During the electrolysis of sodium Chloride solution, hydrogen is discharged at the carbon cathode because

Step 1: Analyze the question In the electrolysis of sodium chloride solution (brine), the cathode attracts two cations: Na⁺ (from NaCl) and H⁺ (from the ionization of water). Only one is discharged. We need the reason hydrogen wins.

Step 2: The concept – selective discharge Which ion is discharged depends mainly on:

1. Position in the electrochemical series (the most important factor)
2. Concentration of the ions
3. Nature of the electrode

At the cathode, the ion lower in the electrochemical series is discharged first, because it gains electrons more easily.

Electrochemical series (top to bottom): K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe²⁺, Pb²⁺, H⁺, Cu²⁺, Ag⁺

Hydrogen sits below sodium, so H⁺ is preferentially discharged.

Step 3: Solution Cathode reaction: 2H⁺(aq) + 2e⁻ → H₂(g)

Na⁺ stays in solution, and together with the OH⁻ left behind from water it forms NaOH. (Cl⁻ is discharged at the anode because of its high concentration, giving Cl₂.)

Step 4: Check the options

• A. Wrong. H⁺ is far less concentrated than Na⁺ in brine.
• B. Wrong. Hydrogen is lower, not higher, than sodium.
• C. Correct. Hydrogen is lower than sodium in the electrochemical series, so it is discharged preferentially.
• D. Wrong. Carbon is an inert electrode and does not determine this.
• E. Wrong. The Na⁺ concentration is indeed greater, but that would favor Na⁺ discharge, not explain H₂ discharge.

Answer: C

Tip: For the cathode, remember "lower in the series is discharged first." Only very unreactive metals (Cu, Ag) beat H⁺, and reactive ones (K, Na, Ca) never get discharged from aqueous solution (unless using a mercury cathode, as in the Castner-Kellner cell).