Question

Which one of the following product is formed at cathode during electrolysis of aqueous sodium chloride solution?

• A. Sodium
• B. Chlorine
• C. Hydrogen
• D. Oxygen

Answer 1

Answer:

C. Hydrogen

Electrolysis of Aqueous Sodium Chloride (Brine)

• Process Overview

  • Electrolysis is a fundamental electrochemical process that utilizes electrical energy to drive non-spontaneous chemical reactions.
  • In the electrolysis of an aqueous solution of sodium chloride (commonly called brine), an electric current is passed through the solution, initiating the decomposition of both water and the dissolved sodium chloride.
  • The setup typically involves an electrolytic cell with two electrodes, the anode (positive) and the cathode (negative), immersed in the electrolyte solution and connected to an external DC power source.

• Components of Brine Solution

  • An aqueous sodium chloride solution contains several species: sodium ions (Na⁺), chloride ions (Cl⁻), water molecules (H₂O).
  • Additionally, due to the autoionization of water, a very small concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) are also present. These minor constituents play a crucial role in determining the products at the electrodes.

• Reactions at the Electrodes

  • During electrolysis, ions migrate towards the electrode of opposite charge.
  • At the Cathode (Negative Electrode) - Reduction Occurs:
    • Possible species that can be reduced at the cathode are Na⁺ ions and H₂O molecules.
    • The standard reduction potential for Na⁺ is: Na⁺(aq) + e⁻ → Na(s) (E° = -2.71 V).
    • The standard reduction potential for water is: 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) (E° = -0.83 V at standard conditions, pH 7).
    • Since water has a significantly less negative (more positive) reduction potential compared to Na⁺, water is preferentially reduced at the cathode.
    • Therefore, the reaction at the cathode is: 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq).
    • This reaction produces Hydrogen gas (H₂) at the cathode and increases the concentration of hydroxide ions (OH⁻) in the solution near the cathode.
  • At the Anode (Positive Electrode) - Oxidation Occurs:
    • Possible species that can be oxidized at the anode are Cl⁻ ions and H₂O molecules.
    • The standard oxidation potential for Cl⁻ is: 2Cl⁻(aq) → Cl₂(g) + 2e⁻ (E° = +1.36 V).
    • The standard oxidation potential for water is: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻ (E° = +1.23 V).
    • Despite water having a slightly lower oxidation potential, chloride ions are preferentially oxidized. This is due to two main reasons: the high concentration of Cl⁻ ions in brine, and the phenomenon of overpotential for oxygen evolution on electrode surfaces, which effectively increases the potential required for water oxidation.
    • Therefore, the reaction at the anode is: 2Cl⁻(aq) → Cl₂(g) + 2e⁻.
    • This results in the formation of Chlorine gas (Cl₂) at the anode.

• Overall Reaction and Major Products

  • The balanced overall chemical equation for the electrolysis of aqueous sodium chloride is: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g).
  • The three primary products obtained from this industrial process are:
    1. Hydrogen gas (H₂): Formed at the cathode.
    2. Chlorine gas (Cl₂): Formed at the anode.
    3. Sodium hydroxide (NaOH): Remains in the solution as Na⁺ ions combine with the OH⁻ ions produced at the cathode, leading to an increase in the pH of the solution.

• Industrial Significance (Chlor-alkali Process)

  • This process is of immense industrial importance and is famously known as the Chlor-alkali process.
  • Hydrogen gas (H₂): Key applications include the synthesis of ammonia via the Haber process, the production of hydrochloric acid, and as a clean-burning fuel.
  • Chlorine gas (Cl₂): Widely used in water purification (as a disinfectant), the production of polyvinyl chloride (PVC), bleaching agents for paper and textiles, and the synthesis of numerous organic and inorganic chemicals.
  • Sodium hydroxide (NaOH): Also known as caustic soda, it is a strong base crucial for manufacturing soaps and detergents, in the paper and pulp industry, textile processing, and alumina production.
  • Different cell technologies are employed for this process, including the Diaphragm cell, Mercury cell, and Membrane cell. Membrane cells are currently favored due to their higher energy efficiency and environmental advantages.