Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

Select the correct statement(s) are: (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{NaHCO}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is a mineral called trona. (b) \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) and \(\mathrm{MgCO}_{3}\) both are thermally stable (c) \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) is only sparingly soluble in water and no \(\mathrm{LiHCO}_{3}\) has been isolated. (d) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) cannot be made by a method similar to the ammonia-soda process.

Step-by-Step Solution

Answer

Statements (a), (c), and (d) are correct.

Step 1: Analyze Statement (a)

Statement (a) states that \(\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{NaHCO}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is a mineral called trona. Trona is indeed composed of sodium carbonate, sodium bicarbonate, and water, matching the chemical composition in the formula. Thus, statement (a) is correct.

Step 2: Analyze Statement (b)

Statement (b) claims that both \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) and \(\mathrm{MgCO}_{3}\) are thermally stable. In reality, \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) decomposes upon heating, while \(\mathrm{MgCO}_{3}\) is more thermally stable than \(\mathrm{Li}_{2} \mathrm{CO}_{3}\). Therefore, statement (b) is incorrect.

Step 3: Analyze Statement (c)

Statement (c) mentions that \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) is only sparingly soluble in water and that no \(\mathrm{LiHCO}_{3}\) has been isolated. It is known that \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) has low solubility and \(\mathrm{LiHCO}_{3}\) tends not to form a stable compound on its own. Thus, statement (c) is correct.

Step 4: Analyze Statement (d)

Statement (d) states that \(\mathrm{K}_{2} \mathrm{CO}_{3}\) cannot be made by a method similar to the ammonia-soda process. The ammonia-soda process is specifically tailored for sodium compounds, and \(\mathrm{K}_{2} \mathrm{CO}_{3}\) typically requires other processes, meaning statement (d) is correct.

Key Concepts

Thermal Stability of Carbonates

In inorganic chemistry, thermal stability of carbonates refers to how a carbonate compound withstands heat without decomposing. For metals, this is an important property. Carbonates decompose into metal oxide and carbon dioxide when heated. The stability is influenced by the metal ion present. For instance:

• Li₂CO₃ (Lithium carbonate): Decomposes at relatively low temperatures.
• MgCO₃ (Magnesium carbonate): More stable and withstands higher temperatures.

This depends on the size and charge of the metal ion. Smaller and higher charged ions tend to form more stable lattices. This explains why MgCO₃ is more stable than Li₂CO₃, as magnesium's 2+ charge and smaller size lead to stronger ionic bonds.

Solubility in Water

Solubility in water indicates how well a substance dissolves in water to form a solution. Lithium carbonate (Li₂CO₃) is only sparingly soluble. This means it dissolves minimally in water. Its solubility is influenced by its ionic structure and the lattice energy. No stable lithium bicarbonate (LiHCO₃) has been isolated, a compound that might form under certain conditions. This lack of isolation is due to its limited stability. Low solubility can affect industrial processes, as not all compounds can be dissolved to be utilized or reacted easily in aqueous solutions.

Ammonia-Soda Process

The ammonia-soda process, also called the Solvay process, is a significant industrial method for producing sodium carbonate (Na₂CO₃). This process involves the reaction of sodium chloride, ammonia, and carbon dioxide in water. It proceeds through several steps:

• Formation of bicarbonate and ammonia gas.
• Precipitation of sodium bicarbonate (NaHCO₃).
• Heating to produce pure sodium carbonate.

While it's effective for sodium carbonate, it doesn't work for potassium carbonate (K₂CO₃) because the different chemical properties and solubilities do not align with the process's conditions, necessitating other methods for its production.

Mineral Trona

Trona is a naturally occurring mineral that serves as a primary source for sodium carbonate. Its composition is given by the formula: \[\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{NaHCO}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}\]This highlights the inclusion of sodium bicarbonate and water within its structure. Trona is extensively mined to produce soda ash (\(\mathrm{Na}_{2}\mathrm{CO}_{3}\)) in commercial quantities. Using trona in industrial applications offers an efficient way to obtain sodium carbonate through relatively simple calcination and purification processes. Its chemical makeup makes it highly valuable as it encapsulates both carbonate and bicarbonate ions, providing versatility in usage.