6.2.6Additional lessons
Secondary Cells — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 89 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Rechargeable Cells
Recharging restores the original reactants, so energy is stored as chemical energy.
Reversible reaction
- Reversible reaction In a secondary cell the discharge reaction can be reversed by passing a current in the opposite direction.
Discharging
- Discharging The cell behaves as a voltaic cell and supplies electrical energy.
Recharging
- Recharging An external supply with a voltage greater than the cell voltage drives the reaction backwards, so it acts as an electrolytic cell.
Lifetime
- Lifetime A secondary cell can be used many times before its materials degrade.
Part 2 of 5Lead–Acid Cell
Lead–Acid Cell
Negative electrode
- Negative electrode Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ on discharge.
Positive electrode
- Positive electrode PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l) on discharge.
Overall
- Overall Pb(s) + PbO₂(s) + 2H₂SO₄(aq) ⇌ 2PbSO₄(s) + 2H₂O(l), with a voltage about 2 V per cell.
Car battery
- 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-ion Lithium ions move between a graphite negative electrode and a metal oxide positive electrode as the cell charges and discharges.
Advantages
- Advantages Lithium-ion cells have high energy density per unit mass, so they suit phones and electric vehicles.
Primary cells
- Primary cells Primary cells are cheap and convenient but are discarded after use.
Environmental issues
- Environmental issues Disposal of lead and the mining of lithium and cobalt raise environmental and ethical concerns.
| Type | Rechargeable? | Example |
|---|---|---|
| Primary | No | Zinc–carbon, alkaline |
| Secondary | Yes | Lead–acid, Li-ion |
| Fuel cell | Continuous fuel supply | H₂–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
- Discharge Chemical energy is converted into electrical energy as the cell supplies current to a device.
Recharge
- Recharge Electrical energy from an external source is converted into chemical energy in the cell.
Lead–acid change
- 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
- 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.