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Difficulty: MediumFactors Affecting Preferential Discharge of Ions

Arrange the following cations in order of INCREASING ease of preferential discharge at an inert cathode during electrolysis, starting from the least easily discharged to the most easily discharged:

  1. 1Sodium ion (Na+Na^+)
  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 Sodium ion (Na+Na^+), Zinc ion (Zn2+Zn^{2+}), Hydrogen ion (H+H^+), and Copper(II) ion (Cu2+Cu^{2+}).
During electrolysis with inert electrodes, cations migrate to the cathode to undergo reduction. The ease with which a cation accepts electrons depends on its position in the electrochemical series; cations situated lower in the series gain electrons more readily than those above them. Since the relative order from top to bottom is Na+Na^+, Zn2+Zn^{2+}, H+H^+, Cu2+Cu^{2+}, the ease of preferential discharge increases in the order: Na+<Zn2+<H+<Cu2+Na^+ < Zn^{2+} < H^+ < Cu^{2+}.

Step-by-Step Solution

1
Identify the relative positions of the cations (Na+Na^+, Zn2+Zn^{2+}, H+H^+, Cu2+Cu^{2+}) in the electrochemical series.
The order from top (most electropositive metal) to bottom is Na+Na^+ > Zn2+Zn^{2+} > H+H^+ > Cu2+Cu^{2+}.
Metals higher up lose electrons more readily, whereas ions lower down accept electrons more readily.
2
Apply the principle of preferential discharge for cations at the cathode.
Cations lower down in the electrochemical series are preferentially reduced over those higher up.
Ions lower in the series have more positive standard reduction potentials, making reduction thermodynamically more favorable.
3
Arrange the cations in increasing order of ease of discharge.
Sequence: Sodium ion (Na+Na^+), Zinc ion (Zn2+Zn^{2+}), Hydrogen ion (H+H^+), Copper(II) ion (Cu2+Cu^{2+}).
The ease of preferential discharge increases progressively down the electrochemical series.

Key Concept

Position of cations in the electrochemical series determines their relative ease of preferential discharge during electrolysis.
Estimated Time:1m 30s
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Factors Affecting Preferential Discharge of Ions