Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Of the following metals, those that cannot be obtained by the electrolysis of the aqueous solution of their salts are (a) \(\mathrm{Ag}\) and \(\mathrm{Mg}\) (b) \(\mathrm{Ag}\) and \(\mathrm{Al}\) (c) \(\mathrm{Mg}\) and \(\mathrm{Al}\) (d) \(\mathrm{Cu}\) and \(\mathrm{Cr}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Mg}\) and \(\mathrm{Al}\) cannot be obtained by electrolysis of their salts.

Step 1: Understanding Electrolysis Concept

Electrolysis involves passing an electric current through an aqueous solution of a salt, causing chemical reactions at the electrodes and separation of materials. Metals with higher reduction potentials than water (0 V) can be obtained by electrolysis, whereas metals with lower reduction potentials are difficult to obtain.

Step 2: Compare Reduction Potentials

Check the reduction potentials of the metals: \(\mathrm{Ag^+ + e^- \rightarrow Ag} \) (0.80 V), \(\mathrm{Cu^{2+} + 2e^- \rightarrow Cu} (0.34 V)\), \(\mathrm{Mg^{2+} + 2e^- \rightarrow Mg} (-2.37 V)\), \(\mathrm{Al^{3+} + 3e^- \rightarrow Al} (-1.66 V)\), \(\mathrm{Cr^{3+} + 3e^- \rightarrow Cr} (-0.74 V)\). The metals \(\mathrm{Mg}\) and \(\mathrm{Al}\) have negative potentials, indicating they cannot be electrodeposited easily from aqueous solutions.

Step 3: Analyze Options

Review the multiple-choice options based on reduction potentials. Identify which metals can't be deposited from aqueous solutions due to their negative reduction potentials: \((a)\ \mathrm{Ag}\ and \mathrm{Mg}\) - Ag can be deposited, \(\mathrm{Mg}\) cannot. \((b)\ \mathrm{Ag}\ and \mathrm{Al}\) - both Ag can be deposited, \(\mathrm{Al}\) cannot. \((c)\ \mathrm{Mg}\ and \mathrm{Al}\) - both cannot be deposited. \((d)\ \mathrm{Cu}\ and \mathrm{Cr}\) - both can be deposited.

Step 4: Solution Conclusion

Metals \(\mathrm{Mg}\) and \(\mathrm{Al}\) are the ones with negative reduction potentials, indicating they cannot be obtained by electrolysis of their aqueous solutions. Thus, the correct answer is option (c), \(\mathrm{Mg}\) and \(\mathrm{Al}\).

Key Concepts

Reduction Potentials

In electrochemistry, reduction potential, also known as redox potential, measures the tendency of a chemical species to acquire electrons and thereby be reduced. Each metal has a specific reduction potential which indicates how easily it gains electrons during a chemical reaction. This potential is usually measured in volts (V) and compared against the standard hydrogen electrode, which is set at 0 V as a reference. Metals with higher reduction potentials are more easily reduced, meaning they can be more readily converted from ions in a solution back to their metallic form.
In the context of electrolysis, metals that have a reduction potential lower than that of water (0 V) struggle to be extracted from an aqueous solution. This is significant because if a metal ion's reduction potential is lower than water's, water will preferentially be reduced to form hydrogen gas, making it difficult to deposit the metal from the solution.

Aqueous Solutions

An aqueous solution is a solution in which the solvent is water. In electrochemical processes, aqueous solutions are commonly used due to water's excellent solvent properties and ability to dissolve various salts.

• In an aqueous solution, electrolytes are dissolved and dissociate into ions which can carry electric current.
• This ability to conduct electricity is essential for processes like electrolysis.

In electrolysis, metal salts dissolved in water create cations and anions, which are attracted to electrodes of opposite charge when an electric current is passed through. Electrolysis allows the extraction of metals from these solutions, but the outcome heavily depends on the reduction potentials of the metals involved. Consequently, metals like magnesium and aluminum, which have negative reduction potentials, cannot be efficiently obtained from aqueous solutions through electrolysis.

Metal Extraction

Metal extraction refers to the process of obtaining metal in its pure form from its ore, often involving steps like mining, crushing, smelting, or electrolysis. In the case of electrolysis, metal ions in an aqueous solution are separated into their constituent ions by running an electric current through the solution. This causes the metallic ions to migrate to the negative electrode, where they undergo reduction to form metal.
However, the feasibility of extracting metals by electrolysis from aqueous solutions is highly dependent on the metal's reduction potential. Metals with positive reduction potentials are easily extracted since they can be reduced before water. Conversely, metals like magnesium (\(-2.37 V\)) and aluminum (\(-1.66 V\)), with negative reduction potentials, cannot be efficiently extracted via this method due to competition with water reduction.

Electrochemical Series

The electrochemical series is a list of elements ranked by their standard electrode potentials. It provides valuable insight into the reactivity of metals and their tendencies to gain or lose electrons in aqueous solutions.

• Metals located higher in the series have lower reduction potentials, and they are more likely to lose electrons and remain in ionic form.
• Conversely, metals lower in the series have higher reduction potentials and are more prone to gain electrons and be reduced to their metallic state.

Understanding the electrochemical series can help predict which metals can be deposited from aqueous solutions using electrolysis. When attempting electrolysis for metal extraction, it's crucial to consult this series to determine which metals can be isolated from the solution based on their position and reduction potential relative to water. Hence, metals like magnesium and aluminum are challenging to extract from aqueous solutions, as indicated by their positions within this series.