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 Concepts

To determine solubility, we must compare the lattice energy and the hydration energy of a compound. If the hydration energy is greater than the lattice energy, the substance is likely to be soluble in water. Conversely, if the lattice energy exceeds the hydration energy, the compound will be less soluble or sparingly soluble.

Step 2: Analyze Sodium Sulphate Solubility

Sodium sulphate is soluble in water, which indicates that its hydration energy is greater than its lattice energy. This is because the ionic bonds in sodium sulphate are broken down easily by water due to strong ion-water interactions.

Step 3: Analyze Barium Sulphate Solubility

Barium sulphate is sparingly soluble, suggesting that its lattice energy is greater than its hydration energy. The strong ionic bonds between the barium and sulphate ions are not sufficiently overcome by interactions with water molecules, making it less soluble.

Step 4: Choose the Correct Option

Based on our analysis, sodium sulphate dissolves well because its hydration energy exceeds the lattice energy (option a), while barium sulphate is sparingly soluble because its lattice energy surpasses its hydration energy (option b).

Key Concepts

Lattice Energy

Lattice energy is an essential concept when considering the solubility of ionic compounds. It refers to the energy required to separate one mole of a solid ionic compound into gaseous ions. Basically, it's a measure of the strength of the forces holding the ions together in a lattice. The greater the lattice energy, the stronger the attraction between the ions.

Here are key points about lattice energy:

• Lattice energy is directly related to the charge and size of the ions. Higher charges and smaller sizes result in greater lattice energy.
• Compounds with high lattice energy tend to be less soluble in water since the energy required to break the ionic bonds is substantial.
• Conversely, compounds with lower lattice energy are often more soluble, as less energy is needed to separate the ions.

Understanding lattice energy is crucial because it helps predict how likely an ionic compound is to dissolve in water based on the strength of the ionic bonds.

Hydration Energy

Hydration energy is another fundamental aspect influencing the solubility of ionic substances. It is the energy released when ions become surrounded by water molecules, forming hydrated ions in the solution. This process helps stabilize the ions in the aqueous environment.

Important points about hydration energy include:

• Hydration energy is influenced by the size and charge of the ions; smaller and more highly charged ions release more energy when hydrated.
• When the hydration energy of a compound exceeds its lattice energy, the compound tends to be soluble because the energy released during hydration can overcome the energy needed to break the ionic lattice.
• Strong hydration indicates a higher attraction between the ions and the water molecules, promoting solubility.

Hydration energy is a significant factor in determining solubility because it reflects how effectively water molecules can integrate ionic compounds into the solvent.

Ionic Bonds

Ionic bonds are the electrostatic forces of attraction between positively charged cations and negatively charged anions. These bonds are fundamental in forming ionic compounds, such as sodium sulphate and barium sulphate.

Understanding ionic bonds is crucial for a few reasons:

• The strength of ionic bonds is determined by lattice energy—the stronger the bond, the higher the lattice energy.
• During dissolution, ionic bonds must be broken for ions to interact with water, a process influenced by both lattice and hydration energies.
• Compounds with weaker ionic bonds typically dissolve more easily, as less energy is required to disrupt the ionic lattice.

Overall, ionic bonds play a critical role in understanding an ionic compound's solubility, as both the ease of breaking these bonds and the energy dynamics involved influence how readily a compound will dissolve in water.