and home inverters.
- 7 · Close in one line. So discharging and charging are the same reaction run in opposite directions.
Scoring tip: Check the cathode equation for O, H and charge. The 4H⁺ and 2H₂O are where most slips happen.
