Q7 / 150 · MHT CET 2025 (27 Apr Shift 2) · Chemistry

Which among the following is common for electrolysis of molten NaCl and aqueous NaCl ?

  • Correct answer
    A
    $\mathrm{H}_{2(\mathrm{~g})}$ is liberated at cathode.
  • B
    $\mathrm{Na}_{(\mathrm{s})}$ is collected at cathode.
  • C
    NaCl is converted to $\mathrm{NaOH}_{(\mathrm{aq})}$.
  • D
    $\mathrm{Cl}_2$ gas is liberated at anode.

Solution

Electrolysis of molten and aqueous NaCl share only one common product: chlorine gas evolved at the anode.

For molten NaCl, the only ions present are Na$^+$ and Cl$^-$. Reduction of Na$^+$ occurs at the cathode: Na$^+$ + e$^-$ → Na(s). Oxidation of Cl$^-$ occurs at the anode: 2Cl$^-$ → Cl2(g) + 2e$^-$.

For aqueous NaCl, the system contains Na$^+$, Cl$^-$, H$^+$, and OH$^-$. At the cathode, water is reduced instead of sodium due to its higher reduction potential: 2H2O + 2e$^-$ → H2(g) + 2OH$^-$. At the anode, chlorine is oxidized despite water’s lower oxidation potential, due to overpotential and high chloride concentration: 2Cl$^-$ → Cl2(g) + 2e$^-$.

Both processes liberate Cl2 gas at the anode, making it the common product. Sodium metal and hydrogen gas are cathode products exclusive to molten and aqueous electrolysis respectively, while NaOH formation is unique to the aqueous case.

Hence, the correct answer is option: 'A'.

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