Question

Which of the following statements is wrong? (1) In Nelson's cell asbestos diaphragm is used because it is permeable to ions and prevents the reaction between \(\mathrm{NaOH}\) and \(\mathrm{Cl}_{2}\). (2) In Castner-Kellner cell, the anode in the middle compartment is mercury. (3) The products of the electrolysis of concentrated aqueous solution of common salt are \(\mathrm{NaOH}, \mathrm{H}_{2}\) and \(\mathrm{Cl}_{2}\). (4) The products formed during the electrolysis of brine at cathode, anode and in the solution are in the mole ratio \(1: 1: 1\).

Short answer

Statement (4) is wrong.

Step-by-step solution

- Understanding Each Statement

Read each statement carefully and understand the context and the components involved. We need to identify the incorrect statement.

- Evaluate Statement (1)

Statement (1): In Nelson's cell asbestos diaphragm is used because it is permeable to ions and prevents the reaction between \(\text{NaOH}\) and \( \text{Cl}_2 \). This statement is true. The asbestos diaphragm in Nelson’s cell indeed allows ions to pass through, preventing the reaction between \( \text{NaOH} \) and \( \text{Cl}_2 \).

- Evaluate Statement (2)

Statement (2): In Castner-Kellner cell, the anode in the middle compartment is mercury. This statement is true. In the Castner-Kellner cell, mercury does indeed serve as the anode in the middle compartment.

- Evaluate Statement (3)

Statement (3): The products of the electrolysis of concentrated aqueous solution of common salt are \( \text{NaOH}\), \( \text{H}_2 \) and \( \text{Cl}_2 \). This statement is true. During electrolysis of brine, concentration of aqueous NaCl produces \( \text{NaOH} \), \( \text{H}_2 \), and \( \text{Cl}_2 \).

- Evaluate Statement (4)

Statement (4): The products formed during the electrolysis of brine at cathode, anode and in the solution are in the mole ratio 1:1:1. This statement appears wrong. During the electrolysis of brine, the mole ratio of the products formed are not generally in a 1:1:1 ratio as the stoichiometry doesn't support this requirement.

Conclusion

Based on the evaluations, it is clear that Statement (4) is incorrect.

Key concepts

Nelson's cell

Nelson's cell is an apparatus used in the industrial production of sodium hydroxide \(\mathrm{(NaOH)}\) through the electrolytic process. In this cell, brine (a concentrated solution of sodium chloride \(\mathrm{(NaCl)}\)) undergoes electrolysis. A key component of Nelson's cell is the asbestos diaphragm.
The diaphragm is essential because it allows ions to pass through, but it prevents mixing of the products formed at the anode and cathode.
This is important because it stops chlorine \(\mathrm{(Cl_2)}\) and sodium hydroxide from reacting with each other to form unwanted by-products.

To summarize, Nelson's cell features:

• An asbestos diaphragm that ensures selective ion permeability.
• The ability to prevent reactions between different electrochemical products.

Understanding Nelson's cell is crucial for grasping the production methods of common industrial chemicals like \(\mathrm{NaOH}}\) and \(\mathrm{Cl_2}}\).

Castner-Kellner cell

The Castner-Kellner cell is another important device used for the electrolysis of brine.
One notable feature of this cell is the presence of mercury, which serves as the anode in the middle compartment.
Mercury acts as a liquid electrode, forming an amalgam (a mercury mixture) with the produced sodium.
This process separates sodium from the brine solution effectively.

The key aspects of a Castner-Kellner cell are:

• Use of a mercury anode that forms an amalgam with sodium.
• Effective separation of sodium from brine.

The amalgam eventually reacts with water to produce pure sodium hydroxide \(\mathrm{(NaOH)}\) and hydrogen \(\mathrm{(H_2)}\).
The chlorine \(\mathrm{(Cl_2)}\) is produced at a different electrode, and this separation is important to avoid unwanted side reactions.

Products of Electrolysis

Electrolysis of brine results in the formation of three main substances: sodium hydroxide \(\mathrm{(NaOH)}\), chlorine \(\mathrm{(Cl_2)}\), and hydrogen \(\mathrm{(H_2)}\). Each product is formed at different electrodes.

Here's a breakdown of where each product forms:

• At the cathode (negative electrode), water is reduced to produce hydrogen gas \(\mathrm{(H_2)}\).
• At the anode (positive electrode), chloride ions are oxidized to form chlorine gas \(\mathrm{(Cl_2)}\).
• In the solution, sodium ions combine with hydroxide ions to form sodium hydroxide \(\mathrm{(NaOH)}\).

The stoichiometry and setup of the electrolysis process mean these products are not always in a 1:1:1 ratio.
It's essential to understand this to avoid misconceptions, like thinking the production ratios are evenly split.

Electrochemical cells

Electrochemical cells are the core of any electrolysis process, converting chemical energy into electrical energy or vice versa.
They are broadly classified into galvanic cells (which produce electricity from chemical reactions) and electrolytic cells (which use electrical energy to drive chemical reactions).
In the context of brine electrolysis, we are dealing with an electrolytic cell.

Key points about electrochemical cells:

• They consist of two electrodes: an anode and a cathode.
• In electrolytic cells, electrical energy is supplied to drive non-spontaneous reactions.

The applied current causes ions in the solution to move towards the corresponding electrodes, resulting in desired chemical transformations.
Electrochemical cells are fundamental to various industrial and scientific applications, making their study very important for chemistry students.