Question

Magnesium cannot be obtained by electrolysis of aqueous MgCl$_2$ or MgSO$_4$ because:

• A. Hydrogen is evolved before Mg is deposited
• B. Reduction potential of Mg$^{2+}$ is higher than water
• C. Mg is dissolved in solution
• D. Use of amalgam cathode increases deposition potential

Hint:

Highly reactive metals like Mg, Na, Al are not obtained from aqueous electrolysis.

Answer 1

The Correct Option is A

Concept: In aqueous electrolysis, the species with higher reduction potential is discharged first. Relevant reduction reactions: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \] \[ Mg^{2+} + 2e^- \rightarrow Mg \]

Step 1: Compare electrode potentials.
- Hydrogen evolution potential is more favorable than Mg deposition in aqueous medium - Therefore water gets reduced first

Step 2: Result.
Hydrogen gas evolves at cathode: \[ H_2 \uparrow \] Magnesium cannot deposit. Final Answer: \[ \boxed{\text{Hydrogen is evolved before Mg is deposited}} \]