Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions. Identify (E)? \(\mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2} \longrightarrow(\mathrm{A}) \stackrel{\Delta, \mathrm{ZnCl}_{2}}{\longrightarrow}\) \((\mathrm{B}) \stackrel{\Delta}{\longrightarrow}(\mathrm{C})+(\mathrm{D}) \uparrow \stackrel{\mathrm{NaOH}}{\longrightarrow}(\mathrm{E})\) (a) \(\mathrm{NaHCO}_{3}\) (b) \(\mathrm{Na}_{2} \mathrm{O}_{2}\) (c) \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\) (d) \(\mathrm{Zn} \mathrm{CO}_{3}^{2}\)

Step-by-Step Solution

Answer

The compound (E) is sodium zincate, option (c) \( \mathrm{Na}_2 \mathrm{ZnO}_2 \).

Step 1: Understanding the Starting Reaction

The initial reaction is \[ \mathrm{Na}_2 \mathrm{CO}_3 + \mathrm{H}_2 \mathrm{O} + \mathrm{CO}_2 \rightarrow (\mathrm{A}) \]This reaction is a formation of sodium bicarbonate (baking soda), \[ \mathrm{NaHCO}_3 \ (\text{A}) \] The insulation of water and passing of carbon dioxide gas through sodium carbonate leads to the formation of sodium bicarbonate.

Step 2: Reaction with Heat and Catalyst

The next reaction is what happens to compound \( (\mathrm{A}) \) when heated with \( \mathrm{ZnCl}_2 \).\[ \mathrm{2NaHCO}_3 \xrightarrow{\Delta, \mathrm{ZnCl}_2} \mathrm{Na}_2 \mathrm{CO}_3 \ (\text{B}) + \mathrm{CO}_2 + \mathrm{H}_2 \mathrm{O} \]This step regenerates sodium carbonate (\( \mathrm{Na}_2 \mathrm{CO}_3 \)) as compound \( (\mathrm{B}) \).

Step 3: Decomposition by Heating

The compound \( (\mathrm{B}) \), the sodium carbonate, when heated further results in:\[ \mathrm{Na}_2 \mathrm{CO}_3 \xrightarrow{\Delta} \mathrm{Na}_2 \mathrm{O} \ (C) + \mathrm{CO}_2 \ (D) \uparrow \]This step decomposes sodium carbonate into sodium oxide \( \mathrm{Na}_2 \mathrm{O} \) and releases carbon dioxide gas.

Step 4: Reaction with Sodium Hydroxide

Finally, the sodium oxide formed \( \mathrm{Na}_2 \mathrm{O} \) reacts with sodium hydroxide \( \mathrm{NaOH} \):\[ \mathrm{Na}_2 \mathrm{O} + \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{ZnO}_2 \ (\mathrm{E}) \]The product formed in the presence of \( \mathrm{NaOH} \) is sodium zincate \( \mathrm{Na}_2 \mathrm{ZnO}_2 \).

Step 5: Identifying the Compound (E)

Compound \( (\mathrm{E}) \) corresponds to \( \mathrm{Na}_2 \mathrm{ZnO}_2 \), which is present in option (c).Therefore, the correct answer is option (c) \( \mathrm{Na}_2 \mathrm{ZnO}_2 \).

Key Concepts

Sodium Bicarbonate Formation

We start with the formation of sodium bicarbonate, which is a critical step in many chemical processes. When sodium carbonate \( \text{Na}_2\text{CO}_3 \) is mixed with water \( \text{H}_2\text{O} \) and carbon dioxide \( \text{CO}_2 \), it undergoes a series of transformations. This combination leads to the creation of sodium bicarbonate, commonly known as baking soda. This reaction is represented as:

• \( \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{NaHCO}_3 \)

Sodium bicarbonate is an important compound with a variety of uses, from baking to cleaning and even in fire extinguishers. It forms when the bicarbonate ion \( \text{HCO}_3^- \) interacts with sodium ions present in \( \text{Na}_2\text{CO}_3 \). The process effectively stabilizes the added carbon dioxide in a solid form as \( \text{NaHCO}_3 \), making it an essential reaction for environmental applications where gas capture is valuable.

Decomposition Reactions

Understanding the principle of decomposition is essential in chemistry. In the case of sodium bicarbonate \( \text{NaHCO}_3 \), it undergoes decomposition when heated. Initially, when sodium bicarbonate is heated in the presence of \( \text{ZnCl}_2 \) (acting as a catalyst), it decomposes back into:

• \( 2\text{NaHCO}_3 \xrightarrow{\Delta, \text{ZnCl}_2} \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \)

The second decomposition occurs as sodium carbonate \( \text{Na}_2\text{CO}_3 \) further breaks down upon heating:

• \( \text{Na}_2\text{CO}_3 \xrightarrow{\Delta} \text{Na}_2\text{O} + \text{CO}_2 \)

These reactions are part of a class of chemical reactions where a complex substance breaks down into simpler parts. Here, energy in the form of heat triggers these transformations, releasing carbon dioxide gas as a byproduct. This fundamental chemical reaction type is notable in industrial applications, where materials are recycled or broken down into usable components under controlled conditions.

Sodium Zincate Formation

The final step introduces sodium hydroxide \( \text{NaOH} \), which reacts with the sodium oxide \( \text{Na}_2\text{O} \) formed from the decomposition process. This reaction creates sodium zincate:

• \( \text{Na}_2\text{O} + \text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 \)

Sodium zincate, \( \text{Na}_2\text{ZnO}_2 \), is part of a family of compounds known as zincates. These are typically formed when zinc or zinc compounds react with strong bases like sodium hydroxide. The process illustrates how chemical transformations can link simple compounds to form more complex materials. This ability to synthesize new materials under specific conditions highlights the versatility and adaptability of chemical reactions to meet different industrial and commercial needs. Sodium zincate, and similar compounds, have potential applications in the field of electroplating, where they provide protective metallic coatings on surfaces.