Chemical cells and batteries (HT)
A chemical cell uses a redox reaction to push electrons through an external circuit, producing a potential difference (voltage). When two or more cells are connected together, they form a battery.
Building a simple cell
Two different metals dipped in an electrolyte (a salt solution). Each metal forms ions in solution at a different rate, leading to a charge imbalance and a voltage.
Example: Zn + Cu cell.
- Zn is more reactive; Zn → Zn²⁺ + 2e⁻ at the negative electrode (oxidation).
- Electrons flow through the external wire to the Cu electrode.
- At the Cu electrode: Cu²⁺ + 2e⁻ → Cu (reduction).
The bigger the reactivity difference between the two metals, the larger the voltage.
Voltage and reactivity
If you replace Cu with a less reactive metal like Ag, the voltage gets bigger. If you swap Zn for the much more reactive Mg, voltage rises further. This is why the reactivity series matters in cell design.
The electrolyte
Different electrolytes (e.g. salt solution vs sulfuric acid) can give different voltages — they affect how readily ions move and which species are involved.
Rechargeable vs non-rechargeable cells
- Non-rechargeable (e.g. alkaline AA): the chemicals are used up. Once one reactant is exhausted, the cell stops working.
- Rechargeable (e.g. Li-ion in phones, lead-acid in cars): the reaction is reversible. Applying a current in the opposite direction restores the original reactants.
Limitations of cells
- Voltage drops over time as reactants are consumed.
- Battery dies when reactants run out (non-rechargeable) or after many charge cycles (rechargeable lose capacity).
- Higher concentration of electrolyte generally gives higher voltage at the start (more ions to support the current).
Simple battery in series
Stacking cells in series adds their voltages:
- 1 alkaline cell ≈ 1.5 V.
- A 6 V "lantern" battery contains four 1.5 V cells in series.
- A 9 V battery contains six small 1.5 V cells.
⚠Common mistakes
- Saying "rechargeable" means "infinite life" — they degrade with cycles.
- Picking two identical metals for a cell — voltage = 0 (no reactivity difference).
- Confusing terminal labels. The more reactive metal is the negative electrode (releases electrons).
- Saying the electrolyte doesn't matter — it does.
Links
Builds on C4.1, C4.9–C4.12 (electrolysis is the reverse process). Sets up C5.5 (fuel cells).
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