A voltaic cell uses a spontaneous redox reaction to produce electrical energy, whereas an electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction. In both cells, oxidation occurs at the anode and reduction occurs at the cathode.
This distinction is part of IB Chemistry Reactivity 3.2 and applies at both SL and HL.
| Feature | Voltaic cell | Electrolytic cell |
|---|---|---|
| Reaction | Spontaneous | Non-spontaneous |
| Energy conversion | Chemical energy to electrical energy | Electrical energy to chemical energy |
| Anode | Negative | Positive |
| Cathode | Positive | Negative |
| Electron flow | Anode to cathode through the external circuit | Anode to cathode through the external circuit |
| Typical purpose | Producing electrical energy in a battery | Electrolysis, extraction, or coating |
At the anode, oxidation releases electrons. At the cathode, reduction consumes electrons. Therefore, electrons always travel through the external circuit from the anode to the cathode, regardless of the type of cell.
The electrode signs differ because of how the reaction is powered. In a voltaic cell, oxidation spontaneously supplies electrons to the anode, making it negative. In an electrolytic cell, the external power supply removes electrons from the anode, making it positive, and pushes electrons toward the negative cathode.
Ions move through the electrolyte to maintain electrical neutrality: cations migrate toward the cathode, while anions migrate toward the anode. A voltaic cell commonly uses two half-cells connected by a salt bridge, although cell designs can vary.
A common misconception is that the anode is always negative. This is incorrect: electrode signs change, but oxidation at the anode and reduction at the cathode never change.
For an IB exam response, compare spontaneity, energy conversion, and electrode signs, then state the invariant rule: oxidation occurs at the anode, reduction at the cathode, and electrons flow from anode to cathode.
