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Difficulty: Very hardFactors Affecting Preferential Discharge of Ions

An electrolytic cell containing concentrated sodium chloride solution initially utilizes inert platinum electrodes, resulting in gas evolution at both the cathode and the anode. If the platinum cathode is replaced with a liquid mercury cathode while maintaining all other experimental conditions, which product is preferentially formed at the cathode, and what primary factor governs this change?

  1. Sodium metal (as sodium amalgam), governed by the nature of the mercury electrodeAnswer
  2. B
    Hydrogen gas, governed by the position of hydrogen below sodium in the electrochemical series
  3. C
    Sodium metal (as sodium amalgam), governed by the high concentration of sodium ions in the electrolyte
  4. D
    Chlorine gas, governed by the high concentration of chloride ions migrating to the cathode

Answer

Sodium metal (as sodium amalgam), governed by the nature of the mercury electrode
When a mercury cathode is used in the electrolysis of concentrated sodium chloride solution, sodium ions (Na+Na^+) are preferentially discharged over hydrogen ions (H+H^+) because sodium dissolves in mercury to form a stable amalgam (Na/HgNa/Hg). This interaction lowers the discharge energy for Na+Na^+, making the nature of the electrode the determining factor.

Step-by-Step Solution

1
Identify the ions present in concentrated NaCl(aq)NaCl(aq)
Cations: Na+Na^+ and H+H^+; Anions: Cl−Cl^- and OH−OH^-
Water partially ionizes to provide H+H^+ and OH−OH^-, while dissolved NaClNaCl provides Na+Na^+ and Cl−Cl^-.
2
Analyze the baseline reaction at an inert platinum cathode
H+H^+ is preferentially discharged over Na+Na^+ to produce H2(g)H_2(g)
According to the electrochemical series, H+H^+ has a higher reduction potential (is easier to reduce) than Na+Na^+ at inert electrodes.
3
Evaluate the effect of replacing the platinum cathode with a liquid mercury cathode
Na+Na^+ is preferentially discharged over H+H^+, forming sodium amalgam (Na/HgNa/Hg)
The chemical affinity between sodium and mercury lowers the overpotential/discharge potential required to reduce Na+Na^+. Thus, the nature of the electrode overrides the standard position in the electrochemical series.

Key Concept

Factors affecting preferential discharge of ions during electrolysis: Nature of electrode
Estimated Time:1m 30s
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Factors Affecting Preferential Discharge of Ions