Electrolysis Principles
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
What is Electrolysis?
Electrolysis is the decomposition of an ionic compound, molten or in aqueous solution, by the passage of an electric current through it.
- Electrical energy breaks the compound down into its elements or simpler substances
- It makes a reaction happen that would not occur naturally
Why Electrolysis Works
- Ionic compounds contain positive ions (cations, e.g. Na⁺, Cu²⁺) and negative ions (anions, e.g. Cl⁻, O²⁻)
- When molten or dissolved in water, these ions are free to move
- Solid ionic compounds cannot be electrolysed - their ions are fixed in place in the lattice
The Electrolytic Cell
| Component | Description |
|---|---|
| Electrolyte | the molten or aqueous ionic substance that undergoes electrolysis |
| Electrodes | two conducting rods that carry current into and out of the electrolyte |
| Anode | the positive electrode |
| Cathode | the negative electrode |
A simple way to remember which is which: PANIC - Positive Anode, Negative Is Cathode.
What Happens at Each Electrode
| Electrode | Attracts | Process | Typical product |
|---|---|---|---|
| Cathode (-) | Cations (+) | Reduction (gain electrons) | a metal, or hydrogen |
| Anode (+) | Anions (-) | Oxidation (lose electrons) | a non-metal (other than hydrogen) |
Remember OIL RIG: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).
Describing the Transfer of Charge
A complete description of electrolysis must describe charge moving all the way around the circuit, in three parts:
- In the external circuit: electrons flow from the anode, through the wires and power supply, to the cathode
- At the electrodes: ions lose electrons at the anode (oxidation) and gain electrons at the cathode (reduction)
- In the electrolyte: ions themselves carry charge by migrating - cations move toward the cathode, anions move toward the anode
Worked Example: Describing Charge Transfer
- Question: in an electrolysis cell, oxidation occurs at electrode 1 and reduction occurs at electrode 2. State the direction of movement of (a) electrons in the external circuit, and (b) positive ions in the electrolyte. [2]
- Answer: oxidation at electrode 1 means electrode 1 is the anode; reduction at electrode 2 means electrode 2 is the cathode [shown by identifying electrodes]; (a) electrons flow from the anode to the cathode in the external circuit, i.e. from electrode 1 to electrode 2 [1]; (b) positive ions migrate toward the cathode, i.e. also from electrode 1's side toward electrode 2 [1]
Conditions for Electrolysis
An ionic compound conducts, and can be electrolysed, only when its ions are free to move: when it is molten (the lattice has broken down) or dissolved in water (an aqueous solution).
Cations migrate to the cathode and are reduced; anions migrate to the anode and are oxidised
Electrode Materials
| Material | Properties |
|---|---|
| Graphite (a non-metal, a form of carbon) | inert, conducts electricity, cheap - the most common inert electrode |
| Platinum (a metal) | inert, very unreactive - used where graphite might react |
| Copper (a metal) | conducts well but can react/dissolve - used as an active electrode, e.g. in electroplating |
Inert electrodes (graphite, platinum) do not react - they simply allow current to flow, so the products come only from ions in the electrolyte.
Common Mistakes
- ❌ Mixing up cathode and anode
- ✓ PANIC: Positive Anode, Negative Is Cathode
- ❌ Thinking a solid ionic compound can be electrolysed
- ✓ the ions must be free to move (molten or aqueous) - in a solid lattice they are fixed in position
Uses of Electrolysis
- Extracting reactive metals, e.g. aluminium from aluminium oxide
- Purifying metals, e.g. copper
- Electroplating objects with a thin layer of metal (see Electroplating, later in this section)
- Producing chemicals such as chlorine, hydrogen, and sodium hydroxide
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