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 Option (a)

This option states that \(\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{NaHCO}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is a mineral called trona. This is true, as trona is a naturally occurring mineral compound consisting of sodium carbonate, bicarbonate, and water as \(2\) water of hydration.

Step 2: Inspect Option (b)

Option (b) claims that both \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) and \(\mathrm{MgCO}_{3}\) are thermally stable. \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) decomposes on heating to form \(\mathrm{Li}_{2} \mathrm{O}\) and \(\mathrm{CO}_{2}\) and is thus not thermally stable. Similarly, \(\mathrm{MgCO}_{3}\) also decomposes upon heating to form \(\mathrm{MgO}\) and \(\mathrm{CO}_{2}\). Thus, neither of these carbonates are considered thermally stable.

Step 3: Evaluate Option (c)

Here, it is stated that \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) is only sparingly soluble in water and that no \(\mathrm{LiHCO}_{3}\) has been isolated. Both parts of this statement are correct: \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) has low solubility in water, and lithium bicarbonate is not known to exist in a solid state and is typically found only in solution if at all.

Step 4: Consider Option (d)

This statement suggests that \(\mathrm{K}_{2} \mathrm{CO}_{3}\) cannot be produced using the ammonia-soda process. This is because the solubility differences required for the precipitation of \(\mathrm{KCl}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\) in the ammonia-soda process do not favor \(\mathrm{K}_{2} \mathrm{CO}_{3}\) formation, making this statement correct.

Key Concepts

Thermal stability of carbonates

Inorganic chemistry often examines the thermal stability of carbonates, which refers to their ability to remain chemically unchanged upon heating. When a carbonate is thermally stable, it does not decompose easily when heated. However, most carbonates, like lithium carbonate \( \mathrm{Li}_2 \mathrm{CO}_3 \) and magnesium carbonate \( \mathrm{MgCO}_3 \), do decompose upon heating.- **Lithium Carbonate (\( \mathrm{Li}_2 \mathrm{CO}_3 \))**: Decomposes to form lithium oxide (\( \mathrm{Li}_2 \mathrm{O} \)) and carbon dioxide (\( \mathrm{CO}_2 \)).- **Magnesium Carbonate (\( \mathrm{MgCO}_3 \))**: Breaks down to magnesium oxide (\( \mathrm{MgO} \)) and carbon dioxide.Thermal decomposition reactions indicate instability. This means that if you heat a carbonate like \( \mathrm{Li}_2 \mathrm{CO}_3 \) or \( \mathrm{MgCO}_3 \), it transforms into another compound and releases \( \mathrm{CO}_2 \).

Solubility of lithium carbonate

Solubility is a crucial factor in chemistry, determining how much of a substance can dissolve in a solvent at a given temperature. Lithium carbonate \( \mathrm{Li}_2 \mathrm{CO}_3 \) is only sparingly soluble in water.- This low solubility means that only a small amount of \( \mathrm{Li}_2 \mathrm{CO}_3 \) can dissolve in water to form a solution.- Consequently, \( \mathrm{Li}_2 \mathrm{CO}_3 \) often forms a precipitate if too much is added to water.Furthermore, lithium bicarbonate (\( \mathrm{LiHCO}_3 \)) does not exist in a solid state as it can't be isolated. It indicates a lack of stability in forming a distinct solid compound under ordinary conditions.

Ammonia-soda process

The ammonia-soda process, also known as the Solvay process, is an industrial method for producing sodium carbonate \( \mathrm{Na}_2 \mathrm{CO}_3 \). This process relies on the reaction of ammonia, carbon dioxide, and brine (saltwater) to create sodium carbonate.- Sodium bicarbonate \( \mathrm{NaHCO}_3 \) precipitates from the solution and can be heated to form sodium carbonate.- This process is efficient for sodium but doesn't work for potassium carbonate \( \mathrm{K}_2 \mathrm{CO}_3 \). The solubility characteristics of potassium compounds prevent them from separating optimally using this method. Thus, ammonium chloride remains soluble while \( \mathrm{K}_2 \mathrm{CO}_3 \) would not precipitate effectively.

Trona mineral

Trona is a naturally occurring mineral with an interesting chemical composition. It is a double salt composed of sodium carbonate, sodium bicarbonate, and water of hydration:- **Chemical Formula**: \( \mathrm{Na}_2 \mathrm{CO}_3 \cdot \mathrm{NaHCO}_3 \cdot 2 \mathrm{H}_2 \mathrm{O} \)Trona is an important source of sodium carbonate, which is essential in various industrial applications.- **Application of Trona**: Used in manufacturing glass, paper, detergents, and textiles.As a key mineral, trona plays a significant role in these industries, owing to its unique properties and availability.

Chemical decomposition

Chemical decomposition refers to the breakdown of a single compound into two or more simpler substances. This is a crucial concept in understanding reactions, especially in inorganic chemistry. It often involves the application of heat.For example, in carbonates:- **Decomposition Reaction**: \( \mathrm{Li}_2 \mathrm{CO}_3 \) decomposes on heating to \( \mathrm{Li}_2 \mathrm{O} \) and \( \mathrm{CO}_2 \).This kind of reaction is endothermic, meaning it needs energy input to proceed.- **Significance**: Decomposition reactions are essential in processes like calcination, which is used in metallurgy and the production of materials like cement.Understanding chemical decomposition can aid in controlling industrial processes and predicting compound behavior in varying conditions.