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This lesson covers electrolysis, a key topic in the AQA GCSE Chemistry specification (4.4.3). You need to understand what electrolysis is, how it works, predict the products of electrolysis for molten compounds and aqueous solutions, and explain electrolysis in terms of ion movement and electron transfer. Electrolysis connects ionic bonding, redox chemistry, and industrial applications in a single, important topic.
Electrolysis is the process of using an electric current to decompose (break down) an ionic compound that is either molten (melted) or dissolved in water (aqueous solution).
| Requirement | Explanation |
|---|---|
| Ionic compound | The substance must contain ions (charged particles) |
| Molten or dissolved | The ions must be free to move — in a solid ionic compound, ions are fixed in a lattice and cannot move |
| Two electrodes | Conductors (usually made of graphite or platinum) dipped into the liquid |
| Direct current (DC) supply | Provides the electrical energy to drive the decomposition |
An electrolyte is the molten or dissolved ionic compound that conducts electricity during electrolysis.
Exam Tip: Electrolysis only works with ionic compounds because they contain ions. Covalent compounds (like sugar or ethanol) do not form ions and therefore cannot be electrolysed. A common mistake is to say any compound can be electrolysed — always specify that the substance must be ionic.
| Electrode | Charge | Name | What happens |
|---|---|---|---|
| Cathode | Negative (-) | The negative electrode | Positive ions (cations) are attracted here and gain electrons (reduction) |
| Anode | Positive (+) | The positive electrode | Negative ions (anions) are attracted here and lose electrons (oxidation) |
graph TD
A["DC Power Supply"] -->|"Negative terminal (-)"|B["CATHODE (-)"]
A -->|"Positive terminal (+)"| C["ANODE (+)"]
D["Positive Ions<br/>(Cations, e.g. Na+, Cu2+)"] -->|"Attracted to<br/>negative electrode"| B
E["Negative Ions<br/>(Anions, e.g. Cl-, O2-)"] -->|"Attracted to<br/>positive electrode"| C
B -->|"Ions GAIN electrons<br/>(REDUCTION)"| F["Metal deposited<br/>or hydrogen gas"]
C -->|"Ions LOSE electrons<br/>(OXIDATION)"| G["Non-metal gas<br/>produced (e.g. Cl2, O2)"]
style A fill:#4a90d9,color:#fff
style B fill:#e74c3c,color:#fff
style C fill:#27ae60,color:#fff
style F fill:#c0392b,color:#fff
style G fill:#2ecc71,color:#fff
Exam Tip: A very common exam error is to mix up which electrode is positive and which is negative. Remember: the cathode is NEGATIVE (it attracts positive ions) and the anode is POSITIVE (it attracts negative ions). Opposite charges attract. Use the mnemonic PANIC if you struggle with this.
When a pure molten ionic compound is electrolysed, the products are straightforward:
| Electrode | Ion attracted | Process | Product |
|---|---|---|---|
| Cathode (-) | Pb2+ (lead ions) | Pb2+ + 2e- → Pb (reduction) | Lead metal (silvery liquid at the bottom) |
| Anode (+) | Br- (bromide ions) | 2Br- → Br2 + 2e- (oxidation) | Bromine gas (orange-brown fumes) |
| Electrode | Ion attracted | Process | Product |
|---|---|---|---|
| Cathode (-) | Na+ (sodium ions) | Na+ + e- → Na (reduction) | Sodium metal |
| Anode (+) | Cl- (chloride ions) | 2Cl- → Cl2 + 2e- (oxidation) | Chlorine gas |
| Electrode | Ion attracted | Process | Product |
|---|---|---|---|
| Cathode (-) | Al3+ (aluminium ions) | Al3+ + 3e- → Al (reduction) | Aluminium metal |
| Anode (+) | O2- (oxide ions) | 2O2- → O2 + 4e- (oxidation) | Oxygen gas |
The oxygen produced at the anode reacts with the carbon electrodes, gradually wearing them away. This is why the carbon anodes need to be replaced regularly in the industrial extraction of aluminium.
When an ionic compound is dissolved in water, the situation is more complex because water also contains ions:
At the cathode (-):
| Condition | Product |
|---|---|
| Metal is less reactive than hydrogen (e.g. copper, silver) | The metal is deposited |
| Metal is more reactive than hydrogen (e.g. sodium, potassium, calcium) | Hydrogen gas is produced (H+ ions are discharged instead) |
At the anode (+):
| Condition | Product |
|---|---|
| Halide ion present (Cl-, Br-, I-) | The halogen is produced (chlorine, bromine, or iodine) |
| No halide ion present (e.g. sulfate, nitrate) | Oxygen gas is produced (from OH- ions) |
| Solution | At cathode (-) | At anode (+) |
|---|---|---|
| Copper sulfate (CuSO4) | Copper (Cu is less reactive than H) | Oxygen (no halide present) |
| Sodium chloride (NaCl) | Hydrogen (Na is more reactive than H) | Chlorine (halide present) |
| Sodium sulfate (Na2SO4) | Hydrogen (Na is more reactive than H) | Oxygen (no halide present) |
| Copper chloride (CuCl2) | Copper (Cu is less reactive than H) | Chlorine (halide present) |
| Silver nitrate (AgNO3) | Silver (Ag is less reactive than H) | Oxygen (no halide present) |
| Potassium bromide (KBr) | Hydrogen (K is more reactive than H) | Bromine (halide present) |
Exam Tip: For predicting cathode products, compare the metal to hydrogen in the reactivity series. For anode products, check if a halide is present. If a halide (Cl, Br, I) is present, the halogen is produced. If not, oxygen is produced. These two simple rules will answer almost every electrolysis prediction question.
Electrolysis is a redox process:
Remember: Reduction at the Cathode, Oxidation at the Anode — or simply, RedCat, AnOx.
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