Problem 9 - Chemistry in 30 days

Question

Of the following metals, the one which cannot be obtained by the electrolysis of the aqueous solution of its salt is. (a) \(\overline{\mathrm{Ag}}\) (b) \(\overline{\mathrm{Mg}}\) (c) \(\mathrm{Cu}\) (d) \(\overline{H g}\)

Step-by-Step Solution

Answer

Magnesium (Mg) cannot be obtained by electrolysis of its aqueous solution.

Step 1: Understanding Electrolysis of Aqueous Solutions

In the electrolysis of aqueous solutions, both water and the dissolved salt can be sources of ions. The cations of the dissolved salt and hydrogen ions from water can both be reduced at the cathode.

Step 2: Considering Ion Reduction Potentials

The reduction potential of a metal ion determines its tendency to gain electrons and be reduced. Hydrogen ions have a standard reduction potential of 0 V. Any metal ion with a reduction potential higher than this will be reduced from the solution.

Step 3: Analyzing Each Metal's Reduction Potential

Look up the standard reduction potentials: - Ag: +0.80 V - Cu: +0.34 V - Mg: -2.37 V - Hg: +0.85 V Comparing these to hydrogen's 0 V potential, we see that Ag, Cu, and Hg have higher reduction potentials than hydrogen.

Step 4: Determining the Metal Not Obtainable by Electrolysis

Magnesium has a reduction potential lower than hydrogen, making the reduction of hydrogen ions from water more favorable than magnesium ions from the salt. Therefore, Mg cannot be retrieved from its aqueous solution via electrolysis.

Key Concepts

Reduction Potential

Reduction potential plays a fundamental role in electrolysis, particularly when determining which metal ions can be reduced in an aqueous solution. A reduction potential is a measure of the tendency of a chemical species to gain electrons. This directly relates to its likelihood of undergoing a reduction reaction.

For a given ion, the higher the reduction potential, the more likely it is to be reduced over other substances present in the solution. Reduction potentials are typically measured in volts (V), and each element has a standard reduction potential.

• A higher positive value indicates a stronger tendency to gain electrons.
• It’s crucial to compare reduction potentials to understand which ion will be reduced first.

During electrolysis, the ion with a higher potential than that of hydrogen (0 V) will typically be reduced, as it is more favorable energetically.

Aqueous Solution

An aqueous solution is one where the solvent is water. In the context of electrolysis, this is important because water can also become a participant in the electrochemical reactions. When salts are dissolved in water, they dissociate into their respective ions.

This results in an environment where water’s own hydrogen ions ( H^+ ) can compete with other ions from dissolved salts for reduction. An aqueous solution supports a dynamic and competitive process during electrolysis, where both the cation of the dissolved salt and H^+ ions from water are present for potential reduction at the cathode.

• The presence of water can lead to competing reactions, influencing which ions are reduced.
• Water can act as a source of hydrogen ions in such solutions, adding complexity to the process.

Metal Ion Reduction

Metal ion reduction is a critical concept when understanding which metals can be obtained via electrolysis of aqueous solutions. Reduction refers to the gain of electrons by a molecule, atom, or ion. During electrolysis, metal ions in a solution can receive electrons at the cathode, potentially depositing as solid metal.

However, not all metal ions are easily reduced. The reduction potential of each metal, which indicates the energy required for this electron gain, plays a pivotal role.

• For metals higher than hydrogen in reduction potential (e.g., Ag, Cu, Hg), these ions are more readily reduced.
• Metals lower in reduction potential than hydrogen (such as Mg) face competition from the H^+ ions from water.

Understanding which metal ions are more likely to be reduced is key in predicting the outcomes of electrolytic processes. Magnesium, for instance, is less likely to be reduced due to its low reduction potential, explaining why it cannot be extracted through electrolysis of its aqueous salts.