Electrolysis · Grade 12
Electrolysis uses electricity to split an ionic compound that is molten or dissolved in water (the electrolyte). Positive ions move to the negative electrode, the cathode, and gain electrons. Negative ions move to the positive electrode, the anode, and lose electrons. In solutions, ions from the water compete, so the products depend on how reactive the metal is and which negative ions are there. Electrolysis is used to extract reactive metals such as aluminium, to make chlorine and sodium hydroxide, and to purify and plate metals.
Key facts
- An electrolyte is a molten or dissolved ionic compound: its ions are free to move.
- Positive ions go to the cathode (−) and are reduced; negative ions go to the anode (+) and are oxidised. Remember PANIC: Positive Anode, Negative Is Cathode.
- A molten compound gives the metal at the cathode and the non-metal at the anode: molten lead(II) bromide gives lead and bromine.
- In a solution, the cathode gives hydrogen unless the metal is less reactive than hydrogen (copper, silver). The anode gives chlorine, bromine or iodine if halide ions are there, otherwise oxygen.
- Half equations: Pb²⁺ + 2e⁻ → Pb at the cathode; 2Br⁻ → Br2 + 2e⁻ at the anode; 4OH⁻ → O2 + 2H2O + 4e⁻ when oxygen forms.
- Aluminium is extracted from aluminium oxide dissolved in molten cryolite (to lower the melting point). Oxygen made at the carbon anodes burns them away, so they are replaced.
- Electrolysis of brine (sodium chloride solution) gives hydrogen, chlorine and sodium hydroxide.