Question

The metal that cannot be obtained by electrolysis of an aqueous solution of its salt is:
(A) Cu
(B) Cr
(C) Ag
(D) Ca

Answer 1

Hint: Think about the electrochemical series of elements. Compare the standard reduction potentials of given metals with that of water. If the metal has more reduction potential than that of water then only the metal ion in electrolyte will get reduced to form metal. The metal having reduction potential less than water is the answer.

Complete step by step answer:
- Let’s have a look at the options.
- According to the electrochemical series, calcium has the highest oxidation potential followed by chromium, copper and silver. So, calcium will have the least reduction potential than chromium, copper and silver.
- The elements which can be extracted by reduction using electrolysis of its aqueous salt solution should have higher reduction potential than water.
- Water has the standard reduction potential of -0.83V.
- So, the water has the tendency to get reduced in the presence of electricity.
\[2{{H}_{2}}{{O}_{\left( l \right)}}+2{{e}^{-}}\to {{H}_{2(g)}}+2O{{H}^{-}}\]

- The standard reduction potential of calcium is -2.87V which is less than the reduction potential of water so the calcium will have the tendency to be present in oxidized form in aqueous solution.
- Copper, silver and chromium have very high standard reduction potentials of +0.52V, +0.79V and +1.33V.
- Therefore, the metal that cannot be obtained by electrolysis of an aqueous solution of its salt is calcium.
So, the correct answer is “Option D”.

Note: Remember in the electrolysis process, for a metal to get reduced from its salt to metal form, the standard reduction potential of the metal should be greater than the standard reduction potential of water. Otherwise, water will get reduced at the cathode and the salt will be present in the solution as the metal electrode will continue to get oxidized and form metal salt which will be present in the solution.