Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Barium carbonate is insoluble in water and is highly stable.
Reason (R) : The thermal stability of the carbonates increases with increasing cationic size.
Choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are true and (R) is the true explanation of (A).
• B. Both (A) and (R) are true but (R) is not the true explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

For Group 2 elements, remember these trends going down the group:
\textbf{Solubility:} Sulphates and Carbonates solubility decreases. Hydroxides solubility increases.
\textbf{Thermal Stability:} Carbonates and Sulphates stability increases.
This is explained by the interplay of lattice energy and hydration energy, and by Fajan's rules regarding polarization.

Answer 1

The Correct Option is A

Step 1: Understanding the Statements
The question presents two statements, an Assertion (A) about the properties of Barium Carbonate (BaCO\(_3\)) and a Reason (R) about a chemical trend for carbonates. We need to evaluate the truthfulness of both statements and determine if (R) correctly explains (A).
Step 2: Analyzing Assertion (A)
Assertion (A) states that Barium carbonate is insoluble in water and is highly stable.
Solubility: Carbonates of alkaline earth metals (Group 2) are generally insoluble in water. As we move down the group, the hydration energy of the cations decreases more rapidly than the lattice energy. For carbonates, the lattice energy does not change significantly because the anion is large. Thus, the solubility of carbonates decreases down the group. BaCO\(_3\) is at the bottom of the group and is indeed insoluble in water.
Stability: Thermal stability refers to the ease of decomposition upon heating. Barium carbonate (BaCO\(_3\)) decomposes at a very high temperature (around 1360\(^{\circ}\)C), making it the most thermally stable carbonate among the alkaline earth metals.
Therefore, Assertion (A) is true.
Step 3: Analyzing Reason (R)
Reason (R) states that the thermal stability of the carbonates increases with increasing cationic size.
This is a correct trend for the carbonates of Group 1 and Group 2 elements. As the size of the cation increases down a group, its polarizing power (ability to distort the electron cloud of the anion) decreases. A smaller cation with a higher charge density will polarize the large carbonate ion (CO\(^{2-}_3\)) more effectively, weakening the C-O bonds within the carbonate ion and making it easier to decompose into the metal oxide and CO\(_2\).
For Group 2: Be\(^{2+}\)<Mg\(^{2+}\)<Ca\(^{2+}\)<Sr\(^{2+}\)<Ba\(^{2+}\) (increasing cationic size).
Thermal Stability Order: BeCO\(_3\)<MgCO\(_3\)<CaCO\(_3\)<SrCO\(_3\)<BaCO\(_3\).
Thus, Reason (R) is true.
Step 4: Connecting Reason (R) and Assertion (A)
Assertion (A) mentions that BaCO\(_3\) is highly stable. Reason (R) states that thermal stability increases with cationic size. Since Barium (Ba\(^{2+}\)) is the largest cation in its group (alkaline earth metals), its carbonate (BaCO\(_3\)) is the most thermally stable. Therefore, the reason correctly explains the high stability part of the assertion.
Final Answer: Both (A) and (R) are true, and (R) is the correct explanation for the stability mentioned in (A).