How electrolysis works
- When an ionic compound is melted or dissolved in water, its ions are free to move. The liquid or solution is called an electrolyte.
- Passing an electric current through the electrolyte makes the ions move to the electrodes.
- Positive ions move to the negative electrode (cathode). Negative ions move to the positive electrode (anode).
- At the electrodes, ions are discharged to form elements.
Molten ionic compounds
- In a molten compound, the metal forms at the cathode and the non-metal at the anode.
- Example: molten lead bromide gives lead at the cathode and bromine at the anode.
Extracting aluminium
- Metals too reactive to be extracted by carbon, such as aluminium, are extracted by electrolysis of their molten compounds. This uses large amounts of energy to melt the compound and make the current.
- Aluminium is made by electrolysing a molten mixture of aluminium oxide and cryolite. Cryolite lowers the melting point, saving energy.
- Aluminium forms at the cathode; oxygen forms at the carbon anode.
- The oxygen reacts with the carbon anodes to form carbon dioxide, so the anodes wear away and must be replaced regularly.
Aqueous solutions
- In aqueous solutions, water also forms some H⁺ and OH⁻ ions, so the products depend on reactivity.
- At the cathode: hydrogen is produced if the metal is more reactive than hydrogen; otherwise the metal forms (for example copper).
- At the anode: oxygen is produced, unless the solution contains halide ions, when the halogen (such as chlorine) is produced.
- Examples: sodium chloride solution gives hydrogen and chlorine, leaving sodium hydroxide solution. Copper sulfate solution gives copper and oxygen.
Half equations (Higher)
- Reduction happens at the cathode: Cu²⁺ + 2e⁻ → Cu, or 2H⁺ + 2e⁻ → H₂.
- Oxidation happens at the anode: 2Cl⁻ → Cl₂ + 2e⁻, or 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Key terms
- Electrolysis
- Breaking down an ionic compound using electricity.
- Electrolyte
- A molten or dissolved ionic compound that conducts electricity.
- Electrode
- A conductor through which current enters or leaves the electrolyte.
- Cathode
- The negative electrode, where positive ions gain electrons.
- Anode
- The positive electrode, where negative ions lose electrons.
- Discharged
- When an ion gains or loses electrons at an electrode to form an element.
- Cryolite
- A compound added to aluminium oxide to lower its melting point.
- Inert electrode
- An electrode, such as graphite or platinum, that does not react.
- Half equation
- An equation showing the electrons gained or lost at one electrode.