Problem 223 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: Solubility of group II hydroxides increases down the group. Reason: Decrease in hydration energy is less than the decrease in lattice energy down the group.

Step-by-Step Solution

Answer

The assertion is true, and the reason correctly explains the assertion.

Step 1: Understanding the Assertion

The assertion states that the solubility of Group II hydroxides increases as we move down the group in the periodic table. This means hydroxides like Be(OH)₂ are less soluble compared to Ba(OH)₂.

Step 2: Understanding Lattice and Hydration Energy

Solubility is influenced by lattice energy, which is the energy released when ions are combined to make a compound, and hydration energy, which is the energy released when ions dissolve in water. As we go down the group, the lattice energy decreases because the cations become larger, which weakens the ionic attractions.

Step 3: Analyzing Hydration Energy

The hydration energy also decreases down the group, as larger cations interact less strongly with water molecules than smaller cations do. However, the decrease in lattice energy is greater than the decrease in hydration energy, making it more energetically favorable for the compound to dissolve.

Step 4: Connecting to Solubility Trend

Since the decrease in hydration energy is less significant than the decrease in lattice energy, the overall solubility increases down the group because the compounds become more readily dissociable in water.

Step 5: Conclusion

Both the decrease in lattice energy and the less significant decrease in hydration energy lead to the increased solubility down the group. Therefore, the assertion is true, and the reason correctly explains the assertion.

Key Concepts

Understanding Lattice Energy in Group II Hydroxides

Lattice energy is a key player when it comes to solubility. It's the energy released when ions bind together to form a solid lattice structure. In the case of Group II hydroxides, as you descend the group on the periodic table, the cations (positively charged ions) become larger. This increased size leads to weaker forces holding the ions together because they are spread further apart.

A few aspects about lattice energy:

• Larger ionic size leads to weaker attraction forces between ions.
• Weaker ion interactions result in lower lattice energy.
• Lower lattice energy means the solid is less stable and easier to dissolve in water.

As lattice energy decreases more significantly than hydration energy, the solubility of the hydroxides increases. This change is due to the balance tipping in favor of dissolution, making it easier for the solid to break apart into its ionic components in the solution.

The Role of Hydration Energy in Solubility

Hydration energy refers to the energy released when water molecules surround and interact with ions from a solute as it dissolves. In Group II hydroxides, hydration energy decreases down the group. This occurs because larger cations have a lower charge density, making them interact less effectively with water molecules.

Key points on hydration energy:

• Smaller cations have higher charge density and stronger interaction with water.
• Larger cations, like those lower down the group, interact more weakly.
• Even though hydration energy decreases, it does not decrease as much as lattice energy does.

The decrease in hydration energy is minor compared to the decrease in lattice energy. This difference allows the compound to still benefit favorably in terms of solubility as you move down the group. The larger cations are easier to hydrate compared to the tight lattice structures they form in the solid state.

Periodic Table Trends Impacting Solubility

Periodic table trends provide insights into solubility changes you see in Group II hydroxides. As you move down the group, elements have increasing atomic numbers, leading to more filled electron shells and larger atomic radii. This trend causes several effects on chemical properties, including solubility.

Understanding periodic trends:

• Atomic and ionic size increases down a group.
• Increasing size decreases the electrostatic forces between ions, lowering lattice energy.
• Larger cations form weaker interactions with water, modestly lowering hydration energy.

Overall, the periodic trend of increasing size down the group has a more significant impact on lattice energy than on hydration energy. This shift makes these hydroxides more soluble as you go from Be(OH)₂ to Ba(OH)₂. Simply put, the compounds become easier to break apart and dissolve as the cations get bigger down the group.