Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

In the Castner process of extracting sodium, which of the following reaction occurs at anode (a) \(\mathrm{Na}^{+}+\mathrm{e}^{-} \longrightarrow \mathrm{Na}\) (b) \(4 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}+\mathrm{e}^{-}\) (c) \(\mathrm{H}_{2} \longrightarrow 2 \mathrm{H}^{+}+2 \mathrm{e}^{-}\) (d) \(2 \mathrm{Cl}^{2} \longrightarrow \mathrm{Cl}_{2}+2 \mathrm{e}^{-}\)

Step-by-Step Solution

Answer

The reaction occurring at the anode in the Castner process is (b).

Step 1: Identifying the Anode Reaction

In electrochemical cells, oxidation occurs at the anode. Thus, we need to find which reaction involves the loss of electrons, which is typical of oxidation.

Step 2: Analyzing Reaction (a)

The reaction \(\mathrm{Na}^{+} + \mathrm{e}^{-} \longrightarrow \mathrm{Na}\) involves the gain of electrons, not the loss. This is a reduction reaction, and therefore cannot occur at the anode.

Step 3: Analyzing Reaction (b)

The reaction \(4 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} + \mathrm{4e}^{-}\) involves the production of electrons, which means oxidation is occurring. This is a potential candidate for the anode reaction.

Step 4: Analyzing Reaction (c)

The reaction \(\mathrm{H}_{2} \longrightarrow 2 \mathrm{H}^{+} + 2 \mathrm{e}^{-}\) results in the loss of electrons. This is an oxidation reaction and thus can occur at the anode. However, it is specific to hydrogen gas and generally not part of the Castner process.

Step 5: Analyzing Reaction (d)

The reaction \(2 \mathrm{Cl}^{-} \longrightarrow \mathrm{Cl}_{2} + 2 \mathrm{e}^{-}\) results in the loss of electrons, which is oxidation. However, this reaction involves chloride ions and is part of the chlor-alkali process, not the Castner process.

Step 6: Selecting the Correct Reaction

In the context of the Castner process, which produces sodium hydroxide and liberates oxygen, Reaction (b) is the correct one: \(4 \mathrm{OH}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} + \mathrm{4e}^{-}\). This occurs at the anode, where oxygen is released.

Key Concepts

Understanding Electrochemical Cells

An electrochemical cell is a device that converts chemical energy into electrical energy or vice versa. This transformation relies on chemical reactions that occur at two electrodes: the anode and the cathode. In the context of the Castner process, an electrochemical cell facilitates the extraction of sodium, where different reactions take place at these electrodes.

The cell comprises two main components:

• **Anode:** Where oxidation occurs. In the Castner process, oxygen is released at this electrode.
• **Cathode:** Where reduction takes place. Sodium is extracted as metal at this electrode.

To generate energy, electrons travel through an external circuit from the anode to the cathode. These electron movements are key to understanding the redox (reduction-oxidation) reactions happening in the cell.

Anode Reaction in Electrochemical Cells

In an electrochemical cell, the anode is where oxidation reactions occur. Essentially, oxidation is a process where a chemical species loses electrons. This loss of electrons results in the generation of free electrons which then move to the cathode.

The Castner process makes use of such anodes for specific chemical reactions. Commonly:

• Oxygen or other gases might be released at the anode due to the splitting of compounds.
• Identifying the suitable anode reaction—is crucial in these processes, ensuring the optimized function of the electrochemical cell.

In our relevant example, the anode reaction producing oxygen gas involves hydroxide ions (OH⁻) losing electrons, which signifies an oxidation process:

\[ 4 \, \mathrm{OH}^{-} \rightarrow 2 \, \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} + 4e^{-} \]

The Role of Oxidation Reaction

Oxidation reactions are central to the function of electrochemical cells, especially in the Castner process. They are characterized by the loss of electrons from a chemical species, which can be identified as electron production in a chemical equation.

When analyzing potential reactions, it becomes evident that the production of electrons confirms an oxidation process. This is seen clearly in reactions where:

• A substance is transformed into a higher oxidation state.
• Electrons are explicitly shown on the product side, like with hydroxide ions transforming during sodium extraction.

This process at the anode ensures that electrons are supplied for the complete operation of the electrochemical cell. Effective anode reactions drive the overall redox mechanism, without which energy conversion or substance extraction (like the formation of sodium hydroxide) wouldn't be viable.

Extracting Sodium in the Castner Process

The Castner process is specifically designed for the extraction and production of sodium. This industrial method provides a pathway to obtain sodium metal from its compounds, primarily sodium hydroxide.

The pathway operates through an electrochemical setup:

• **Cathodic Reaction:** Sodium ions gain electrons at the cathode, becoming pure sodium metal.
• **Anodic Reaction:** As examined, hydroxide ions lose electrons at the anode.

The efficiency of the Castner process relies heavily on maintaining the balance between these reactions. Sodium is often prized for its reactivity and applications, such as creating other important chemical compounds and applications in various industries. The accurate execution of the Castner process, including the specific anode reaction in question, ensures the successful extraction of this crucial element.