Problem 40 - A Complete Resource Book in Chemistry for JEE Main

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)

Step-by-Step Solution

Answer

(c) High lattice energy.

Step 1: Understanding Solubility in Water

To determine why a compound like \(\mathrm{BaSO}_{4}\) is insoluble in water, we need to understand the concepts of lattice energy and hydration energy. Lattice energy is the energy required to separate the ions of the compound, while hydration energy is the energy released when ions interact with water molecules.

Step 2: Evaluating Lattice Energy

BaSO_4 has a high lattice energy because it forms strong ionic bonds due to the large charge and the small size of its ions. This high energy makes it difficult for water to break these bonds.

Step 3: Analyzing Hydration Energy

Even though \(\mathrm{BaSO}_{4}\) ions could interact with water molecules, the hydration energy it releases is relatively low compared to the lattice energy. This means not enough energy is released during hydration to overcome the strong bonds between ions.

Step 4: Finalizing the Conclusion

Since the high lattice energy of \(\mathrm{BaSO}_{4}\) is not compensated by sufficient hydration energy, \(\mathrm{BaSO}_{4}\) remains insoluble in water because water cannot effectively separate the ions.

Key Concepts

Lattice Energy

Lattice energy is a vital concept in understanding why some ionic compounds, like \( \mathrm{BaSO}_{4} \), are insoluble in water. It refers to the energy required to break apart the ions in an ionic compound and convert them into a gas. Consider lattice energy like the glue holding the ions together in a crystal lattice structure. The stronger this glue, the more energy is required to break it. Key factors influencing lattice energy include:

• Charge of the Ions: Higher charges result in a stronger attraction between ions. For \( \mathrm{BaSO}_{4} \), barium (Ba) has a +2 charge and sulfate (SO₄) has a -2 charge, leading to a very strong bond between them.
• Size of the Ions: Smaller ions can pack closely together, resulting in stronger interactions. Barium and sulfate ions are quite compact, further increasing the lattice energy.

In \( \mathrm{BaSO}_{4} \), the high lattice energy means a substantial amount of energy would be needed to separate these ions, making it hard for water to dissolve the compound.

Hydration Energy

When an ionic compound dissolves in water, its ions interact with the water molecules, releasing energy, termed as hydration energy. This energy is crucial because it works against lattice energy to break the ionic bonds and facilitate solubility. For \( \mathrm{BaSO}_{4} \), the hydration energy is relatively low when compared to the lattice energy.

• Interaction with Water Molecules: Although barium and sulfate ions can interact with water, the resultant energy from this interaction isn't substantial enough.
• Balancing Lattice Energy: For a compound to dissolve, hydration energy must offset or overcome the lattice energy. In the case of \( \mathrm{BaSO}_{4} \), the low hydration energy cannot balance out the high lattice energy, thereby preventing dissolution.

Hence, even though some energy is released during the hydration of \( \mathrm{BaSO}_{4} \), it's insufficient to break the ionic lattice apart.

Ionic Bonds

Ionic bonds form when there is an electrostatic attraction between positively and negatively charged ions. This type of bond is typically strong, especially in compounds like \( \mathrm{BaSO}_{4} \).In \( \mathrm{BaSO}_{4} \), the ionic bond lies between barium ions (\( \text{Ba}^{2+} \)) and sulfate ions (\( \text{SO}_{4}^{2-} \)).

• Electrostatic Attraction: The strong attraction stems from the charge difference between the ions. Here, each \( \text{Ba}^{2+} \) ion is strongly attracted to the opposite charge of the \( \text{SO}_{4}^{2-} \) ion, forming a robust bond.
• Crystal Lattice Formation: These bonds create a tightly packed lattice, contributing to the compound's high lattice energy. This tight lattice makes it difficult for water to pull the ions apart.

Thus, the strength of the ionic bonds in \( \mathrm{BaSO}_{4} \) plays a significant role in its insolubility, as water cannot effectively disrupt these bonds to dissolve the compound.