Question

Why do ionic substances with higher lattice energies tend to be less soluble in water than those with lower lattice energies? [Section 13.1]

Short answer

Ionic substances with higher lattice energies are less soluble in water because their stronger electrostatic forces require a greater amount of energy to be broken. As a result, the hydration energy released during the interaction with water molecules may not be sufficient to overcome the lattice energy, preventing the substance from dissolving effectively. Substances with lower lattice energies generally have weaker electrostatic forces, which are more easily overcome by hydration energy, leading to higher solubility in water.

Step-by-step solution

Understand lattice energy

Lattice energy is the energy required to separate the ions in an ionic crystal. It is a measure of the strength of the electrostatic forces that hold the ions together in the crystal lattice. Higher lattice energy indicates stronger electrostatic forces, making it more difficult to separate the ions in the crystal.

Understand hydration energy

When an ionic substance dissolves in water, its ions are surrounded by water molecules through a process called hydration. In this process, water molecules are attracted to the charged ions, and hydration energy is released. Hydration energy is the energy released when ions are solvated (surrounded) by water molecules. The energy released during hydration is greater when the ions have a higher charge and/or a smaller size, as they will have a stronger attraction to the polar water molecules.

Understand the relation between lattice energy and solubility

For an ionic substance to dissolve in water, it needs to overcome its lattice energy. The solubility of an ionic substance in water is determined by the competition between its lattice energy (which opposes solubility) and its hydration energy (which favors solubility). If the hydration energy is higher than the lattice energy, the substance will dissolve in water; if not, it will remain insoluble.

Solubility of high lattice energy substances

Ionic substances with higher lattice energies tend to be less soluble in water because their stronger electrostatic forces require more energy to be broken. If the hydration energy released by an ionic substance is not enough to overcome the lattice energy, it will not dissolve in water. In general, substances with lower lattice energies, which have weaker electrostatic forces, are more likely to dissolve in water, as their hydration energy is more likely to exceed their lattice energy. In conclusion, ionic substances with higher lattice energies tend to be less soluble in water because their stronger electrostatic forces require significant energy to be broken, making it more difficult for these substances to dissolve in water.