Factors Influencing Discharge:
* Electrode Potential: The ease with which an ion is discharged depends on its standard electrode potential (E°). Chloride ions have a more positive standard electrode potential than hydroxide ions. This means chloride ions are more readily oxidized (lose electrons) at the anode.
* Concentration: The concentration of ions in the solution also plays a role. In a brine solution, the concentration of chloride ions is significantly higher than that of hydroxide ions.
* Overpotential: The actual potential required for a reaction to occur is often higher than the standard electrode potential due to overpotential. This is a resistance to the reaction occurring at the electrode surface. However, in this case, the overpotential for chloride oxidation is lower than that for hydroxide oxidation.
The Reaction at the Anode:
At the anode, the chloride ions are oxidized to chlorine gas:
2Cl⁻ (aq) → Cl₂(g) + 2e⁻
Why Hydroxide Ions Don't Discharge:
Hydroxide ions have a more negative standard electrode potential, making them less likely to be oxidized. In addition, the presence of a large concentration of chloride ions means they are more readily available for reaction at the anode.
Summary:
In the electrolysis of brine, the combination of a more positive standard electrode potential, higher concentration, and lower overpotential for chloride ions makes them the preferred species to be discharged at the anode. This results in the production of chlorine gas.
