Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

In Solvay process of manufacture of \(\mathrm{Na}_{2} \mathrm{CO}_{3}\), the by products are (a) \(\mathrm{CaCl}_{2}, \mathrm{CO}_{2}, \mathrm{NH}_{3}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}, \mathrm{CO}_{2}\) (c) \(\mathrm{NH}_{4} \mathrm{Cl}, \mathrm{CaO}\) (d) \(\mathrm{CaO}, \mathrm{Na}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

The by-products are NH₄Cl and CaO, option (c).

Step 1: Understanding the Solvay Process

The Solvay process is used to produce sodium carbonate (Na₂CO₃), commonly known as soda ash. This industrial process involves the reaction of sodium chloride (NaCl), ammonia (NH₃), and carbon dioxide (CO₂) in water. The main steps produce sodium carbonate while generating by-products.

Step 2: Identifying the Reactions Involved

In the Solvay process, there are several key reactions: \[\text{NH}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{NH}_4\text{HCO}_3 \\text{NH}_4\text{HCO}_3 + \text{NaCl} \rightarrow \text{NaHCO}_3 + \text{NH}_4\text{Cl} \2 \text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \]Another part of the process involves regenerating ammonia: \[\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2\]The ammonia is recycled with the help of calcium hydroxide (Ca(OH)₂).

Step 3: Determining By-products

From the identified reactions, the main products of the Solvay process include sodium carbonate (Na₂CO₃) and by-products such as ammonium chloride (NH₄Cl) and calcium chloride (CaCl₂). Although CO₂ and NH₃ are involved, they are reused in the cycle rather than as final by-products.

Step 4: Choosing the Correct Answer

After analyzing the production and processes involved, we can determine that the by-products are ammonium chloride (NH₄Cl). This matches option (c) NH₄Cl, CaO, as calcium oxide (CaO) is used in regenerating ammonia, playing an indirect role as a by-product of calcium carbonate decomposition.

Key Concepts

Sodium Carbonate Production

The production of sodium carbonate, also known as soda ash, is predominantly carried out via the Solvay process. This process is a major industrial method for the production of sodium carbonate. It utilizes raw materials such as sodium chloride (common salt), ammonia, and carbon dioxide, and facilitates the formation of sodium carbonate through a series of chemical reactions.
Initially, ammonia is absorbed into a salt solution, and then this mixture is treated with carbon dioxide. This reaction forms ammonium bicarbonate, a key intermediate. When ammonium bicarbonate reacts with the sodium chloride in the solution, sodium bicarbonate precipitates out. This is later heated to yield sodium carbonate, releasing water and more carbon dioxide.
The Solvay process is favored on an industrial scale due to its cost-effectiveness and efficiency, utilizing easily obtainable raw materials and recycling chemicals like ammonia within the reaction scheme.

By-products of Solvay Process

In the course of the Solvay process, several by-products are generated. Understanding these by-products is crucial for anyone studying industrial chemistry.
The primary by-products include:

• Ammonium chloride \((\mathrm{NH}_{4}\mathrm{Cl})\)
• Calcium chloride \((\mathrm{CaCl}_{2})\)

During the reaction, ammonium chloride is formed when sodium bicarbonate is created. Ammonium chloride is crystalline and soluble in water, and it can have various uses in industry.On the other hand, calcium chloride is a result of the reaction involving calcium compounds, particularly when limestone \((\mathrm{CaCO}_{3})\) is converted to quicklime \((\mathrm{CaO})\). This calcium chloride is more of a waste by-product in terms of the main objective of the Solvay process, but it is often used as a drying agent for gases and as a de-icing agent for roads.While not a by-product itself, it's important to understand that both carbon dioxide \((\mathrm{CO}_{2})\) and ammonia \((\mathrm{NH}_{3})\) are repeatedly used rather than disposed of, enhancing the efficiency of the process. These substances contribute to its environmentally friendly characteristic by minimizing waste.

Industrial Chemistry Reactions

Industrial chemistry, particularly processes like the Solvay process, rely on a series of reactions that are interconnected. Understanding these reactions helps us appreciate the complexity and beauty of industrial processes.
For instance, in the Solvay process, an intricate dance of reactions is carefully choreographed:

• Ammonia reacts with carbon dioxide and water to form ammonium bicarbonate.
• This compound then reacts with sodium chloride to yield sodium bicarbonate, precipitating it out.
• Sodium bicarbonate is heated, decomposing into sodium carbonate, while releasing water and carbon dioxide, which are recycled in the system thanks to the continuous regeneration of ammonia.

Another vital aspect is the regeneration step, where lime \((\mathrm{CaO})\) produced from decomposing limestone is used to recover ammonia from ammonium chloride. This cycle is essential, not only to minimize costs by recycling ammonia but also to highlight the process's efficiency.
These reactions are not just steps in a chemical process; they reveal how chemistry enables massive production of crucial materials we often take for granted. They also demonstrate the importance of developing sustainable chemical processes that minimize waste and enhance the productivity of raw materials.