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Difficulty: MediumFactors Affecting Preferential Discharge of Ions

During the electrolysis of concentrated sodium chloride solution (brine) using inert platinum electrodes, chlorine gas is liberated at the anode instead of oxygen gas. Which factor primarily accounts for the preferential discharge of chloride ions over hydroxide ions in this process?

  1. The high concentration of chloride ions relative to hydroxide ions in the solutionAnswer
  2. B
    The lower position of chloride ions relative to hydroxide ions in the electrochemical series
  3. C
    The chemical affinity between the platinum electrode and chloride ions
  4. D
    The greater ionic mass of chloride ions compared to hydroxide ions

Answer

The high concentration of chloride ions relative to hydroxide ions in the solution
Under standard conditions, hydroxide ions are discharged preferentially to chloride ions because hydroxide ions lie lower in the electrochemical series of anions. However, in concentrated brine, the concentration of chloride ions is vastly higher than that of hydroxide ions, causing concentration to become the decisive factor leading to chlorine gas evolution.

Step-by-Step Solution

1
Identify the ions present at the anode during electrolysis of concentrated sodium chloride solution.
The anions attracted to the anode are chloride ions (Cl−Cl^-) and hydroxide ions (OH−OH^-).
Anions migrate to the positively charged anode.
2
Compare the factors governing preferential discharge of these anions.
Based on position in the electrochemical series, OH−OH^- is discharged more easily than Cl−Cl^-. However, because the solution is concentrated, Cl−Cl^- ions far outnumber OH−OH^- ions.
High concentration of a particular ion can overcome its disadvantage in electrochemical series position.
3
Deduce the discharged product and the dominant factor.
Chloride ions (Cl−Cl^-) are discharged to form chlorine gas (Cl2Cl_2) due to the concentration effect.
The concentration factor overrides the standard electrochemical series order in concentrated brine.

Key Concept

Effect of concentration on preferential discharge of ions during electrolysis
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Factors Affecting Preferential Discharge of Ions