ElectrolysisOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is electrolysis?
Electrolysis is the process of passing an electric current through an ionic substance that is molten (melted) or dissolved in water (in solution), breaking it down into its elements. The substance being broken down is called the electrolyte.
In a solid ionic compound, the ions are held in fixed positions in a lattice and cannot move, so no current can flow. When the substance is melted or dissolved, the ions become free to move and can carry electrical charge through the liquid, and can be discharged at the electrodes.
Always explain conductivity in terms of ions being 'free to move' — the word 'free' is a required mark point, not just 'the ions can move'.
Section 2
How do ions move during electrolysis?
Two electrodes are placed in the electrolyte and connected to a power supply:
- The cathode is the negative electrode
- The anode is the positive electrode
During electrolysis, positively charged ions (cations) are attracted to and move towards the negative electrode (cathode), while negatively charged ions (anions) are attracted to and move towards the positive electrode (anode). Opposite charges attract.
Section 3
What happens at each electrode?
When ions reach an electrode, they are discharged — converted into neutral atoms or molecules:
| Electrode | Charge | Ions attracted | What happens | Term |
|---|---|---|---|---|
| Cathode | Negative | Positive ions | Gain electrons | Reduction |
| Anode | Positive | Negative ions | Lose electrons | Oxidation |
This links to the electron-transfer definitions: oxidation is loss of electrons, reduction is gain of electrons (remember with the mnemonic OIL RIG).
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Section 4
How do you write half-equations for electrode reactions?
A half-equation shows the ions being discharged and the electrons transferred at one electrode. For example, the electrolysis of molten sodium chloride:
- At the cathode (reduction): Na⁺ + e⁻ → Na
- At the anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻
To balance a half-equation:
- Write the ion and the element or molecule it forms
- Balance the atoms first
- Balance the charge by adding the correct number of electrons on the side with the ions
For chloride ions forming chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻ — two chloride ions combine and each loses one electron, giving two electrons released overall.
Must Know
- Electrolysis breaks down an ionic substance (the electrolyte) using an electric current, while molten or in solution
- Ions must be free to move to conduct electricity and be discharged — this doesn't happen in the solid state
- Positive ions move to the cathode (negative electrode); negative ions move to the anode (positive electrode)
- At the cathode, positive ions gain electrons (reduction); at the anode, negative ions lose electrons (oxidation)
- Oxidation is loss of electrons, reduction is gain of electrons (OIL RIG)
- Half-equations must balance both atoms and charge, e.g. 2Cl⁻ → Cl₂ + 2e⁻
That's the notes covered.
Carry on to the next subtopic.