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Difficulty: EasyFactors Affecting Preferential Discharge of Ions

During the electrolysis of an aqueous solution containing equal concentrations of cations using inert carbon electrodes, ions migrate to the cathode where discharge occurs based on their position in the electrochemical series. Arrange the following cations in increasing order of their ease of preferential discharge at the cathode (from the least easily discharged to the most easily discharged):

  1. 1Potassium ion (K+K^+)
  2. 2Zinc ion (Zn2+Zn^{2+})
  3. 3Hydrogen ion (H+H^+)
  4. 4Copper(II) ion (Cu2+Cu^{2+})

Answer

The correct sequence from least easily discharged to most easily discharged is Potassium ion (K+K^+), Zinc ion (Zn2+Zn^{2+}), Hydrogen ion (H+H^+), and Copper(II) ion (Cu2+Cu^{2+}).
In electrolysis at inert electrodes under equal concentrations, the primary factor determining preferential discharge of cations at the cathode is their position in the electrochemical series. Cations positioned lower in the series gain electrons more easily (have higher reduction potentials). Therefore, Potassium ion (K+K^+) is the hardest to discharge, followed by Zinc ion (Zn2+Zn^{2+}), Hydrogen ion (H+H^+), and finally Copper(II) ion (Cu2+Cu^{2+}), which is the most easily discharged.

Step-by-Step Solution

1
Recall the electrochemical series position for cations
The order of cations from top (most reactive / hardest to discharge) to bottom (least reactive / easiest to discharge) is K+K^+, Zn2+Zn^{2+}, H+H^+, Cu2+Cu^{2+}.
Ions lower in the electrochemical series have higher standard reduction potentials and accept electrons more readily at the cathode.
2
Arrange the cations in increasing order of ease of discharge
Potassium ion (K+K^+) < Zinc ion (Zn2+Zn^{2+}) < Hydrogen ion (H+H^+) < Copper(II) ion (Cu2+Cu^{2+}).
Ease of discharge increases down the electrochemical series.

Key Concept

Position of cations in the electrochemical series determines their relative ease of discharge at the cathode.
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Factors Affecting Preferential Discharge of Ions