A primary (voltaic) cell generates electricity by using a spontaneous redox reaction to convert chemical energy directly into electrical energy. Oxidation and reduction occur at separate electrodes, forcing electrons to travel through an external circuit.
The Mechanism
In IB Chemistry Reactivity 3.2, oxidation is defined as loss of electrons, while reduction is gain of electrons. Consider a zinc-copper voltaic cell:
| Electrode | Process | Half-equation |
|---|---|---|
| Anode (negative) | Zinc is oxidized and releases electrons | Zn(s) → Zn²⁺(aq) + 2e⁻ |
| Cathode (positive) | Copper(II) ions are reduced by accepting electrons | Cu²⁺(aq) + 2e⁻ → Cu(s) |
The zinc electrode supplies electrons, so electrons flow through the external wire from the anode to the cathode. This movement of charge through the circuit produces an electric current. Conventional current is defined in the opposite direction.
At the cathode, copper(II) ions remove electrons from the circuit and form copper atoms. The overall reaction is:
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
Ion movement inside the cell completes the circuit and maintains electrical neutrality. In a two-half-cell model, a salt bridge allows ions to migrate without directly mixing the solutions. Commercial primary cells instead use an electrolyte and separator that provide an internal ionic pathway.
A primary cell eventually stops producing electricity because its reactants are consumed and its reactions are not readily reversed by applying an external current. This distinguishes it from a rechargeable secondary cell.
A common misconception is that the cell creates electrons. It does not: the redox reaction transfers existing electrons, and the separation of the two half-reactions directs that transfer through the wire.
Exam Technique
For an IB response, label the anode and cathode, identify oxidation and reduction, write balanced half-equations with state symbols, and show electron flow from negative anode to positive cathode. Remember: oxidation always occurs at the anode and reduction always occurs at the cathode.
