Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following orders are correct? (i) Thermal stability \(\mathrm{BeCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (ii) Basic nature \(\mathrm{LiOH}>\mathrm{NaOH}>\mathrm{KOH}>\mathrm{RbOH}>\mathrm{CsOH}\) (iii) Solubility in water \(\mathrm{BeSO}_{4}<\mathrm{MgSO}_{4}<\mathrm{CaSO}_{4}<\mathrm{SrSO}_{4}<\mathrm{BaSO}_{4}\) (iv) Melting point \(\mathrm{NaCl}>\mathrm{KCl}>\mathrm{RbCI}>\mathrm{CsCl}>\mathrm{LiCl}\) (a) (i), (iv) (b) (i), (ii), (iv) (c) (ii), (iii) (d) (i), (ii) (iii), (iv)

Step-by-Step Solution

Answer

The correct orders are (i) and (iv).

Step 1: Analyze order (i): Thermal Stability

Thermal stability generally increases down the group in the carbonate series due to increasing lattice energy and the size of the cation. Hence, the given order (i) \( \mathrm{BeCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) is correct.

Step 2: Review order (ii): Basic Nature

Basic nature generally increases down the group in alkali metal hydroxides as the metallic character increases and the bond between the metal and hydroxide weakens. The given order (ii) \( \mathrm{LiOH} > \mathrm{NaOH} > \mathrm{KOH} > \mathrm{RbOH} > \mathrm{CsOH} \) is incorrect; it should be \( \mathrm{CsOH} > \mathrm{RbOH} > \mathrm{KOH} > \mathrm{NaOH} > \mathrm{LiOH} \).

Step 3: Assess order (iii): Solubility in Water

Solubility in water generally decreases down the group for sulfates due to decreasing lattice energy being overshadowed by the increase in hydration energy. Therefore, the given order \( \mathrm{BeSO}_{4}<\mathrm{MgSO}_{4}<\mathrm{CaSO}_{4}<\mathrm{SrSO}_{4}<\mathrm{BaSO}_{4} \) is incorrect and should be \( \mathrm{BaSO}_{4} < \mathrm{SrSO}_{4} < \mathrm{CaSO}_{4} < \mathrm{MgSO}_{4} < \mathrm{BeSO}_{4} \).

Step 4: Evaluate order (iv): Melting Point

The melting point typically decreases down the group for alkali chlorides as cations increase in size, less lattice energy is released, and bonds are weaker. The given order (iv) \( \mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl} > \mathrm{LiCl} \) is correct since \( \mathrm{LiCl} \) is an exception with a lower melting point due to covalent character.

Key Concepts

Thermal stability

In the study of ionic compounds, thermal stability increases as we move down a group in the periodic table. This is particularly evident in the carbonate series. This stability is predominantly influenced by two factors: lattice energy and the size of the cation.
Lattice energy is the energy released when ions bond to form an ionic solid. In larger cations, like in the case of barium carbonate (\( \mathrm{BaCO}_{3} \)), the lattice energy is greater due to the larger ionic radius. This results in more energy required to decompose the compound, hence greater stability. Conversely, beryllium carbonate (\( \mathrm{BeCO}_{3} \)) has a smaller cation, resulting in less lattice energy and lower thermal stability.
To sum up, as we move down the carbonate group from beryllium to barium, the thermal stability of carbonates progressively increases.

Basic nature of hydroxides

The basic nature of alkali metal hydroxides is an interesting concept where the basicity increases as we move down the group. This trend is primarily due to their increasing metallic character and the weakening of the bond between the metal and the hydroxide ion.
To elaborate, as you move from lithium hydroxide (\( \mathrm{LiOH} \)) to cesium hydroxide (\( \mathrm{CsOH} \)), the atomic number of the metals increases, which increases their metallic character. Additionally, the bond between the alkali metal and hydroxide weakens due to the larger size of the alkali metals, making it easier for the compound to release hydroxide ions in solution. This release of hydroxide ions enhances the basic nature of hydroxides.
Therefore, contrary to order (ii) in the original exercise, \( \mathrm{CsOH} \) exhibits greater basicity than \( \mathrm{LiOH} \), with \( \mathrm{CsOH} > \mathrm{RbOH} > \mathrm{KOH} > \mathrm{NaOH} > \mathrm{LiOH} \).

Solubility of sulfates

When discussing the solubility of sulfates, such as those in group 2 of the periodic table, the trend is a bit different. Generally, the solubility of the sulfates decreases as you move down the group. This behavior is due to the interplay between lattice energy and hydration energy.
The lattice energy, the energy needed to separate the ions, decreases as the size of the ions increases down the group. However, the decrease in lattice energy is outweighed by a greater decrease in hydration energy as cations become larger. Smaller ions like beryllium sulfate (\( \mathrm{BeSO}_{4} \)) have more hydration energy and higher solubility compared to larger ions like barium sulfate (\( \mathrm{BaSO}_{4} \)).
Thus, the correct order for solubility of sulfates should be \( \mathrm{BaSO}_{4} < \mathrm{SrSO}_{4} < \mathrm{CaSO}_{4} < \mathrm{MgSO}_{4} < \mathrm{BeSO}_{4} \), making the initial order in the original exercise incorrect.

Melting points of chlorides

The melting points of alkali metal chlorides generally decrease as we move down the group. This trend is primarily influenced by the increase in size of the cations, which affects lattice energy and bond strength.
Larger cations possess lower lattice energy because the larger size of these ions results in a weaker electrostatic attraction between the ions, requiring less energy to disrupt the crystal lattice. For example, cesium chloride (\( \mathrm{CsCl} \)) has a much lower melting point compared to sodium chloride (\( \mathrm{NaCl} \)).
However, it's worth noting exceptions like lithium chloride (\( \mathrm{LiCl} \)), where the melting point is lower than expected due to the covalent character of the bond. Hence, while \( \mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl} > \mathrm{LiCl} \) is mostly correct, \( \mathrm{LiCl} \)'s lower melting point is an interesting deviation from the norm, due to its specific ionic-covalent balance.