Account for the following:
On the basis of \( E^\circ \) values, O\(_2\) gas should be liberated at anode but it is Cl\(_2\) gas which is liberated in the electrolysis of aqueous NaCl.
On the basis of \( E^\circ \) values, O\(_2\) gas should be liberated at anode but it is Cl\(_2\) gas which is liberated in the electrolysis of aqueous NaCl.
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Solution and Explanation
To solve the problem, we need to explain why chlorine gas (Cl₂) is liberated at the anode during the electrolysis of aqueous NaCl, even though oxygen gas (O₂) has a more positive standard electrode potential (\(E^\circ\)).
1. Electrolysis of Aqueous NaCl:
In the electrolysis of aqueous NaCl, two reactions take place at the electrodes: - At the anode (positive electrode), oxidation occurs (loss of electrons). - At the cathode (negative electrode), reduction occurs (gain of electrons). The ions present in aqueous NaCl are: - Na⁺ (sodium ions) - Cl⁻ (chloride ions) - H₂O (water molecules, which dissociate into H⁺ and OH⁻ ions)
2. Standard Electrode Potentials:
We are given that oxygen gas (O₂) has a more positive standard electrode potential compared to chlorine gas (Cl₂). The standard electrode potentials are as follows: - \( E^\circ_{\text{O}_2/\text{H}_2\text{O}} = +1.23 \, \text{V} \) (for the oxidation of water to oxygen) - \( E^\circ_{\text{Cl}_2/\text{Cl}^-} = +1.36 \, \text{V} \) (for the oxidation of chloride ions to chlorine gas) From these values, we see that chlorine gas (Cl₂) has a higher (more positive) electrode potential than oxygen gas (O₂), which suggests that Cl₂ should be favored at the anode.
3. Effect of Concentration and Overpotentials:
The reason chlorine gas (Cl₂) is actually liberated at the anode in the electrolysis of aqueous NaCl, rather than oxygen gas (O₂), can be explained by the following factors:
- Concentration Effect: In aqueous NaCl, the concentration of chloride ions (Cl⁻) is much higher than the concentration of water molecules (H₂O). This higher concentration of Cl⁻ ions means that the chloride ion oxidation reaction is more likely to occur at the anode, as there are more Cl⁻ ions available for oxidation.
- Overpotential: Overpotential refers to the extra voltage required to drive a particular oxidation or reduction reaction at an electrode, beyond the theoretical electrode potential. The overpotential for the evolution of oxygen gas (O₂) is relatively high, which makes the oxidation of chloride ions to chlorine gas (Cl₂) easier and more favorable under normal electrolysis conditions.
4. Final Answer:
Although oxygen gas (O₂) has a more positive standard electrode potential, chlorine gas (Cl₂) is liberated at the anode in the electrolysis of aqueous NaCl because: - Chloride ions (Cl⁻) are present in higher concentration in the solution. - The overpotential for oxygen gas (O₂) is high, making the oxidation of chloride ions to chlorine gas (Cl₂) more favorable. Thus, chlorine gas is preferentially produced at the anode.
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