How do primary and secondary cells store and supply electrical energy?
Distinguish primary and secondary cells, describe their electrode reactions, and evaluate them as energy resources.
How primary cells provide single-use energy and secondary cells are rechargeable, the electrode reactions involved, and how batteries are evaluated as energy resources.
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What this dot point is asking
You must distinguish primary and secondary cells, describe their operation, and evaluate them as energy resources.
Primary cells
A common example is the alkaline cell, in which zinc is oxidised at the anode and manganese(IV) oxide is reduced at the cathode. Primary cells are cheap and convenient for low-drain devices but generate waste because they are single-use.
Secondary cells
When discharging, a secondary cell is a galvanic cell (spontaneous, oxidation at the anode). When recharging, an external power supply drives the reverse, non-spontaneous reaction - the cell now operates as an electrolytic cell, regenerating the original reactants.
Example: the lead-acid cell
The lead-acid car battery is the classic secondary cell. During discharge, lead is oxidised at the anode and lead(IV) oxide is reduced at the cathode, both forming lead(II) sulfate in sulfuric acid; recharging reverses these reactions. It delivers high current, which is why it is used to start engines.
Evaluating batteries as resources
When comparing batteries as energy resources, consider:
- Cost and lifespan: secondary cells cost more initially but are cheaper over many cycles.
- Energy density: energy stored per unit mass (important for portable devices and vehicles).
- Environmental impact: disposal of toxic metals, recyclability, and the resources needed to manufacture them.