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 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: Understanding the Problem Statement

The task is to determine which of the given statements about certain chemical compounds are correct. Each statement must be evaluated for its accuracy based on chemical knowledge.

Step 1: Evaluating Statement (a)

Statement (a) claims that \(\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot \mathrm{NaHCO}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is called trona. Trona is indeed a mineral known as sodium sesquicarbonate, which fits the given chemical formula. Therefore, statement (a) is correct.

Step 2: Evaluating Statement (b)

Statement (b) suggests that \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) and \(\mathrm{MgCO}_{3}\) are both thermally stable. However, \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) decomposes upon heating to form \(\mathrm{Li}_{2}\mathrm{O}\) and \(\mathrm{CO}_{2}\). Therefore, this statement is incorrect as \(\mathrm{Li}_{2}\mathrm{CO}_{3}\) is not thermally stable.

Step 3: Evaluating Statement (c)

Statement (c) claims that \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) has limited solubility in water and that \(\mathrm{LiHCO}_{3}\) has not been isolated. Both these claims are true: \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) is sparingly soluble and lithium bicarbonate is unstable and cannot be isolated. Hence, statement (c) is correct.

Step 4: Evaluating Statement (d)

Statement (d) asserts that \(\mathrm{K}_{2} \mathrm{CO}_{3}\) cannot be prepared by a process similar to the ammonia-soda process. The ammonia-soda (Solvay) process is specifically tailored for sodium carbonate production due to solubility differences, and it doesn't work for potassium carbonate. Thus, statement (d) is correct.

Key Concepts

Thermal Stability

Thermal stability refers to how a compound withstands heat without decomposing. In ionic compounds, different elements react uniquely when subjected to high temperatures. For instance, while many carbonates decompose upon heating, not all do so at the same rate or temperature.
Take lithium carbonate (\(\text{Li}_2\text{CO}_3\)), for example. Despite being a carbonate, it isn't very stable when heated. It breaks down into lithium oxide (\(\text{Li}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)). This decomposition occurs because lithium ions have small sizes and high charge densities, which make their bonds weaker under heat.
On the other hand, magnesium carbonate (\(\text{MgCO}_3\)) is more thermally stable in comparison. Larger magnesium ions maintain the structural integrity of the compound better under heat. Thus, not all carbonates respond to heat in the same way.

Solubility in Water

The solubility of a compound in water tells us how easily it dissolves. This depends on factors like ionic size, charge, and the nature of bonds within the compound.
Lithium carbonate (\(\text{Li}_2\text{CO}_3\)) is noted for being sparingly soluble in water. This means it does not dissolve well. The reasons include its strongly bonded lattice structure and the relatively low solubility product.
Moreover, lithium bicarbonate (\(\text{LiHCO}_3\)) is unstable in an isolated form. It does not easily form a solid under normal conditions and tends to revert back to carbonates or decompose. This limited solubility and instability impact how lithium salts are used in processes and applications requiring water as a solvent.

Chemical Processes

Chemical processes in chemistry often refer to methods for breaking down or synthesizing compounds. These processes sometimes rely on exploiting solubility differences and reaction mechanics.
For instance, the Solvay process is a well-known technique for producing sodium carbonate (\(\text{Na}_2\text{CO}_3\)), also known as soda ash. It leverages solubility differences and specific reactions, such as with ammonia, to precipitate insoluble bicarbonate forms.
However, the same method isn't efficient for potassium carbonate (\(\text{K}_2\text{CO}_3\)) production. The solubility properties of potassium compounds differ significantly, making the precipitative part of the process ineffective. Thus, understanding solubility and reactivity is crucial in adapting processes for different compounds.

Minerals

Minerals are naturally occurring substances from which many ionic compounds originate. Often, minerals are sources of economically valuable compounds used in various industries.
For example, trona is a vital mineral from which sodium carbonate compounds can be derived. It has the chemical formula \(\text{Na}_2\text{CO}_3 \cdot \text{NaHCO}_3 \cdot 2 \text{H}_2\text{O}\), and serves as a significant source of soda ash. The presence of both sodium carbonate and sodium bicarbonate structures makes it a unique dual-acting mineral.
Understanding these minerals is crucial not only for academic purposes but also for practical extraction and use in industry. They provide essential raw materials for chemical manufacturing, agriculture, and environmental projects, underlining their broad significance.