Electrolysis of molten salts
New substances form when a moltenA term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid. or dissolved ionic compoundAn ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). conducts electricity. For example, lead(II) bromide can be used to produce bromine and lead using the following process:
- place solid lead(II) bromide in a crucible and heat over a Bunsen burner until it melts
- insert two carbon electrodeA conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. into the molten electrolyte and pass a direct current between them
Carbon electrodes are chosen because they have a high melting point and are inert (they will not react with the reactants and products during electrolysis). The observations are summarised in the table:
| Observation | Explanation | |
| Anode (positive) | Bubbles of brown gas | Bromine vapour (Br2) is produced |
| Cathode (negative) | Silvery liquid metal present at this electrode underneath the remaining molten electrolyte | Lead (Pb) is produced |
| Anode (positive) | |
|---|---|
| Observation | Bubbles of brown gas |
| Explanation | Bromine vapour (Br2) is produced |
| Cathode (negative) | |
|---|---|
| Observation | Silvery liquid metal present at this electrode underneath the remaining molten electrolyte |
| Explanation | Lead (Pb) is produced |
It is easy to predict the products of electrolysis of molten electrolytes because they simply split into their elementA substance made of one type of atom only.:
- the metal is formed at the negative electrode because that is where the positive metal ions are attracted
- the non-metal element is formed at the positive electrode where the negative non-metal ions are attracted
For example:
| Compound | Anode (positive) | Cathode (negative) |
| Zinc chloride (ZnCl2) | Chlorine (Cl2) | Zinc (Zn) |
| Aluminium oxide (Al2O3) | Oxygen (O2) | Aluminium (Al) |
| Compound | Zinc chloride (ZnCl2) |
|---|---|
| Anode (positive) | Chlorine (Cl2) |
| Cathode (negative) | Zinc (Zn) |
| Compound | Aluminium oxide (Al2O3) |
|---|---|
| Anode (positive) | Oxygen (O2) |
| Cathode (negative) | Aluminium (Al) |