Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

When sodium chloride is electrolyzed in Nelson's cell, hydrogen is liberated at cathode and not sodium, because (a) \(\mathrm{H}^{+}\)does not react with water (b) \(\mathrm{H}^{+}\)has lower electrode potential (c) \(\mathrm{H}^{+}\)has greater mobility than \(\mathrm{Na}^{+}\) (d) \(\mathrm{H}^{+}\)is smaller than \(\mathrm{Na}^{+}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{H}^{+}\) has lower electrode potential.

Step 1: Understanding the Problem

We need to determine why hydrogen is released at the cathode during the electrolysis of sodium chloride in Nelson's cell, rather than sodium. Each given option provides different reasoning related to the behavior of hydrogen ions (\(\mathrm{H}^{+}\)).

Step 2: Analyzing Electrode Potential

The release of a substance at the cathode during electrolysis depends on the electrode potentials. Generally, substances with lower (more negative) electrode potentials are preferentially discharged. Thus, comparing the standard electrode potentials: \(\mathrm{Na}^{+} + e^- \rightarrow \mathrm{Na} \) is approximately \(-2.71\, \text{V} \) and \(2\mathrm{H}^{+} + 2e^- \rightarrow \mathrm{H}_2 \) is \(0\, \text{V} \). \(\mathrm{Na}^{+}\) has a lower electrode potential than \(\mathrm{H}^{+}\), meaning \(\mathrm{Na}^{+}\) should theoretically deposit, but it doesn't happen due to overpotential and the practical conditions of the electrolysis setup.

Step 3: Selecting the Correct Explanation

In practical electrolysis, \(\mathrm{H}^{+}\) is preferentially discharged due to having a more favorable reaction condition than sodium, despite the lower electrode potential of sodium. Thus, the statement (b) \(\mathrm{H}^{+}\) has a lower electrode potential is the correct reasoning since it contrasts the theoretical expectation.

Key Concepts

Sodium Chloride Electrolysis

The process of sodium chloride electrolysis involves breaking down sodium chloride (\( \text{NaCl} \)) into its constituent elements using electrical energy. This process occurs in an electrolytic cell where an electric current is passed through an aqueous solution or molten form of sodium chloride. As the current flows, \( \text{Na}^+ \) ions and \( \text{Cl}^- \) ions migrate toward the cathode and anode, respectively.

• At the cathode, reduction occurs. \( \text{H}^+ \) ions from water are reduced to form hydrogen gas (\( \text{H}_2 \)).
• At the anode, oxidation occurs. \( \text{Cl}^- \) ions are oxidized to form chlorine gas (\( \text{Cl}_2 \)).

This process is energetically demanding but is crucial for producing important substances like chlorine, hydrogen, and sodium hydroxide. It illustrates how chemical industries utilize electricity to drive otherwise non-spontaneous chemical reactions.

Nelson's Cell

Nelson's Cell is a type of electrolytic cell specifically designed for the production of chlorine gas, sodium hydroxide, and hydrogen gas via the electrolysis of brine (concentrated sodium chloride solution).
This design features distinctive components that help optimize the electrolysis process:

• The anode is typically made of titanium and coated with an oxide of ruthenium or iridium. These materials are chosen for their ability to withstand corrosion and efficiently conduct electricity.
• The cathode is commonly made from steel, ensuring it is robust and capable of handling the desired chemical changes.
• An important feature is the asbestos diaphragm separating the anode from the cathode. This diaphragm prevents the products from mixing and allows the selective passage of ions.

Nelson's Cell showcases the tailored engineering needed to maximize the extraction of desired products, leading to its significant role in large-scale industrial applications.

Electrode Potential

Electrode potential, also known as standard electrode potential, is a crucial concept in electrochemistry. It helps predict and understand the outcome of oxidation-reduction reactions. The electrochemical series lists metals and their ions arranged in order of their standard electrode potentials.
The electrode potential can give insights into an element's ability to donate or accept electrons:

• A lower (more negative) electrode potential indicates a substance is more likely to donate electrons, thus being oxidized.
• A higher electrode potential suggests a substance is more likely to accept electrons, thus being reduced.

During the electrolysis of sodium chloride, although sodium (\(\text{Na}^+ \)) has a lower electrode potential than hydrogen (\(\text{H}^+ \)), hydrogen is discharged preferentially. This result is due to the specific conditions within the electrolysis setup where overpotential and other kinetic factors affect the actual discharge process.

Hydrogen Evolution

Hydrogen evolution refers to the generation of hydrogen gas during a chemical reaction. In the context of electrolysis, this occurs at the cathode, where hydrogen ions (\(\text{H}^+ \)) gain electrons and form hydrogen gas (\(\text{H}_2 \)).
This process is vital for several reasons:

• It demonstrates the transformation of electrical energy into chemical energy, a fundamental aspect of electrolysis.
• Hydrogen gas is a valuable product, used in industries for things like fuel production and as a reducing agent.
• Hydrogen evolution is favored over other possible reactions because of the kinetic and thermodynamic conditions within the cell. Even with sodium having a more negative standard electrode potential, hydrogen is still preferentially evolved due to these conditions.

Understanding hydrogen evolution helps clarify why sodium is not deposited on the cathode, despite initial theoretical expectations.