Question

\(\mathrm{BaSO}_{4}\) is insoluble in water because of: (a) Low hydration energy (b) Low lattice energy (c) High lattice energy (d) Both (a) and (c)

Short answer

BaSO₄ is insoluble in water due to high lattice energy.

Step-by-step solution

Understanding Solubility

Solubility is influenced by two main factors: lattice energy and hydration energy. Lattice energy is the energy released when ions form a crystal lattice, while hydration energy is the energy released when ions dissolve in water and are surrounded by water molecules.

Defining Lattice and Hydration Energies

For a compound to dissolve in water, its hydration energy must be greater than its lattice energy. High lattice energy means that the compound forms a stable crystal lattice, which is harder to break apart.

Analyzing BaSO_4

Barium sulfate (BaSO_4 ) has a high lattice energy, making the crystal lattice very stable and difficult to break apart. Additionally, its hydration energy is not sufficient to overcome the lattice energy, contributing to its insolubility.

Conclusion on Insolubility

Given that BaSO_4 has a high lattice energy and a relatively low hydration energy, it is insoluble in water primarily due to the high lattice energy. Thus, the correct choice is (c) High lattice energy.

Key concepts

Lattice Energy

Lattice energy is the cornerstone for understanding why certain compounds are hard to dissolve in water.
It refers to the amount of energy released when ions are brought together to form a solid crystal lattice from gaseous ions.
This process is exothermic, meaning energy is released, which contributes to the stability of the solid compound.
For example, in the case of barium sulfate (BaSO), it is known for its high lattice energy.
This high energy indicates that a lot of energy must be overcome for the solid to break apart and go into solution.
The more stable the lattice, the more energy it takes to separate the ions, thus making the compound less likely to dissolve in water.
Lattice energy is often influenced by:

• The charges of the ions: Higher charges result in stronger attractions and thus higher lattice energy.
• The size of the ions: Smaller ions can get closer together and have stronger attractions.

The combination of these factors makes it significantly challenging for the compound to break apart in a solvent like water.

Hydration Energy

Hydration energy is the energy change that occurs when ions from a solid compound are surrounded by water molecules.
This is also an exothermic process, releasing energy as ionic bonds form between the water molecules and the ions.
For a compound to dissolve, the hydration energy needs to be high enough to offset the lattice energy.
This means the energy released when water surrounds the ions must be greater than the energy used to break the ions apart from the crystal.
However, in the case of BaSO, the hydration energy is relatively low compared to its high lattice energy.
Factors affecting hydration energy include:

• The charge of the ions: Higher-charge ions usually have higher hydration energies due to stronger interactions with water.
• The size of the ions: Smaller ions tend to have higher hydration energies, as they can be surrounded more closely by water molecules.

When the hydration energy is less than the lattice energy, like in BaSO, the compound remains insoluble in water.

Insolubility of Compounds

Understanding why a compound like BaSO is insoluble involves looking at both lattice and hydration energies.
The interplay between these energies dictates whether a compound can dissolve in a solvent like water.
Insolubility arises primarily when the lattice energy is significantly higher than the hydration energy.
This indicates that more energy is required to disrupt the stable crystal lattice than the energy that can be recovered by hydration.
This results in compounds that remain stable and solid rather than mixing into the solution.
Key reasons for this insolubility:

• High lattice energy, due to factors such as high ionic charges and small ionic sizes, stabilizes the solid form.
• Low hydration energy fails to compete, resulting in an inability to break apart the lattice effectively.

Thus, in compounds like BaSO, the balance tips towards retaining the solid form and staying insoluble in water.