Water may be oxidized or reduced instead of ions, depending on potentials.
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In aqueous solution, water molecules compete with dissolved ions for discharge at the electrodes, so the products depend on relative standard potentials and kinetic factors. At the cathode, water is often reduced to hydrogen gas in preference to very reactive metal ions such as sodium or potassium. At the anode, oxidation of water to oxygen competes with oxidation of chloride, and overpotential and concentration determine which is favored. Concentrated chloride solutions therefore give chlorine rather than oxygen. Predicting the product requires considering every species present.
Key ideas: Water competes: at the cathode, species with the least negative E is reduced (Cu2+ before H2O; Na+ never, so H2 and OH- form). At the anode, the most easily oxidized species wins, but overpotential on O2 often favors Cl- over water in concentrated chloride. Inert electrodes, active electrodes such as Cu dissolve themselves.
Common mistakes and connections: Mistakes: predicting Na or Al metal from aqueous solutions, ignoring concentration and overpotential, and forgetting the anode metal can oxidize. Brine gives H2, Cl2 and NaOH; dilute Na2SO4 gives just H2 and O2 (2:1 volume ratio). The pH changes near each electrode: acidic at the anode, basic at the cathode.
In aqueous NaCl, water is reduced to H2 at the cathode instead of Na+, while chloride is oxidized to Cl2 at the anode, a competition decided by potentials. The interactive lesson shows this as a live animation or chart.
Electrolysis of concentrated brine gives hydrogen at the cathode and chlorine at the anode, leaving sodium hydroxide in solution.
Water may be oxidized or reduced instead of ions, depending on potentials.
Electrolysis of Aqueous Solutions is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Acids, Bases & Electrochemistry first: Faraday's First Law, Faraday's Second Law, Electrolysis of Molten Salts.
The next topics in Acids, Bases & Electrochemistry are Electroplating and Electrorefining.
Yes. The interactive lesson includes a self-check question on Electrolysis of Aqueous Solutions, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Electrolysis of Aqueous Solutions is topic 63 of 105 in Acids, Bases & Electrochemistry.