Electrolysis
Electrolysis
- Electrolysis breaks down an ionic compound (when molten or in solution) using an electric current.
- It only works when molten or dissolved — then the ions are free to move and carry charge. A solid can't be electrolysed.
Practice
Electrolysis works only when the ionic compound is molten or dissolved because then:
Free-moving ions carry the current; in a solid the ions are fixed, so it can't be electrolysed.
The electrolytic cell
- Two electrodes dip into the electrolyte (the substance broken down).
- The anode is positive ($+$); the cathode is negative ($-$).
- Electrodes are often inert (platinum or carbon/graphite).
Practice
The anode is the:
The anode is positive (+); the cathode is negative (−).
What forms at each electrode
- Rule: metals or hydrogen at the cathode; non-metals (other than hydrogen) at the anode.
| Electrolyte | Cathode ($-$) | Anode ($+$) |
|---|---|---|
| molten $\text{PbBr}_2$ | lead | bromine |
| concentrated $\text{NaCl}$ (aq) | hydrogen | chlorine |
| dilute sulfuric acid | hydrogen | oxygen |
Practice
Electrolysing molten lead(II) bromide gives:
Metals form at the cathode (lead), non-metals at the anode (bromine).
You've got it
Key idea
- electrolysis = breaking an ionic compound (molten/aqueous) with current; ions must be free to move
- anode +, cathode −, often inert (carbon/platinum)
- metals/hydrogen at the cathode; non-metals at the anode