In the extraction of sodium from fused sodium chloride, the anode is made of platinum because

Analysis of the Question

This question tests your understanding of the Downs Cell process, which is the industrial method used for the extraction of sodium metal from molten (fused) sodium chloride ($NaCl$). You are specifically asked why platinum is chosen as the material for the anode.

Concept Explanation

In the electrolysis of fused sodium chloride:

• The Electrolyte: Molten $NaCl$ dissociates into ions: $NaCl \rightarrow Na^+ + Cl^-$.
• The Cathode (Negative Electrode): Sodium ions ($Na^+$) migrate to the cathode, where they gain electrons to form sodium metal ($Na^+ + e^- \rightarrow Na$).
• The Anode (Positive Electrode): Chloride ions ($Cl^-$) migrate to the anode, where they lose electrons to form chlorine gas ($2Cl^- \rightarrow Cl_2 + 2e^-$).

Because chlorine gas is a highly reactive non-metal and a strong oxidizing agent, it can easily corrode many common metals. Therefore, the material chosen for the anode must be chemically inert (unreactive) toward the chlorine gas being produced at its surface.

Step-by-Step Solution

1. Identify the product at the anode: As established, chlorine gas ($Cl_2$) is liberated at the anode.
2. Evaluate the reactivity: If the anode were made of a reactive metal, the chlorine gas would react with the electrode, causing it to dissolve or corrode, which would contaminate the product and destroy the electrode.
3. Why Platinum? Platinum is a noble metal. It is highly resistant to chemical attack and oxidation. Even in the presence of hot, corrosive chlorine gas, platinum remains stable and does not react.
4. Reviewing the options: • Option A is incorrect because sodium is formed at the cathode, not the anode. • Option B is a true statement (chlorine is formed at the anode), but it does not explain why platinum is used as the material. • Option C is irrelevant because sodium is not formed at the anode. • Option D explains the chemical necessity: the anode must be made of a material that does not react with the chlorine gas produced.

Conclusion

The choice of platinum is specifically to prevent the corrosion of the anode by the chlorine gas being evolved.

Final Answer: The correct option is D.

Exam Tip: In electrolysis questions, always remember the mnemonic PANIC: Positive Anode, Negative Is Cathode. Also, remember that in the Downs Cell, the anode is actually made of graphite in industrial practice to save costs, but if platinum is mentioned in a theoretical context, it is because of its chemical inertness toward chlorine.