Cl₂ is being produced at the anode instead of O₂. Chlorine gas is toxic and corrosive — in a real laboratory, this reaction must be performed in a fume hood with appropriate safety precautions.
Why is this happening? — Kinetics vs Thermodynamics
The electrochemical series predicts that O₂ evolution (E° = +1.23 V) should be favoured over Cl₂ evolution (E° = +1.36 V) at the anode, since oxygen requires a lower oxidation potential. However, three factors override this thermodynamic prediction:
1. Overpotential: O₂ evolution has a very high kinetic activation barrier (overpotential) on most electrode surfaces. The four-electron transfer needed to produce O₂ from water is mechanistically complex, adding ~0.5–0.8 V of extra voltage in practice.
2. Concentration effect: In concentrated NaCl solution, Cl⁻ ions are abundant at the electrode surface and readily available for the simpler two-electron oxidation to Cl₂.
3. Electrode kinetics: The two-electron transfer for Cl₂ production is kinetically faster than the four-electron O₂ pathway. This is a textbook example of kinetic control overriding thermodynamic control.
eWater Membrane Cell: A selective ion-exchange membrane separates the anode and cathode chambers. Cl⁻ ions migrate to the anode producing HOCl (sanitiser), while Na⁺ ions migrate to the cathode producing NaOH (cleaner). Despite Cl₂ having a less favourable E° (+1.36 V) than O₂ evolution (+1.23 V), the high Cl⁻ concentration and kinetic overpotential of O₂ on the electrode surface favour chlorine production — a classic example of kinetics overriding thermodynamics.