Question

The incorrect statement about Castner-Kellner cell process is

• A. Sodium hydroxide is prepared
• B. Brine solution is the electrolyte
• C. Mercury acts as anode and carbon rod acts as cathode
• D. Chlorine gas liberates at anode

Hint:

Remember the electrode definitions: AN-OX (Anode is Oxidation) and RED-CAT (Reduction is at Cathode). In the Castner-Kellner cell, Cl⁻ is oxidized to Cl₂ at the anode, and Na⁺ is reduced to Na at the mercury cathode.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Castner-Kellner process is an electrolytic method used for the large-scale production of very pure sodium hydroxide (NaOH) from brine (concentrated aqueous NaCl solution). It uses a mercury cathode.
Step 2: Analyzing the Process and Statements:
Let's review the key features of the Castner-Kellner cell:
- Overall Goal: To produce NaOH. So, statement (A) is correct.
- Electrolyte: The starting material is a saturated solution of sodium chloride, known as brine. So, statement (B) is correct.
- Anode: The anode is typically made of graphite or titanium. Here, chloride ions are oxidized to produce chlorine gas.
\[ 2Cl^-(aq) \rightarrow Cl_2(g) + 2e^- \] This means statement (D) is correct.
- Cathode: The cathode is a flowing layer of mercury (Hg). Here, sodium ions are reduced (not water, due to the high overpotential of hydrogen on mercury) to form sodium metal, which immediately dissolves in the mercury to form sodium amalgam.
\[ Na^+(aq) + e^- \xrightarrow{Hg} Na(Hg) \] - The sodium amalgam then flows into a separate chamber (a denuder) where it reacts with water to form NaOH, hydrogen gas, and pure mercury, which is recycled.
\[ 2Na(Hg) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g) + 2Hg(l) \] Step 3: Evaluating the Incorrect Statement:
Let's examine statement (C): "Mercury acts as anode and carbon rod acts as cathode".
Based on our analysis, Mercury (Hg) acts as the cathode (where reduction occurs), and the carbon (graphite) rod acts as the anode (where oxidation occurs). The statement has the roles of the electrodes reversed.
Step 4: Final Answer:
Statement (C) is incorrect. Therefore, it is the correct answer to the question.