Question

The metal that cannot be obtained by electrolysis of an aqueous solution of its salts is:

• A. \( \text{Ag} \)
• B. \( \text{Ca} \)
• C. \( \text{Cu} \)
• D. \( \text{Cr} \)

Hint:

Metals with highly negative reduction potentials, such as alkali and alkaline earth metals, cannot be obtained by electrolysis of aqueous solutions. These metals are usually extracted using molten salt electrolysis instead.

Answer 1

The Correct Option is B

Electrolysis of aqueous metal salts to obtain metals is contingent upon the comparison of metal ion reduction potentials against water's reduction potential. If a metal ion's reduction potential is more negative than that of water, water will undergo reduction instead, preventing metal deposition from the aqueous solution.

Step 1: Reduction Potential Comparison.

Consider the reduction potentials for calcium (\( \text{Ca} \)) and water:

• Calcium reduction potential: \[ \text{Ca}^{2+} + 2e^- \rightarrow \text{Ca}, \quad E^\circ = -2.87 \, \text{V}. \]
• Water reduction potential: \[ 2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2\text{OH}^-, \quad E^\circ = -0.83 \, \text{V}. \]

As calcium's reduction potential (\( -2.87 \, \text{V} \)) is significantly more negative than water's (\( -0.83 \, \text{V} \)), the reduction of calcium from its aqueous solution is unfavorable. Consequently, hydrogen gas is produced at the cathode during electrolysis, and no calcium metal is deposited.

Step 2: Evaluation of Other Metals.

Silver (Ag): Possesses a positive reduction potential, facilitating its reduction from aqueous solutions.

Copper (Cu): Also exhibits a positive reduction potential, making its reduction favorable in aqueous solutions.

Chromium (Cr): Reduction of chromium is achievable under specific electrolytic conditions.

Conclusion:

Due to its highly negative reduction potential relative to water, calcium cannot be obtained via electrolysis of its aqueous salt solution. Therefore, \( \mathbf{(B)} \) is the correct answer, identifying \( \mathbf{Ca} \) as the metal unobtainable through electrolysis of its aqueous salt solution.