Concentrated sodium chloride solution is electrolyzed using mercury cathode and graphite anode. The products at the anode and the cathode respectively are

Analyze the question: Brine (concentrated NaCl) is electrolyzed in a mercury cathode cell (the Castner-Kellner cell). We need the product at the anode first, then the cathode.

Concept:

• Anode = positive electrode, where oxidation happens (anions lose electrons).
• Cathode = negative electrode, where reduction happens (cations gain electrons).
• In brine, the ions present are Na⁺, H⁺, Cl⁻, and OH⁻.
• Selective discharge depends on three factors: position in the electrochemical series, concentration, and the nature of the electrode.

Solution:

At the graphite anode: Cl⁻ is in high concentration, so it is discharged in preference to OH⁻: 2Cl⁻ → Cl₂ + 2e⁻ Chlorine gas is produced.

At the mercury cathode: Normally H⁺ would be discharged before Na⁺, since hydrogen is lower in the electrochemical series. However, mercury has a very high overpotential for hydrogen discharge, so H⁺ is not discharged. Na⁺ is discharged instead and dissolves in the mercury to form sodium amalgam: Na⁺ + e⁻ → Na (which forms Na–Hg amalgam) So sodium is the product at the cathode.

(In the next stage of the process, the amalgam reacts with water to give NaOH and H₂, but the question asks only about the electrolysis products.)

Answer: Anode: chlorine. Cathode: sodium. This matches option C.

Tip: Remember "Mercury cathode = Sodium wins." With a mercury cathode, sodium is discharged instead of hydrogen. With a different cathode, such as a steel or graphite cathode in a diaphragm cell, hydrogen is the cathode product (option B would apply there). Always check which electrode the question names.