Examkin

Question

Difficulty: MediumFactors Affecting Preferential Discharge of Ions

During the electrolysis of a very dilute solution of sodium chloride using inert platinum electrodes, which gas is evolved at the anode and what primary factor determines its discharge?

  1. Oxygen gas, because the position of OH−OH^- in the electrochemical series is lower than that of Cl−Cl^-Answer
  2. B
    Chlorine gas, because chloride ions are present in higher concentration than hydroxide ions
  3. C
    Chlorine gas, because chloride ions are lower than hydroxide ions in the electrochemical series
  4. D
    Hydrogen gas, because the nature of the platinum electrode promotes cation oxidation

Answer

Oxygen gas is evolved at the anode because the position of hydroxide ions (OH−OH^-) in the electrochemical series is lower than that of chloride ions (Cl−Cl^-).
In a very dilute solution of sodium chloride, the position of ions in the electrochemical series is the primary factor determining preferential discharge. Because hydroxide ions (OH−OH^-) lie below chloride ions (Cl−Cl^-) in the electrochemical series, OH−OH^- ions are preferentially discharged at the anode to produce oxygen gas.

Step-by-Step Solution

1
Identify all anions migrating to the positive electrode (anode).
The anions present in aqueous NaClNaCl are Cl−Cl^- from sodium chloride and OH−OH^- from the auto-ionization of water.
Negatively charged ions migrate to the anode during electrolysis.
2
Determine which factor governs preferential discharge in this setup.
Because the solution is explicitly stated as very dilute, position in the electrochemical series predominates over concentration.
Ion concentration only overrides standard discharge potential when the concentration of a competing ion is significantly high.
3
Compare the electrochemical series positions of the anions.
OH−OH^- lies below Cl−Cl^- in the electrochemical series, so OH−OH^- is oxidized preferentially to produce oxygen gas: 4OH−→2H2O+O2+4e−4OH^- \rightarrow 2H_2O + O_2 + 4e^-.
Anions positioned lower in the series require less energy to give up electrons.

Key Concept

Factors affecting preferential discharge of ions in electrolysis: position in the electrochemical series vs. ion concentration.
Rate this question

Topics

Factors Affecting Preferential Discharge of Ions