In Castner-Kellner cell for production of sodium hydroxide :

Options

A. Brine is electrolyzed with Pt electrodes
B. Brine is electrolyzed using graphite electrodes
C. Molten magnesium chloride is electrolysed
D. Sodium amalgam is formed at mercury cathode

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Castner-Kellner Cell**: The Castner-Kellner cell is an electrolytic cell used for the production of sodium hydroxide (NaOH) from molten sodium chloride (NaCl).

2. **Dissociation of Sodium Chloride**: When NaCl is heated and melted, it dissociates into sodium ions (Na⁺) and chloride ions (Cl⁻).

3. **Electrolysis Process**: The cell consists of a cathode and an anode. The cathode is typically made of mercury, which allows for the formation of sodium amalgam.

4. **Reduction at the Cathode**: At the cathode, sodium ions (Na⁺) gain electrons (reduction process) to form sodium metal (Na). This sodium metal then combines with mercury to form sodium amalgam.

5. **Water Reaction**: Water (H₂O) present in the cell also plays a crucial role. It undergoes reduction at the cathode, where it accepts electrons to form hydroxide ions (OH⁻) and hydrogen gas (H₂).

6. **Formation of Sodium Hydroxide**: The sodium metal (from sodium amalgam) reacts with the hydroxide ions (OH⁻) produced from the water to form sodium hydroxide (NaOH).

7. **Overall Reaction**: The overall reaction in the Castner-Kellner cell can be summarized as:
- NaCl (molten) → Na⁺ + Cl⁻
- At cathode: Na⁺ + e⁻ → Na (sodium amalgam)
- At cathode: 2H₂O + 2e⁻ → 2OH⁻ + H₂ (gas)
- Final product: Na + OH⁻ → NaOH

8. **Conclusion**: The Castner-Kellner cell effectively produces sodium hydroxide through the electrolysis of molten sodium chloride, with sodium amalgam formation and water reduction playing key roles.

---