Electrolysis — common mistakes

Always include state symbols in your half-equations. They are crucial and often worth a mark. Pay special attention to (l) for molten substances, (aq) for aqueous species, and (g) or (s) for products. Forgetting them is a common way to lose easy marks.

Inert electrodes are used because they do not react with the electrolyte or the products of electrolysis. This ensures that the only reactions occurring are the ones we want to study or use, preventing contamination of the products. If an active electrode (like copper) were used as the anode in CuSO₄ electrolysis, the electrode itself would oxidise instead of the water or sulfate ions.

The sign convention is reversed. In an electrolytic cell, the external power supply makes the anode positive and the cathode negative. In a voltaic cell, which generates electricity from a spontaneous reaction, the anode (site of oxidation) is the source of electrons and is therefore the negative terminal, while the cathode (site of reduction) is the positive terminal. Crucially, in both cell types, oxidation always occurs at the anode and reduction always occurs at the cathode.

Sodium is a very reactive metal. In an aqueous solution, water is present and can also be reduced at the cathode. The reduction of H⁺ ions from water (2H₂O + 2e⁻ → H₂ + 2OH⁻) requires less energy (has a less negative E^⦵ value) than the reduction of Na⁺ ions (Na⁺ + e⁻ → Na). Therefore, hydrogen gas is produced at the cathode instead of sodium metal.