Question

On the basis of \( E^\circ \) values, O\(_2\) gas should be liberated at the anode, but it is Cl\(_2\) gas that is liberated in the electrolysis of aqueous NaCl.

Hint:

Even though oxygen has a more positive reduction potential, chlorine is preferentially produced at the anode due to the high concentration of chloride ions.

Answer 1

Electrolysis of Aqueous NaCl

1. Standard Electrode Potentials:

The standard electrode potential for the reduction of oxygen to water is: \[ E^\circ_{\text{O}_2/\text{H}_2\text{O}} = +1.23 \, \text{V} \]. For the reduction of chloride ions to chlorine gas: \[ E^\circ_{\text{Cl}^-/\text{Cl}_2} = +1.36 \, \text{V} \]

2. Anode Product Determination:

The standard electrode potential for chloride oxidation (+1.36 V) is slightly less positive than that for oxygen reduction (+1.23 V). However, in aqueous NaCl electrolysis, oxygen is not preferentially reduced at the anode.

The liberation of chlorine gas (\( \text{Cl}_2 \)) at the anode is primarily due to the ionic concentration. Chloride ions (\( \text{Cl}^- \)) are present in significantly higher concentrations than oxygen in the solution.

The concentration effect dictates preferential oxidation. Despite oxygen's more positive reduction potential, the high abundance of chloride ions facilitates their oxidation to chlorine gas at the anode.

3. Process Description:

During aqueous NaCl electrolysis, oxidation occurs at the anode. Chloride ions (\( \text{Cl}^- \)) are oxidized to chlorine gas (\( \text{Cl}_2 \)): \[ 2\text{Cl}^- (aq) \rightarrow \text{Cl}_2 (g) + 2e^- \] At the cathode, water is reduced, producing hydroxide ions (\( \text{OH}^- \)) and hydrogen gas (\( \text{H}_2 \)): \[ 2\text{H}_2\text{O} (l) + 2e^- \rightarrow \text{H}_2 (g) + 2\text{OH}^- \]

4. Summary:

Chlorine is preferentially liberated at the anode during aqueous NaCl electrolysis, overriding the standard electrode potential favoring oxygen reduction. This is because the high concentration of chloride ions makes them more readily oxidized.