Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

Sodium sulphate is soluble in water whereas barium sulphate is sparingly soluble because (a) the hydration energy of sodium sulphate is more than its lattice energy (b) the lattice energy of barium sulphate is less than its hydration energy (c) the lattice energy has no role to play in solubility (d) the hydration energy of sodium sulphate is less than its lattice energy

Step-by-Step Solution

Answer

Option (a): The hydration energy of sodium sulfate is more than its lattice energy.

Step 1: Understanding Solubility

Solubility in water depends on the interplay between the lattice energy of the compound and its hydration energy. A compound will generally dissolve if its hydration energy is greater than its lattice energy.

Step 2: Analyzing Sodium Sulfate

For sodium sulfate, its solubility in water suggests that the hydration energy is greater than the lattice energy, making option (a) possible.

Step 3: Analyzing Barium Sulfate

Barium sulfate's sparing solubility indicates that its lattice energy is greater than its hydration energy, so option (b) is ruled out.

Step 4: Elimination of Misleading Choices

Since lattice energy does play a crucial role, option (c) can be eliminated. Option (d) contradicts sodium sulfate's solubility, so it is also incorrect.

Step 5: Conclusion

Given the information, option (a) correctly explains that sodium sulfate is soluble because its hydration energy is greater than its lattice energy.

Key Concepts

Lattice Energy

Lattice energy is the energy released when ions in the gaseous state come together to form a crystalline lattice. It's a measure of the strength of the ionic bonds in a solid. Generally, the larger the charges of the ions, the greater the lattice energy.

• It is influenced by the size and charge of the constituent ions. Smaller ions with higher charges typically have higher lattice energies.
• Lattice energy is always an exothermic process, meaning it releases energy.

Lattice energy plays a crucial role in determining the solubility of ionic compounds in water. If the lattice energy is high, it indicates that the compound has strong ionic bonds, which can make the compound more difficult to dissolve.

Hydration Energy

Hydration energy is the energy released when ions are surrounded by water molecules. In an aqueous solution, the water molecules are attracted to the ions, stabilizing them and helping to pull them into solution.

• Hydration energy is an endothermic process, meaning it releases energy as ions are solvated.
• This energy is crucial in the dissolution process and can be seen as the counterpart to lattice energy.

A compound will likely dissolve if its hydration energy is greater than its lattice energy. This energy difference determines whether the overall interaction with water is energetically favorable.

Sodium Sulphate Solubility

Sodium sulfate is known to be quite soluble in water. This solubility can be attributed to its relatively high hydration energy surpassing its lattice energy. The strong attraction between the water molecules and the sodium and sulfate ions offsets the energy needed to break the ionic bonds.

• Sodium sulfate dissociates into sodium (\( Na^+ \)) and sulfate (\( SO_4^{2-} \)) ions in water.
• The hydration energy released is sufficient to overcome the lattice energy.

This makes dissolving sodium sulfate in water an energetically favorable process, as the interactions with water lead to a net release of energy.

Barium Sulphate Solubility

Barium sulfate is sparingly soluble in water, which means it dissolves only slightly. This limited solubility can be explained by its high lattice energy which exceeds its hydration energy. As a result, the ionic bonds in barium sulfate are not sufficiently overcome when in contact with water.

• The compound does not easily dissociate into barium (\( Ba^{2+} \)) and sulfate (\( SO_4^{2-} \)) ions.
• The strength of the ionic bonds makes dissolving the compound less energetically favorable.

In simple terms, the strong internal ionic forces in barium sulfate do not allow water molecules to solvate the ions effectively, leading to its low solubility.