A chemical cell turns the energy of a redox reaction into electricity. The metal that loses electrons more readily becomes the negative terminal, and electrons flow from it through the wire to the other metal.
This lesson is part of SPM Chemistry redox equilibrium. It reuses the half-equations from writing half-equations.
In what order should you explain a cell?
Follow the same order each time and you will rarely miss a point.
- Decide which metal is higher in the electrochemical series. This is the negative terminal.
- Write the half-equation at the negative terminal (oxidation) and at the positive terminal (reduction).
- State the direction of electron flow through the external wire.
- Give the observations at each electrode and in the solutions.
- Say what the salt bridge or porous pot does.
Worked example: zinc and copper
The cell uses a zinc plate in zinc sulfate solution, a copper plate in copper(II) sulfate solution and a salt bridge.
Terminals. Zinc is higher in the series, so zinc is the negative terminal.
Half-equations. Negative: Zn → Zn²⁺ + 2e⁻. Positive: Cu²⁺ + 2e⁻ → Cu.
Electron flow. Electrons flow from the zinc plate through the external wire to the copper plate.
Observations. The zinc plate becomes thinner. A brown deposit forms on the copper plate. The blue colour of the copper(II) sulfate solution fades.
Salt bridge. It lets ions move between the solutions so that the circuit is complete and charge stays balanced.
A second pair, to test the method
Magnesium and silver. Magnesium is higher in the series, so it is the negative terminal: Mg → Mg²⁺ + 2e⁻. Silver is positive: Ag⁺ + e⁻ → Ag.
This pair is further apart in the series than zinc and copper, so the voltage is larger.
The mistake that costs marks
The common slip is writing that electrons flow through the solution, or from copper to zinc. Electrons travel through the wire, and they start at the negative terminal.
| Point | Wrong | Right |
|---|---|---|
| Electron flow | Copper to zinc, through the solution | Zinc to copper, through the wire |
| Positive terminal | Zinc | Copper |
A second slip is carrying over the electrode names from electrolysis. In a chemical cell the negative terminal is where oxidation takes place, and this is not the same arrangement as in electrolysis.
Check yourself
A cell is made from magnesium and copper. State the negative terminal, write the half-equation at each terminal, and predict whether its voltage is larger or smaller than the zinc and copper cell.
Answer
Negative terminal: magnesium, Mg → Mg²⁺ + 2e⁻.
Positive terminal: copper, Cu²⁺ + 2e⁻ → Cu.
Magnesium is further from copper in the series than zinc is, so the voltage is larger.
What to study next
The same idea of two metals and electrons explains rusting. Continue with interpreting corrosion and prevention, then use the redox practice set.
The mistake log and paper-error review tool helps you track errors in cell diagrams. If you want a teacher to go through diagrams you have drawn, see online one-to-one Chemistry tuition.