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

(a) the hydration energy of sodium sulphate is more than its lattice energy

Step 1: Understanding Solubility in Water

Solubility depends on the balance between lattice energy (the energy required to separate ions in a solid) and hydration energy (the energy released when ions interact with water). If the hydration energy is greater than the lattice energy, the substance is likely to be soluble.

Step 2: Analyzing Sodium Sulphate Solubility

Sodium sulphate is soluble in water, which indicates that the hydration energy of sodium sulphate is greater than its lattice energy. This means it releases more energy interacting with water than it requires to break apart its ionic lattice.

Step 3: Understanding Barium Sulphate Solubility

Barium sulphate is sparingly soluble in water, which suggests that its lattice energy is greater than its hydration energy. This means it does not release enough energy when dissolving to overcome the energy needed to break apart its lattice.

Step 4: Evaluating the Options

Option (a) states that the hydration energy of sodium sulphate is more than its lattice energy, which aligns correctly with its solubility in water. Options (b), (c), and (d) do not correctly encapsulate the energetic reasons behind the solubility behaviors of these compounds.

Step 5: Selecting the Correct Option

Based on the factors influencing solubility, the correct choice is (a), which is the only option that accurately reflects the energy relationship responsible for the solubility of sodium sulphate.

Key Concepts

Lattice Energy

Lattice energy plays a crucial role in the solubility of compounds. It refers to the energy required to break the ionic bonds in a solid crystal lattice to liberate individual ions. This energy measurement helps us understand how tightly ions are held together in a crystalline form.

• When a substance has high lattice energy, it means the ions are strongly attracted to each other, making the solid more stable and harder to dissolve.
• If the lattice energy is low, the ionic bonds are weaker, which increases the likelihood of the compound dissolving in a solvent, like water.

The balance between lattice energy and hydration energy is key to determining whether a substance will be soluble or sparingly soluble. In the case of sodium sulphate versus barium sulphate, sodium sulphate's lower lattice energy compared to its hydration energy allows it to dissolve readily in water.

Hydration Energy

Hydration energy is an essential factor when considering the solubility of ionic compounds in water. It is the energy released when ions become surrounded by water molecules. This process helps stabilize the ions in solution, facilitating solubility.

• A high hydration energy means that a large amount of energy is released when the ions hydrate, favoring the dissolution of the solid into the liquid phase.
• Conversely, a lower hydration energy makes it harder for the compound to solubilize, as insufficient energy is released to overcome the lattice energy.

In the context of sodium and barium sulphate, the hydration energy of sodium sulphate surpasses its lattice energy, which is why it readily dissolves in water. On the other hand, barium sulphate's hydration energy is insufficient to overcome its higher lattice energy, leading to its sparing solubility.

Ionic Interactions

Ionic interactions are pivotal in understanding the strength of ionic compounds and their behavior in solutions. These interactions refer to the electrostatic forces holding the positively and negatively charged ions together in a lattice.

• Strong ionic interactions suggest strong ionic bonds, contributing to high lattice energy and thus less solubility in water.
• Weaker ionic interactions imply a lower lattice energy, which potentially increases solubility as the ions separate more easily.

In soluble salts, like sodium sulphate, the ionic interactions are sufficiently weak to be overcome by the hydration process, allowing the ions to disperse in the solution. In the case of barium sulphate, the strong ionic interactions prevent favorable dissolution, keeping the ions in a more stable lattice.