Problem 49 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Of the following, the metals that cannot be obtained by electrolysis of the aqueous solution of their salts are: [1990 - 1 Mark] (a) \(\mathrm{Ag}\) (b) \(\mathrm{Mg}\) (c) \(\mathrm{Cu}\) (d) \(\mathrm{Al}\) (e) \(\mathrm{Cr}\).

Step-by-Step Solution

Answer

Mg and Al cannot be obtained by electrolysis of their aqueous solutions.

Step 1: Understanding Electrolysis of Aqueous Solutions

In the electrolysis of an aqueous solution, both the cations from the salt and the water can be reduced at the cathode. The cation that is reduced will be the one that requires less energy, or in other words, has a more positive reduction potential.

Step 2: Consult the Electrochemical Series

In the electrochemical series, metals with more negative standard reduction potentials are less likely to be reduced than water. Metals such as Mg, Al, and Cr have highly negative reduction potentials and are therefore not deposited from aqueous solutions using electrolysis because water will reduce before these metals.

Step 3: Identify Metals with Highly Negative Reduction Potentials

Check the reduction potentials: Magnesium \((\mathrm{Mg}^{2+} + 2\mathrm{e}^- \to \mathrm{Mg})\) and aluminum \( (\mathrm{Al}^{3+} + 3\mathrm{e}^- \to \mathrm{Al}) \) have highly negative values, resulting in water being preferentially reduced over these metals when using aqueous solutions.

Step 4: Determine Metals Obtainable by Electrolysis

Silver \( (\mathrm{Ag}^+ + \mathrm{e}^- \to \mathrm{Ag}) \) and copper \( (\mathrm{Cu}^{2+} + 2\mathrm{e}^- \to \mathrm{Cu}) \) have reduction potentials greater than water, making them easier to reduce during the electrolysis of their aqueous solutions.

Step 5: Conclusion

From our analysis, Mg \((\mathrm{Mg})\) and Al \((\mathrm{Al})\) cannot be obtained by electrolysis of aqueous solutions because water is reduced instead of these metals.

Key Concepts

Reduction Potential

Reduction potential is a measure of the tendency of a chemical species to acquire electrons and be reduced. It is an essential concept in electrochemistry, often expressed in volts. Every element has a characteristic reduction potential, which helps determine its behavior during electrolysis.

In electrolysis, the reduction potential is crucial because it influences which species will gain electrons at the cathode. Generally, the more positive the reduction potential, the more likely that species will be reduced. This is because it requires less energy to gain electrons.

For example, in an aqueous solution, both metal cations and water molecules are potential candidates for reduction. The species with a higher (more positive) reduction potential will be reduced first. This explains why not all metals are deposited during electrolysis. Metals like \( ext{Mg}\) and \( ext{Al}\) have very negative reduction potentials. Water, with its higher reduction potential, will get reduced instead, preventing the deposition of these metals.

Aqueous Solutions

An aqueous solution is a homogeneous mixture of water (the solvent) and one or more substances dissolved in it (the solutes). In the context of electrolysis, we often deal with aqueous solutions of metal salts.

When performing electrolysis on these solutions, water plays a significant role as it is abundant and can itself undergo reduction. This is especially important when considering metals with low reduction potentials. In such cases, instead of reducing the metal cation, water is more likely to be reduced to form hydrogen gas.

The presence of water thus complicates the electrolysis process due to the competing reactions at the cathode. The competition arises because both the cation from the salt and the water can accept electrons. Thus, understanding the nature of aqueous solutions helps explain why some metals cannot be effectively extracted from their aqueous salt solutions through electrolysis.

Electrochemical Series

The electrochemical series, also known as the activity series, is a list of elements ordered by their standard reduction potentials. This series is a critical tool for predicting the outcome of electrochemical reactions, like those in electrolysis.

The series provides insights into which species will be reduced or oxidized during a reaction. Elements with higher (less negative/more positive) reduction potentials will typically gain electrons before those with lower potentials when competing in a reaction.

When applied to the electrolysis of aqueous solutions, the electrochemical series helps identify which metal cations can be reduced. For instance, \( ext{Ag}\) and \( ext{Cu}\) have higher reduction potentials than water, making them more easily reduced during electrolysis. In contrast, metals like \( ext{Mg}\), \( ext{Al}\), and \( ext{Cr}\) appear lower in the series with more negative potentials. Thus, in aqueous solutions, these metals are less likely to be reduced than water, illustrating why they cannot be extracted via electrolysis.