1. Chloride Ion Concentration: The concentration of chloride ions (Cl-) in a concentrated NaCl solution is significantly higher compared to other ions, such as hydroxide ions (OH-) or water molecules (H2O). As a result, chloride ions are more likely to be oxidized and form chlorine gas at the anode.
2. Oxidation Potential: The oxidation potential of chloride ions (Cl- to Cl2) is lower than that of hydroxide ions (OH- to O2) and water molecules (H2O to O2). This means that it requires less energy to oxidize chloride ions and produce chlorine gas.
The standard oxidation potentials at 25°C are:
- Cl- → Cl2 (g) + 2 e- E° = 1.36 V
- 2 H2O (l) → O2 (g) + 4 H+ (aq) + 4 e- E° = 1.23 V
Therefore, under the same conditions, chloride ions are preferentially oxidized, leading to the formation of chlorine gas at the anode.
3. Competing Reactions: In the electrolysis of water, oxygen evolution from water molecules (2H2O → O2 + 4H+ + 4e-) is a competing reaction to the oxidation of chloride ions. However, the presence of concentrated chloride ions shifts the equilibrium towards chlorine evolution due to the higher concentration of chloride ions and their lower oxidation potential.
In summary, the combination of a high concentration of chloride ions, a lower oxidation potential for chloride oxidation, and the presence of competing reactions from water molecules results in the liberation of chlorine gas rather than oxygen at the anode during the electrolysis of concentrated aqueous sodium chloride solution.
