6.2.6Additional lessons
Secondary Cells — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 137 of 172
Part 1 of 4Rechargeable Cells
Secondary cells can be used again and again, unlike primary cells.
Secondary cell
- A cell whose chemical reaction can be reversed by passing a current through it in the opposite direction.
Discharging
- The cell behaves as a voltaic cell, and a spontaneous redox reaction supplies electrical energy.
Recharging
- An external supply forces electrons through the cell and reverses the discharge reaction.
Re-use
- Because the products stay on the electrodes, the original reactants can be regenerated many times.
Part 2 of 4The Lead-Acid Cell
1
The negative electrode is lead and the positive electrode is lead(IV) oxide, both in sulfuric acid.
2
On discharge lead is oxidised: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻.
3
Lead(IV) oxide is reduced: PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l).
4
The overall discharge is Pb + PbO₂ + 4H⁺ + 2SO₄²⁻ → 2PbSO₄ + 2H₂O, so the acid becomes more dilute.
5
Recharging forces the reaction in reverse, restoring lead, lead(IV) oxide and the sulfuric acid concentration.
Part 3 of 4Lithium-Ion and Comparisons
Lithium-Ion and Comparisons
Lithium-ion cell
- Lithium ions move between the two electrodes during charging and discharging while electrons flow in the circuit.
Energy density
- Lithium-ion cells store more energy per unit mass than lead-acid cells, so they suit portable devices.
Lead-acid use
- Lead-acid cells are cheap and deliver large currents, so they start vehicle engines.
Limitations
- Both types lose capacity after many cycles, and recycling is needed to deal with hazardous materials.
| Property | Primary cell | Secondary cell |
|---|---|---|
| Rechargeable | No | Yes |
| Reaction reversible | Not in practice | Yes |
| Example | Alkaline battery | Lead-acid, lithium-ion |
Part 4 of 4Exam Checklist & Tips
6.2.6 – Key Points for the Exam
- Secondary cell A rechargeable cell in which the cell reaction can be reversed.
- Discharge The process in which a cell supplies current as the redox reaction proceeds.
- Recharge Using an external supply to reverse the redox reaction.
- Energy density The amount of energy stored per unit mass or volume of the cell.
Common Mistakes
- Believing a secondary cell never changes composition; the electrode materials do change.
- Writing the discharge equations in the wrong direction when describing recharging.
- Forgetting that sulfuric acid is consumed during discharge in a lead-acid cell.
- Saying electrons flow through the electrolyte rather than ions.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.