Examkin

Question

Difficulty: MediumIndustrial Applications of Electrolysis

In the industrial Chlor-Alkali process, concentrated sodium chloride solution (brine) is electrolyzed using inert carbon electrodes. Which substance is liberated at the cathode, and what is the reason for its discharge?

  1. Hydrogen gas, because H+H^+ ions are lower than Na+Na^+ ions in the electrochemical series and require less energy to gain electrons.Answer
  2. B
    Sodium metal, because Na+Na^+ ions are present in a significantly higher concentration in brine than H+H^+ ions.
  3. C
    Chlorine gas, because Cl−Cl^- ions migrate to the cathode to undergo reduction.
  4. D
    Oxygen gas, because OH−OH^- ions are preferentially discharged over Cl−Cl^- ions at the cathode.

Answer

Hydrogen gas is liberated at the cathode because hydrogen ions (H+H^+) lie lower in the electrochemical series than sodium ions (Na+Na^+) and are thus preferentially reduced.
In the electrolysis of concentrated aqueous sodium chloride (brine), water provides H+H^+ and OH−OH^- ions alongside Na+Na^+ and Cl−Cl^- ions. At the cathode (the negative electrode), cations Na+Na^+ and H+H^+ compete for discharge. Hydrogen (H+H^+) is much lower than sodium (Na+Na^+) in the electrochemical series, meaning it accepts electrons far more easily. Therefore, hydrogen ions are preferentially reduced to yield hydrogen gas (H2H_2).

Step-by-Step Solution

1
Identify the ions present in the electrolyte
In aqueous NaClNaCl (brine), the ions present are Na+Na^+ and Cl−Cl^- from sodium chloride, and H+H^+ and OH−OH^- from the auto-ionization of water.
Electrolysis of aqueous solutions involves ions from both the solute and the solvent.
2
Determine which ions migrate to the cathode
Cations (Na+Na^+ and H+H^+) migrate to the negatively charged cathode.
Oppositely charged ions are attracted to the electrodes.
3
Apply the principles of preferential discharge at the cathode
H+H^+ is placed much lower than Na+Na^+ in the reactivity/electrochemical series, so 2H++2e−→H2(g)2H^+ + 2e^- \rightarrow H_2(g) occurs.
Ions lower in the electrochemical series gain electrons (are reduced) more readily than ions higher up.

Key Concept

Preferential discharge of cations during electrolysis of aqueous brine in the Chlor-Alkali industry
Rate this question

Topics

Industrial Applications of Electrolysis

More questions from this topic