Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following reactions: 1\. \(2 \mathrm{Na}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{NaOH}+\mathrm{H}_{2}\) 2\. \(2 \mathrm{NaOH}+\mathrm{Cl}_{2} \longrightarrow \mathrm{NaCl}+\mathrm{NaOCl}+\mathrm{H}_{2} \mathrm{O}\) 3\. \(4 \mathrm{OH}^{-} \longrightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{e}^{-}\) 4\. \(2 \mathrm{Cl} \longrightarrow \mathrm{Cl}_{2}^{2}+2 \mathrm{e}\) In the diaphragm cell used for the electrolysis of brine, the reactions that occur would include (a) \(2,3,4\) (b) \(1,3,4\) (c) \(1,2,3\) (d) \(1,2,4\)

Step-by-Step Solution

Answer

The reactions are 2, 3, and 4, so option (a) is correct.

Step 1: Identify Electrolysis Reactions

Electrolysis of brine primarily involves the decomposition and reactions involving sodium chloride (NaCl, or brine). We must identify which reactions are directly associated with the process of electrolysis.

Step 2: Evaluate Reaction 2

Reaction 2: \(2 \mathrm{NaOH} + \mathrm{Cl}_{2} \rightarrow \mathrm{NaCl} + \mathrm{NaOCl} + \mathrm{H}_{2} \mathrm{O}\) is associated with the electrolysis of brine where chlorine gas is involved. This indicates it can occur in a diaphragm cell.

Step 3: Analyze Reaction 3

Reaction 3: \(4 \mathrm{OH}^{-} \rightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} + 4 \mathrm{e}^{-}\) represents the generation of oxygen gas during electrolysis. Such a reaction typically takes place at the anode during electrolysis and is therefore relevant.

Step 4: Consider Reaction 4

Reaction 4: \(2 \mathrm{Cl}^- \rightarrow \mathrm{Cl}_{2} + 2 \mathrm{e}^{-}\) describes the oxidation of chloride ions to chlorine gas during electrolysis, common in a diaphragm cell.

Step 5: Exclude Reaction 1

Reaction 1: \(2 \mathrm{Na} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_{2}\) does not involve chlorine and does not reflect an electrolysis process, as it does not involve electricity or electrolytic decomposition in the context of brine electrolysis.

Step 6: Conclude the Valid Set

Reactions 2, 3, and 4 involve elements and processes typical of brine electrolysis (especially the diaphragm cell method). Therefore, the valid set of reactions occurring in the diaphragm cell includes 2, 3, and 4.

Key Concepts

Diaphragm Cell

The diaphragm cell is a vital piece of technology used in the industrial process known as the electrolysis of brine. This method is employed to produce key chemicals such as chlorine, hydrogen, and sodium hydroxide, which are essential in various industrial applications.
This cell serves the important function of separating the anode and cathode chambers with a semi-permeable membrane or diaphragm. This separation is crucial to ensure that the products of each electrode do not mix. This prevents undesirable side reactions that could compromise the purity of the final products.

• In the anode chamber, oxidation reactions typically occur where chlorine gas is formed.
• On the cathode side, reduction reactions lead to the production of hydrogen gas and hydroxide ions.

By using a diaphragm cell, the efficiency and safety of the electrolysis process are enhanced, allowing for higher purity of the resultant chemicals.

Oxidation Reactions

Oxidation reactions play a critical role in the electrolysis of brine, especially in its execution within a diaphragm cell. Oxidation involves the loss of electrons by a substance, which in the case of electrolysis, occurs at the anode.
In the brine electrolysis process, the key oxidation reaction involves the transformation of chloride ions to chlorine gas. This reaction can be illustrated by the equation:\[2 ext{Cl}^- ightarrow ext{Cl}_2 + 2 ext{e}^-\]This reaction is important not only because it produces chlorine gas, a valuable industrial chemical, but also because it exemplifies the fundamental principle of oxidation in electrochemical reactions.

• Chloride ions lose electrons and become chlorine gas at the anode.
• This reaction necessitates a flow of electricity to drive the conversion.

Understanding oxidation in this context is critical as it is one-half of the reaction process that allows for the entire system of brine electrolysis to function effectively.

Sodium Hydroxide Production

Sodium hydroxide, often referred to as caustic soda, is a key product of the electrolysis of brine using a diaphragm cell. It is a compound that holds immense industrial significance due to its wide range of applications, from paper manufacturing to soap production.
During electrolysis, sodium ions migrate through the diaphragm towards the cathode compartment, where they react with hydroxide ions forming sodium hydroxide. The presence of the diaphragm is vital; it prevents the chlorine gas formed at the anode from mixing with the hydroxide ions produced at the cathode.

• The process carefully manages the flow of ions across the diaphragm.
• This ensures the formation of NaOH is uncontaminated by chlorine.

Effective sodium hydroxide production through electrolysis relies heavily on the containment and control provided by the diaphragm cell, emphasizing this technology's critical role in chemical manufacturing.

Chloride Ion Oxidation

Chloride ion oxidation is a crucial process in the electrolysis of brine, typically occurring at the anode within a diaphragm cell setup. This step not only emphasizes the electrochemical principles at work but also the economic importance of the products formed.
The oxidation of chloride ions is represented by the reaction:\[2 ext{Cl}^- ightarrow ext{Cl}_2 + 2 ext{e}^-\]This reaction showcases the loss of electrons as chloride ions are converted to chlorine gas. Chlorine gas itself is a significant industrial chemical, extensively used in water treatment, the production of disinfectants, and in the synthesis of various organic compounds.

• Chlorine production via this process highlights the benefit of controlled electrolysis.
• The diaphragm cell ensures separation of product gases to maintain product purity.

By comprehending the mechanism and importance of chloride ion oxidation, one gains a deeper insight into the electrochemical applications driving modern industry.