Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

Sodium sulphate is soluble in water whereas barium sulphate is sparingly soluble because [IIT 1988] (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 and Energy

To understand why compounds are soluble or sparingly soluble in water, we need to compare two types of energy: lattice energy and hydration energy. Lattice energy is the energy released when ions come together to form a crystal lattice. Hydration energy is the energy released when ions interact with water molecules.

Step 2: Sodium Sulphate's Solubility Explanation

For sodium sulphate ( ext{Na}_2 ext{SO}_4), the hydration energy is greater than the lattice energy. This means that the energy gained from soluble interactions with water compensates more than the energy lost from breaking the lattice. Thus, sodium sulphate is soluble in water.

Step 3: Barium Sulphate's Solubility Explanation

For barium sulphate ( ext{BaSO}_4), the lattice energy is greater than the hydration energy. This indicates that breaking the lattice interactions costs more energy than is gained from forming interactions with water. As a result, barium sulphate is only sparingly soluble in water.

Step 4: Connecting to Answer Choices

The explanation in steps 2 and 3 indicates that the correct option attributes solubility differences to the comparison between lattice and hydration energy. Option (b) states that the lattice energy of barium sulphate is more than its hydration energy, which aligns with our explanation for why barium sulphate is sparingly soluble.

Key Concepts

Lattice Energy

Lattice energy is a fundamental concept in chemistry, particularly when studying the solubility of ionic compounds. It is the energy released when oppositely charged ions, like sodium ions and sulfate ions, come together to form a solid crystal lattice from a gaseous state. The stronger the attraction between the ions, the higher the lattice energy will be.

When considering why certain compounds are more soluble than others, lattice energy plays a crucial role. If the lattice energy is very high, it indicates that the ionic bonds within the solid are extremely strong. This makes it challenging for water molecules to break those bonds and dissolve the compound. In other words, high lattice energy makes a compound less likely to dissolve. On the other hand, lower lattice energy suggests weaker ionic bonds which water can break more easily, leading to higher solubility.

Understanding lattice energy helps us to comprehend why compounds like barium sulfate remain largely undissolved in water. Its lattice energy is high, outweighing the benefits of interaction with water molecules, making it sparingly soluble.

Hydration Energy

Hydration energy pertains to the energy released when ions are surrounded by water molecules, a critical factor influencing the solubility of substances. When an ionic compound dissolves in water, each ion becomes surrounded by water molecules in a process called hydration.

The amount of energy released during this process is the hydration energy. Factors such as ion size and charge affect the extent of hydration energy. Smaller and highly charged ions typically have higher hydration energies because they interact more effectively with the water molecules.

• High hydration energy suggests that the interactions between ions and water are strong, favoring solubility.
• When hydration energy surpasses lattice energy, substances like sodium sulfate dissolve readily in water.

In the case of sodium sulfate, the hydration energy is significantly higher than its lattice energy, which results in it being soluble in water. Hence, understanding hydration energy is key to predicting and explaining why some ionic compounds dissolve more easily than others.

Ionic Compounds Solubility

The solubility of ionic compounds is a fascinating topic as it involves a balance between two vital energy concepts: lattice energy and hydration energy. To predict whether a compound will dissolve in water, these two energies are compared.

Solubility essentially boils down to this comparison:

• When hydration energy exceeds lattice energy, the compound is more likely to be soluble. This means that the energy reward from the ions interacting with water compensates for the energy "cost" of breaking apart the ionic lattice.
• Conversely, if lattice energy is greater than hydration energy, the compound tends to be sparingly soluble or insoluble. The energy needed to disrupt the ionic framework is not compensated by the energy gained from hydration.

Let's take sodium sulfate and barium sulfate as examples. Sodium sulfate dissolves easily in water (high solubility) because its hydration energy surpasses its lattice energy. Barium sulfate, however, is sparingly soluble as its lattice energy is much higher than the hydration energy, indicating stronger ionic bonds that water cannot easily separate.
Understanding these energy interactions gives insight into how and why different ionic compounds behave in water, making this a crucial concept in chemistry.