Problem 3 - Chemistry The Central Science In Si Units

Question

Consider two ionic solids, both composed of singly charged ions, that have different lattice energies. (a) Will the solids have the same solubility in water? (b) If not, which solid will be more soluble in water, the one with the larger lattice energy or the one with the smaller lattice energy? Assume that solute-solvent interactions are the same for both solids.

Step-by-Step Solution

Answer

(a) No, the solids will not have the same solubility. (b) The solid with the smaller lattice energy will be more soluble in water.

Step 1: Understanding Lattice Energy

Lattice energy is the energy required to separate one mole of a solid ionic compound into gaseous ions. It is a measure of the strength of the forces holding the ions together. High lattice energy indicates a strong attraction between ions.

Step 2: Linking Lattice Energy to Solubility

The solubility of an ionic solid in water depends on the balance between the lattice energy and hydration energy. Hydration energy is the energy released when ions interact with water molecules. If the lattice energy is significantly greater than the hydration energy, the solid is less likely to dissolve. Conversely, a lower lattice energy compared to hydration energy enhances solubility.

Step 3: Analyzing Solids with Different Lattice Energies

Given that both solids are composed of singly charged ions but have different lattice energies, the solid with the smaller lattice energy requires less energy for the ions to separate and can more easily interact with water molecules.

Step 4: Conclusion on Solubility

The solid with the smaller lattice energy will be more soluble in water because it requires less energy to separate its ions, allowing them to interact more readily with water molecules. Thus, differences in lattice energy affect solubility.

Key Concepts

Ionic Solids

Ionic solids are fascinating structures formed by the orderly arrangement of ions. They consist of positively charged ions, known as cations, and negatively charged ions, called anions. These ions are held together by strong electrostatic forces in a repeating three-dimensional lattice pattern.

This structure gives ionic solids some distinct properties. For instance, they are typically hard and brittle, with high melting and boiling points. This is because a lot of energy is required to overcome the ionic bonds during phase changes. Ionic solids are also known for their ability to conduct electricity when molten or dissolved in water, as the ions are free to move and carry charge. However, in solid form, they do not conduct electricity because the ions are fixed in place within the lattice.

• Examples: Table salt (NaCl) and magnesium oxide (MgO) are common ionic solids.
• Characteristics: High melting points, brittleness, and electrical conductivity when dissolved or melted.

Solubility in Water

Solubility is the ability of a solute to dissolve in a solvent, forming a homogeneous mixture. For ionic solids, solubility in water depends largely on the interplay between lattice energy and hydration energy.

In water, the solvent molecules interact with the ions of the solute, facilitating its dissolution. Water's polar nature allows it to effectively surround and separate the ions in an ionic solid. However, the force holding the ions together, known as lattice energy, plays a critical role in determining how easily the ions can be separated.

• Solubility Factors: If lattice energy is lower or comparable to hydration energy, solubility is favored.
• Impact: Solids with lower lattice energy are generally more soluble in water because they require less energy to separate the ions.

Hydration Energy

Hydration energy is an essential concept when explaining why and how ionic solids dissolve in water. It is the energy released when ions are surrounded and stabilized by water molecules.

This energy can vary depending on the size and charge of the ions. Smaller and more highly charged ions usually have higher hydration energies due to stronger interactions with water molecules. The overall solubility of ionic solids is determined by the balance between its hydration energy and its lattice energy.

• Role: If hydration energy exceeds lattice energy, an ionic solid is more likely to dissolve.
• Thermodynamics: A higher hydration energy generally facilitates the dissolving process, making ionic compounds more soluble.