Electrolysis cell
Choose an electrolyte and watch the ions migrate, then check the products predicted at each electrode.
- Ion migration
- Anode and cathode products
- Half equations
At the cathode — reduction
Pb²⁺ + 2e⁻ → Pb
At the anode — oxidation
2Br⁻ → Br₂ + 2e⁻
Predict
Concentrated sodium chloride solution is electrolysed. What forms at the cathode?
Experiment
- 1Compare molten zinc chloride with concentrated sodium chloride solution. Why do the cathode products differ?
- 2Switch to dilute sulfuric acid and look at the anode product.
- 3Try copper(II) sulfate — check whether the metal or hydrogen is deposited, and explain why.
Explain
Electrolysis is the breakdown of an ionic compound, molten or in solution, by the passage of electricity. The ions must be free to move, which is why a solid ionic lattice does not conduct.
At the cathode, positive ions gain electrons (reduction). At the anode, negative ions lose electrons (oxidation). OIL RIG.
In aqueous solutions water supplies H⁺ and OH⁻, so there is competition. Hydrogen is discharged at the cathode unless the metal is less reactive than hydrogen. At the anode, a halogen wins if a halide is present in reasonable concentration; otherwise oxygen is discharged.