Electrolysis Principles
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
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
Electrolysis of a Molten Binary Compound (Supplement)
A binary ionic compound contains just two elements, e.g. NaCl, MgCl₂, PbBr₂. When molten, only the compound's own ions are present - there is no water, so the products are fully predictable: the metal forms at the cathode and the non-metal forms at the anode.
Worked Example: Molten Lead(II) Bromide
Molten PbBr₂ is electrolysed using inert graphite electrodes. Ions present: Pb²⁺ and Br⁻.
- Question: name the product at each electrode and describe the observations. [4]
- Answer - cathode: Pb²⁺ + 2e⁻ → Pb; grey molten lead forms [2]
- Answer - anode: 2Br⁻ → Br₂ + 2e⁻; brown bromine vapour forms [2]
Overall: PbBr₂ → Pb + Br₂
In a molten binary compound, the metal always forms at the cathode and the non-metal at the anode
| Molten compound | Cathode product | Anode product |
|---|---|---|
| NaCl | Na (sodium) | Cl₂ (chlorine) |
| MgCl₂ | Mg (magnesium) | Cl₂ (chlorine) |
| PbBr₂ | Pb (lead) | Br₂ (bromine) |
Electrolysis of Concentrated Aqueous Sodium Chloride
Concentrated aqueous NaCl (brine) is electrolysed using inert electrodes. Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
- Cathode: sodium is far more reactive than hydrogen, so H⁺ is discharged instead: 2H⁺ + 2e⁻ → H₂ - colourless gas bubbles (hydrogen)
- Anode: Cl⁻ is present in high concentration, so it is discharged: 2Cl⁻ → Cl₂ + 2e⁻ - greenish-yellow gas forms (chlorine)
Na⁺ and OH⁻ are left behind in solution, so the solution becomes sodium hydroxide.
Electrolysis of Dilute Sulfuric Acid
Dilute H₂SO₄ is electrolysed using inert electrodes. Ions present: H⁺, SO₄²⁻, plus the small amount of H⁺ and OH⁻ from water.
- Cathode: 2H⁺ + 2e⁻ → H₂ - colourless gas (hydrogen)
- Anode: SO₄²⁻ is not discharged, so OH⁻ is discharged instead: 4OH⁻ → O₂ + 2H₂O + 4e⁻ - colourless gas (oxygen)
Overall, water is being decomposed (2H₂O → 2H₂ + O₂).
| Gas | Test | Result |
|---|---|---|
| Hydrogen | lit splint | burns with a squeaky pop |
| Oxygen | glowing splint | relights |
| Chlorine | damp litmus paper | bleaches it white |
Uses of Electrolysis
- Extracting reactive metals, e.g. aluminium from aluminium oxide
- Purifying metals, e.g. copper
- Producing chemicals such as chlorine, hydrogen, and sodium hydroxide
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