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Difficulty: MediumFactors Affecting Preferential Discharge of Ions

Arrange the following anions in order of INCREASING ease of preferential discharge at an inert platinum anode during the electrolysis of dilute solutions, starting from the least easily discharged to the most easily discharged.

  1. 1NO3−NO_3^- (Nitrate ion)
  2. 2Cl−Cl^- (Chloride ion)
  3. 3Br−Br^- (Bromide ion)
  4. 4OH−OH^- (Hydroxide ion)

Answer

The correct sequence of anions in order of increasing ease of discharge at an inert anode is: NO3−NO_3^- followed by Cl−Cl^-, then Br−Br^-, and finally OH−OH^-.
In dilute aqueous solutions using inert electrodes, preferential discharge of anions at the anode is determined by their relative positions in the electrochemical series. The order of increasing ease of discharge (from hardest to easiest) is nitrate (NO3−NO_3^-), chloride (Cl−Cl^-), bromide (Br−Br^-), and hydroxide (OH−OH^-).

Step-by-Step Solution

1
Identify the factor governing preferential discharge
Since the solutions are dilute and electrodes are inert (platinum), preferential discharge depends entirely on the positions of the anions in the electrochemical series.
Concentration effects and electrode nature do not alter the standard relative discharge order in dilute solutions with inert electrodes.
2
Recall the electrochemical series position for anions
The relative positions from highest (hardest to discharge) to lowest (easiest to discharge) are F−<SO42−<NO3−<Cl−<Br−<I−<OH−F^- < SO_4^{2-} < NO_3^- < Cl^- < Br^- < I^- < OH^-.
Anions lower in the series lose electrons (get oxidized) more readily due to lower standard oxidation potentials.
3
Sequence the given anions from least easily discharged to most easily discharged
The correct sequence is NO3−→Cl−→Br−→OH−NO_3^- \rightarrow Cl^- \rightarrow Br^- \rightarrow OH^-.
Nitrate is positioned highest among the listed ions, followed by chloride, then bromide, with hydroxide positioned lowest.

Key Concept

Position of anions in the electrochemical series determines preferential discharge at the anode in dilute solutions.
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Factors Affecting Preferential Discharge of Ions