Question

How does the lattice energy of an ionic solid affect its solubility in water? [Section 13.1\(]\)

Short answer

The lattice energy of an ionic solid affects its solubility in water by determining the strength of the electrostatic forces holding the ions together in the crystal lattice. If the lattice energy is high, indicating stronger attractive forces between ions, the solubility will be lower as it is more difficult for the solid to dissolve in water. Conversely, if the lattice energy is low, indicating weaker attractive forces between ions, the solubility will be higher as it is easier for the solid to dissolve in water. The solubility of an ionic solid in water depends on the balance between the lattice energy and the hydration energy, which is the energy released when the ions become surrounded by water molecules. If the hydration energy is greater than or equal to the lattice energy, the ionic solid will have high solubility, while low hydration energy compared to lattice energy results in low solubility.

Step-by-step solution

1. Define lattice energy

Lattice energy is the energy required to separate one mole of an ionic solid into its constituent gaseous ions. It is a measure of the strength of the electrostatic forces holding the ions together in the crystal lattice. A higher lattice energy indicates stronger attractive forces between ions, making it more difficult to separate them.

2. Define solubility

Solubility refers to the maximum amount of a solute (in this case, an ionic solid) that can dissolve in a given amount of solvent (in this case, water) to form a homogeneous solution. Solubility is influenced by factors such as temperature, pressure, and the nature of the solute and solvent.

3. Understand the solvation process

When an ionic solid dissolves in water, the ions in the solid become surrounded by water molecules, which forms a sphere of hydration around the ions. The energy released during this process is called hydration energy. The hydration energy is the sum of the ion-dipole interactions between the ions and the polar water molecules.

4. Examine the relationship between lattice energy and solubility

For an ionic solid to dissolve in water, the attractive forces between the ions in the solid (lattice energy) must be overcome by the attractive forces between the ions and water molecules (hydration energy). If the hydration energy is greater than or equal to the lattice energy, the ionic solid will dissolve in water, and the solubility will be high. On the other hand, if the hydration energy is less than the lattice energy, the ionic solid will not dissolve easily in water, resulting in low solubility.

5. Conclusion

In summary, the lattice energy of an ionic solid affects its solubility in water. If the lattice energy is high, meaning the ionic bonds are very strong, it will be more difficult for the solid to dissolve in water, and solubility will be lower. If the lattice energy is low, meaning the ionic bonds are weaker, it will be easier for the solid to dissolve in water, and solubility will be higher. Therefore, a higher lattice energy generally leads to a lower solubility, while a lower lattice energy leads to a higher solubility in water.

Key concepts

Solubility

Solubility is a key concept in chemistry that explains how different substances dissolve in solvents, like water. When discussing solubility, it's essential to consider how much of a substance, known as the solute, can be dissolved in a given amount of solvent. This forms a solution where everything is evenly mixed.

The maximum amount of solute that can dissolve in a solvent is the solubility. This can be affected by several factors:

• **Temperature**: Generally, increasing temperature increases solubility for solids. However, for gases, higher temperatures often decrease solubility.
• **Pressure**: While pressure influences gas solubility significantly, it's less impactful on solids and liquids.
• **Nature of solute and solvent**: "Like dissolves like," meaning polar solvents dissolve polar solutes well and vice versa for nonpolar substances.

Understanding solubility helps explain why some ionic solids dissolve easily in water, while others do not.

Ionic Solids

Ionic solids are made up of ions that are held together by strong electrostatic forces in a lattice structure. These solids are typically characterized by high melting points and hardness, since a lot of energy is required to overcome the attractions between ions in the lattice.

Ionic solids dissolve in solvent due to interactions between the ions and solvent molecules. The solubility of these solids is heavily influenced by their lattice energy, which is the energy needed to separate these ions within the solid. A higher lattice energy means stronger forces between ions, making it more challenging for them to dissolve. Consequently, if the solvent, like water, has sufficient energy (hydration energy) to break these forces, the ionic solid will dissolve.

The process of dissolving an ionic solid involves replacing the strong ionic interactions with interactions between ions and solvent molecules. This is why understanding ionic solids and their behaviors in solution is essential for predicting solubility.

Hydration Energy

Hydration energy is the energy released when ions from a solid become surrounded by water molecules during the solvation process. This energy is crucial because it helps determine whether an ionic solid will dissolve in water.

When an ionic solid dissolves, the ions are attracted to the polar water molecules. The positive (hydrogen) side of water attracts negative ions, while the negative (oxygen) side attracts positive ions. This interaction forms a hydration shell, stabilizing the ions in solution.
If the hydration energy exceeds the lattice energy—the energy required to separate the ions in the solid lattice—solubility is high, and the solid dissolves well. However, if the lattice energy is greater than the hydration energy, the solid does not dissolve easily, indicating low solubility.

Hydration energy is thus essential for understanding how ionic compounds interact with water and influences the solubility of the compound.