The big idea: A redox reaction moves electrons from one substance to another. An electrochemical cell puts those electrons to work by forcing them through a wire.
There are two kinds:
- Voltaic (galvanic) cell — a spontaneous reaction pushes electrons through the wire, so it makes electricity (chemical → electrical). This is a battery. - Electrolytic cell — an external power supply drives a non-spontaneous reaction (electrical → chemical). This is electrolysis.
A voltaic cell: the more reactive metal (Zn) is the anode (oxidation, −); electrons flow through the external wire to the cathode (Cu, reduction, +); the salt bridge completes the circuit.
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Two electrodes, two jobs: Every cell has two electrodes dipping into a solution (the electrolyte):
- Anode — where oxidation happens (loss of electrons). - Cathode — where reduction happens (gain of electrons).
Memory aid: AN OX (ANode = OXidation) and RED CAT (REDuction = CAThode). This is true for both cell types.
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Both cells use an anode and a cathode, but they run in opposite directions. A voltaic cell releases energy from a spontaneous reaction; an electrolytic cell absorbs energy to drive a reaction that would not happen on its own.
| Voltaic (galvanic) cell | Electrolytic cell | |
|---|---|---|
| Energy change | chemical → electrical | electrical → chemical |
| Reaction | spontaneous redox | non-spontaneous (driven by a power supply) |
| Power supply | none — the cell IS the source | required (a battery/external source) |
| Anode (oxidation) | negative (−) | positive (+) |
| Cathode (reduction) | positive (+) | negative (−) |
| Example | a battery delivering current | electrolysis (e.g. molten NaCl, electroplating) |
The electrode signs flip: The anode is always oxidation and the cathode always reduction — but the signs swap between the two cell types:
- Voltaic: anode is negative, cathode is positive. - Electrolytic: anode is positive, cathode is negative.
This is the most common exam trap. Decide the cell type first, then assign the signs.
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In both cells, electrons leave the anode (where oxidation releases them) and travel through the external wire to the cathode (where reduction consumes them). So the direction is always anode → cathode in the wire.
What the salt bridge does: In a voltaic cell the two half-cells are joined by a salt bridge (an inert ionic solution, e.g. KNO3).
It completes the circuit and keeps each solution electrically neutral: as ions are made and used up, negative ions move toward the anode and positive ions move toward the cathode. Without it, charge would build up and the current would stop.
Worked example — describe the electron flow
In the zinc–copper voltaic cell shown above, state where oxidation occurs and describe the direction of electron flow in the external wire.
How to score the marks
- Oxidation site: zinc is more reactive, so the zinc electrode is the anode, where oxidation occurs (Zn loses electrons).
- Electron direction: electrons flow through the wire from the zinc anode to the copper cathode (anode → cathode).
- Link it: the electrons released by oxidation at the anode are exactly the ones gained by reduction at the cathode.
Final answer
Oxidation occurs at the zinc anode; electrons flow through the external wire from the zinc anode to the copper cathode.
How this is tested: R3.2 cells show up as a quick Paper 1A identify ('give the sign and name of this electrode') and a Paper 2 annotate/write task on a labelled cell diagram.
The markers want you to: name the anode and cathode correctly, give the right sign for that cell type, show the electron-flow direction (anode → cathode in the wire), and write the electrode half-equations.
Sure marks: Decide voltaic or electrolytic before you write anything — it fixes every sign. Then state AN OX / RED CAT, draw electrons anode → cathode, and balance each half-equation for charge as well as atoms.
IB-style question — silver–iron voltaic cell (a)
A voltaic cell is built from an iron electrode in Fe2+(aq) and a silver electrode in Ag+(aq). Iron is the more reactive metal. (a) Identify the name and sign of the silver electrode. [2]
How to score the marks
- Which electrode? Iron is more reactive, so iron is oxidised — iron is the anode. The silver electrode is therefore the cathode (reduction).
- Sign: in a voltaic cell the cathode is the positive (+) electrode.
Final answer
The silver electrode is the cathode and is positive (+).
IB-style question — silver–iron voltaic cell (b)
(b) Write the half-equation for the reaction occurring at the silver electrode. [1]
How to score the mark
- Reduction (gain of electrons) happens at the cathode — silver ions are reduced to silver metal.
- Balance charge: one electron per Ag+.
Final answer
Ag⁺(aq) + e⁻ → Ag(s)