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6.2.6Additional lessons

Secondary Cells — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 89 of 98

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Part 1 of 5Rechargeable Cells

6.2.6 Secondary Cells

Recharging restores the original reactants, so energy is stored as chemical energy.

Reversible reaction
  • In a secondary cell the discharge reaction can be reversed by passing a current in the opposite direction.
Discharging
  • The cell behaves as a voltaic cell and supplies electrical energy.
Recharging
  • An external supply with a voltage greater than the cell voltage drives the reaction backwards, so it acts as an electrolytic cell.
Lifetime
  • A secondary cell can be used many times before its materials degrade.

Part 2 of 5Lead–Acid Cell

Lead–Acid Cell
Negative electrode
  • Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ on discharge.
Positive electrode
  • PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l) on discharge.
Overall
  • Pb(s) + PbO₂(s) + 2H₂SO₄(aq) ⇌ 2PbSO₄(s) + 2H₂O(l), with a voltage about 2 V per cell.
Car battery
  • Six cells in series give about 12 V, with high current for starting the engine.

Part 3 of 5Lithium-Ion Cell and Comparisons

Lithium-ion
  • Lithium ions move between a graphite negative electrode and a metal oxide positive electrode as the cell charges and discharges.
Advantages
  • Lithium-ion cells have high energy density per unit mass, so they suit phones and electric vehicles.
Primary cells
  • Primary cells are cheap and convenient but are discarded after use.
Environmental issues
  • Disposal of lead and the mining of lithium and cobalt raise environmental and ethical concerns.
TypeRechargeable?Example
PrimaryNoZinc–carbon, alkaline
SecondaryYesLead–acid, Li-ion
Fuel cellContinuous fuel supplyH₂–O₂

Part 4 of 5Charging and Discharging

The same electrodes work in both directions, which is what makes the cell rechargeable.

Charging and Discharging
Discharge
  • Chemical energy is converted into electrical energy as the cell supplies current to a device.
Recharge
  • Electrical energy from an external source is converted into chemical energy in the cell.
Lead–acid change
  • During discharge sulfuric acid is used up, so its density falls, which can be used to check the charge.
Lithium-ion movement
  • On discharge lithium ions move from the negative electrode through the electrolyte to the positive electrode,
    • and the process reverses during charging.

Part 5 of 5Exam Checklist & Tips

6.2.6 – Key Points for the Exam

  • Secondary cell A voltaic cell whose reaction can be reversed to recharge it.
  • Discharge The process of producing current from the cell.
  • Energy density The energy stored per unit mass or volume.
  • Lead–acid battery A rechargeable cell using lead, lead(IV) oxide and sulfuric acid.

Common Mistakes

  • Writing the recharge reaction as spontaneous; it needs an external supply.
  • Forgetting that during recharging the positive terminal of the supply is connected to the cell's positive electrode.
  • Stating that rechargeable cells never wear out.
  • Forgetting that the electrolyte in the lead–acid cell takes part in the reaction.

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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.