Electrolysis of Molten and Aqueous Solutions
What Is Electrolysis?
Electrolysis is the process of using an electric current to decompose (break down) an ionic compound into its elements. The ionic compound must be molten (melted) or dissolved in water (aqueous) so that the ions are free to move and carry the current.
Ionic compounds cannot be electrolysed when solid because the ions are fixed in the lattice and cannot move.
Key Terms
- Electrolyte: the molten or dissolved ionic compound being decomposed
- Electrode: a conductor (usually graphite or platinum) dipped into the electrolyte to allow current to flow
- Anode: the positive electrode (connected to the positive terminal of the power supply)
- Cathode: the negative electrode (connected to the negative terminal)
- Cation: a positive ion (e.g. Na⁺, Cu²⁺, H⁺) — attracted to the cathode
- Anion: a negative ion (e.g. Cl⁻, O²⁻, OH⁻) — attracted to the anode
Memory aid: CAtions go to the CAthode. PANIC — Positive Anode, Negative Is Cathode.
Electrolysis of Molten Compounds
When a molten ionic compound is electrolysed, only two ions are present (the metal cation and the non-metal anion).
Example: Molten Lead Bromide (PbBr₂)
At the cathode (−): Pb²⁺ ions gain electrons → lead metal is deposited
Pb²⁺ + 2e⁻ → Pb (reduction)
At the anode (+): Br⁻ ions lose electrons → bromine gas is produced
2Br⁻ → Br₂ + 2e⁻ (oxidation)
OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
Example: Molten Aluminium Oxide (Al₂O₃)
This is the industrial extraction of aluminium (see Reactivity Series notes).
At the cathode: Al³⁺ + 3e⁻ → Al
At the anode: 2O²⁻ → O₂ + 4e⁻
Electrolysis of Aqueous Solutions
When an ionic compound is dissolved in water, there are extra ions present from the water itself:
- H⁺ ions (from water)
- OH⁻ ions (from water)
So at each electrode, there is a competition between ions.
Rules for the Cathode (Which Cation Is Discharged?)
The metal is produced if it is less reactive than hydrogen. If the metal is more reactive than hydrogen, hydrogen gas is produced instead.
- Metals below hydrogen in the reactivity series (e.g. Cu, Ag) → metal is deposited
- Metals above hydrogen (e.g. Na, K, Ca, Mg, Al) → hydrogen gas is produced
Example:
- CuSO₄(aq): Cu is less reactive than H → copper metal deposited
- NaCl(aq): Na is more reactive than H → hydrogen gas produced
Rules for the Anode (Which Anion Is Discharged?)
- If the solution contains a halide ion (Cl⁻, Br⁻, I⁻) → the halogen is produced
- If no halide is present → oxygen is produced (from OH⁻ ions)
Example:
- NaCl(aq): Cl⁻ present → chlorine gas at anode
- Na₂SO₄(aq): no halide → oxygen gas at anode
- CuSO₄(aq): no halide → oxygen gas at anode
Summary Table: Electrolysis of Aqueous Solutions
| Electrolyte | Cathode product | Anode product |
|---|---|---|
| Copper sulfate (CuSO₄) | Copper | Oxygen |
| Sodium chloride (NaCl) | Hydrogen | Chlorine |
| Sodium sulfate (Na₂SO₄) | Hydrogen | Oxygen |
| Copper chloride (CuCl₂) | Copper | Chlorine |
Required Practical: Electrolysis of Aqueous Solutions
Method
1. Pour the solution into a beaker
2. Place two inert electrodes (e.g. graphite) into the solution, connected to a DC power supply
3. Turn on the power supply
4. Observe what forms at each electrode
5. Test any gases produced
Gas Tests
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Hold a burning splint near the gas | Squeaky pop |
| Chlorine | Hold damp litmus paper near the gas | Bleaches it white |
| Oxygen | Hold a glowing splint in the gas | Relights |
Industrial Electrolysis: Brine (Sodium Chloride Solution)
The electrolysis of concentrated sodium chloride solution (brine) is commercially important. It produces three useful products:
- Cathode: hydrogen gas
- Anode: chlorine gas
- Solution: sodium hydroxide (NaOH) is left in solution
Uses of These Products
| Product | Uses |
|---|---|
| Chlorine | Water purification, making bleach, making PVC |
| Hydrogen | Making margarine (hydrogenation), fuel, making ammonia |
| Sodium hydroxide | Making soap, bleach, paper, ceramics |
Exam Tips
- Know the difference between molten and aqueous electrolysis — aqueous has competing H⁺ and OH⁻ ions
- Learn the rules: cathode depends on reactivity vs hydrogen; anode depends on halide presence
- Oxidation occurs at the anode, reduction at the cathode — always
- Learn the half equations for common examples
- State symbols matter: (l) for molten, (aq) for dissolved