Problem 47 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Which of the following substances: Na, \(\mathrm{Hg}, \mathrm{S}, \mathrm{Pt}\) and graphite can be used as electrodes in electrolytic cells having aqueous solution? (a) \(\mathrm{Na}\), \(\mathrm{Pt}\) and graphite (b) \(\mathrm{Na}\) and \(\mathrm{Hg}\) (c) \(\mathrm{Hg}\), \(\mathrm{Pt}\) and graphite (d) \(\mathrm{Na}\) and \(\mathrm{S}\)

Step-by-Step Solution

Answer

The correct combination of substances that can be used as electrodes in electrolytic cells having aqueous solution is Mercury (Hg), Platinum (Pt), and graphite; hence the answer is (c).

Step 1: Understanding Electrodes in Aqueous Solutions

Identify the properties of suitable electrode materials in aqueous solutions. Good electrodes must be chemically inert (non-reactive) in the electrolytic environment. They should not participate in the electrolysis reaction, which eliminates active metals like sodium (Na) because they react with water.

Step 2: Evaluate Listed Materials as Electrodes

Check the reactivity of the listed substances with water. Sodium (Na) reacts with water and therefore cannot be used as an electrode in aqueous solutions. Mercury (Hg), Platinum (Pt), sulfur (S), and graphite are chemically inert in aqueous solutions. Sulfur, however, is a non-conductor and cannot be used as an electrode.

Step 3: Determine the Correct Combination

Among the non-reactive conductive materials listed, eliminate Sodium (Na) and Sulfur (S) as they do not meet the requirements of a good electrode for aqueous solutions. This leaves us with Mercury (Hg), Platinum (Pt), and graphite as suitable materials.

Key Concepts

Chemical Inertness of Electrodes

When it comes to choosing electrode materials for electrolytic cells, especially those with aqueous solutions, chemical inertness is a crucial feature. An electrode is considered chemically inert if it does not react with the electrolyte or the products of electrolysis during the process.

For instance, Platinum (Pt) and graphite are commonly used electrode materials because they exhibit remarkable chemical inertness. This makes them ideal for use in various electrolysis applications where the stability of the electrode material is essential to prevent contamination of the products or degradation of the electrodes themselves.

On the other hand, materials like Sodium (Na) are incredibly reactive in aqueous environments - they can react with the water to form hydrogen gas and a hydroxide, thereby disqualifying them as suitable electrode materials in this context due to their lack of chemical inertness.

Electrolysis in Aqueous Solutions

Electrolysis in aqueous solutions involves the movement of ions in a solution upon the application of an electrical current. The process can lead to a variety of reactions at the electrodes, which is why the choice of electrode material is significant.

An electrolytic cell with an aqueous solution can decompose the water itself, leading to the evolution of hydrogen and oxygen gases at the respective electrodes. Apart from the chemical inertness of the electrode materials, their ability to facilitate these reactions efficiently without participating in them is essential. Materials such as Mercury (Hg) and Platinum (Pt) perform well in these environments, as they do not react with the water or the other substances present in the solution.

As an educational note, students should be aware that just because an electrode is inert, it doesn't automatically guarantee its effectiveness in all electrolytic cells. The specific reactions and the electrolyte matter greatly in choosing the appropriate electrode material.

Conductivity of Electrode Materials

The conductivity of electrode materials is paramount for their function in electrolytic cells. Conductivity refers to the ability of a material to allow the flow of electrical current, which is necessary for inducing the electrolysis reactions.

While materials such as graphite and Platinum (Pt) are excellent conductors of electricity and also chemically inert, making them ideal for this purpose, materials like Sulfur (S) do not conduct electricity and are therefore unsuitable for use as electrodes despite their inertness.

The choice of a conductive material is not just about facilitating the flow of current, but also about the efficiency of the energy used during electrolysis. Poor conductors can result in significant energy losses, making the process less efficient and more costly. Therefore, the right balance between chemical inertness and high conductivity is essential for optimizing the performance of electrolytic cells.