Problem 57 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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 more 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

(b) The lattice energy of barium sulphate is more than its hydration energy.

Step 1: Understanding Solubility Concepts

Solubility of a compound in water depends on the competition between lattice energy and hydration energy. Lattice energy is the energy required to separate the ions of an ionic compound. Hydration energy is the energy released when ions are solvated by water molecules.

Step 2: Analyzing Sodium Sulphate Solubility

Sodium sulphate dissolves in water because the hydration energy, which is the energy released when water molecules surround and interact with ions, is greater than the energy needed to break the ionic bonds (lattice energy). This means sodium sulphate's hydration energy exceeds its lattice energy, making it soluble.

Step 3: Analyzing Barium Sulphate Solubility

Barium sulphate is sparingly soluble because its lattice energy, the energy needed to break its ionic bonds, is greater than the hydration energy. Thus, the lattice energy makes it difficult for barium sulphate to dissolve in water, classifying it as sparingly soluble.

Step 4: Verifying Against Provided Options

Considering the above analysis, sodium sulphate's solubility is due to its hydration energy being greater than its lattice energy, matching option (a). Barium sulphate's sparing solubility is due to its lattice energy being greater than its hydration energy, matching option (b).

Key Concepts

Lattice Energy

Lattice energy is a fundamental concept when it comes to understanding solubility in ionic compounds. This energy is defined as the amount of energy required to separate the ions in a crystalline ionic compound from each other to an infinite distance, effectively turning the solid into gas-phase ions.
This means it represents the strength of the forces holding the ions together within the compound. Generally, the higher the lattice energy, the stronger the ionic bonds, and thus, the compound will be less likely to dissolve in water.

• An example of a compound with high lattice energy is barium sulphate. Its strong ionic bonds mean more energy is needed to break them, resulting in its low solubility in water.
• Lattice energy is influenced by the size of the ions and the charge on them. Smaller ions with higher charges tend to have higher lattice energies.

This concept helps explain the differing solubility of compounds like sodium sulphate and barium sulphate. With higher lattice energy, a compound is less soluble as it requires more energy to disrupt its structure.

Hydration Energy

Hydration energy, on the other hand, is the energy released when ions from an ionic compound are surrounded by water molecules. This process is also known as solvation. When you dissolve an ionic compound in water, the ions are attracted to the polar water molecules, and they spread out into the solution, releasing this energy.
Hydration energy is a key player in the solubility of ionic compounds. When the hydration energy exceeds the lattice energy required to break the ionic bonds, the compound will dissolve.

• Sodium sulphate is a classic case where hydration energy surpasses lattice energy, leading to high solubility in water.
• The magnitude of hydration energy depends on factors like the size of the ions and their charge. Smaller, highly charged ions have higher hydration energies because they can attract more water molecules closely.

This energy counteracts the lattice energy and is vital in determining whether a compound will dissolve in water.

Ionic Compounds

Ionic compounds consist of positive and negative ions held together by strong electrostatic forces, commonly referred to as ionic bonds. These are often crystalline solids with high melting and boiling points due to the strength of these bonds.
When considering solubility, one must think about the delicate balance between lattice energy and hydration energy. Ionic compounds' ability to dissolve in water largely depends on whether their structure can be overcome by the energy of hydration.

• Sodium sulphate, an ionic compound, dissolves well in water because the energy released from water interaction (hydration) is greater than that required to break the solid crystal structure (lattice).
• Barium sulphate, another ionic compound, remains mostly undissolved as its high lattice energy prevents the dissolution despite the release of hydration energy.

Understanding these dynamics is crucial for predicting and explaining the behavior of various ionic compounds in solutions, providing insight into why some are freely soluble and others are not.