What is an electrolytic cell?
An electrolytic cell is a set-up that uses electrical energy to bring about a chemical change. This process is called electrolysis. It has three main parts: a source of direct current, two electrodes dipped into the liquid, and an electrolyte — a molten or aqueous substance that conducts electricity and is decomposed by it.
Key idea
In an electrolytic cell electrical energy is converted into chemical energy. The anode is the positive electrode and the cathode is the negative electrode.
How the ions move
An electrolyte contains freely moving ions, which are the charged particles that carry the current through the liquid. In a solid the ions are locked in place and cannot move, so a solid ionic compound does not conduct. Only when the compound is melted or dissolved in water are the ions free to move. When the current is switched on:
- Cations (positive ions) are attracted to the cathode, where they gain electrons — this is reduction.
- Anions (negative ions) are attracted to the anode, where they lose electrons — this is oxidation.
Example
When molten lead(II) bromide is electrolysed, lead ions move to the cathode and are reduced to a silvery bead of lead metal, while bromide ions move to the anode and are oxidised to brown bromine vapour.
Uses of electrolysis
Electrolysis is used widely in industry:
- Electroplating — coating an object with a thin layer of metal such as silver or chromium for protection or beauty.
- Purifying copper — impure copper is made the anode and dissolves, while pure copper deposits on the cathode.
- Extracting reactive metals — aluminium is obtained by electrolysing molten aluminium oxide, because it is too reactive to be extracted by heating with carbon.
Remember
A memory aid: reduction happens at the cathode (both begin like "red-cat"), and oxidation happens at the anode.