Chemistry into electricity
A voltaic cell (also called a chemical cell) produces electricity from a spontaneous redox reaction. It uses two different metals as electrodes dipped in an electrolyte. The difference in their tendency to release electrons drives electrons through the external wire, producing a voltage.
The more electropositive metal (higher in the reactivity series) becomes the negative terminal — it is oxidised and releases electrons. The less reactive metal becomes the positive terminal.
Worked example
In a magnesium–copper cell in copper(II) sulfate, magnesium is more reactive and forms the negative terminal:
Negative: Mg → Mg2+ + 2e− (oxidation)
Positive: Cu2+ + 2e− → Cu (reduction)
Electrons flow from Mg to Cu in the wire; the magnesium electrode dissolves and copper is deposited.
Voltage and the reactivity series
The bigger the difference in reactivity between the two electrodes, the higher the voltage produced. A Mg/Cu pair gives a larger voltage than a Zn/Cu pair because Mg and Cu are further apart in the series.
Remember
- Negative terminal = more reactive metal = oxidation (electron source).
- Positive terminal = less reactive metal.
- Electrons flow through the wire from negative to positive; conventional current is opposite.
- Voltage increases as the gap in reactivity widens.
Everyday examples include the dry cell, alkaline cell and lead–acid accumulator, all storing energy in a spontaneous redox system.
In a two-beaker cell the two half-cells are joined by a salt bridge, a tube of saturated salt solution that completes the circuit and keeps each solution electrically neutral as ions are formed or removed. Inside the cell, positive ions drift towards the positive electrode and negative ions towards the negative electrode. The reading on the voltmeter is the potential difference between the two electrodes, and it gradually falls as the reactants are used up and the cell goes flat.