Question

How does each of the following affect the solubility of an ionic compound: (a) lattice energy, (b) solvent (polar versus nonpolar), (c) enthalpies of hydration of cation and anion?

Short answer

High lattice energy reduces solubility; polar solvents increase it; high hydration enthalpies improve solubility.

Step-by-step solution

Understanding Lattice Energy

Lattice energy is the energy required to separate one mole of an ionic solid into its gaseous ions. Higher lattice energy means the ions are more strongly attracted to each other in the solid, making it less likely to dissolve. Therefore, if the lattice energy is high, the solubility of the compound generally decreases.

Role of the Solvent

The type of solvent affects solubility through its polarity. Polar solvents, like water, can stabilize ions in solution and are better at dissolving ionic compounds than nonpolar solvents. This is due to the ability of polar solvents to interact with the charged ions, overcoming the lattice energy holding the ions together.

Impact of Enthalpies of Hydration

Enthalpies of hydration refer to the energy released when gaseous ions dissolve in a solvent. If the enthalpies of hydration for the cation and anion are high, the solubility in a polar solvent like water increases because the energy released compensates for the energy required to break the lattice structure.

Key concepts

Lattice Energy

Lattice energy is a critical concept in understanding the solubility of ionic compounds. It represents the energy needed to break apart the ionic bonds in a solid compound to form gaseous ions, essentially reflecting how tightly the ions hold together.
\[\text{Lattice Energy} = \text{Energy to separate ions}\]
When the lattice energy is high, it requires more energy to dismantle the solid structure. This strong attraction between the ions means they are less likely to drift apart and dissolve, hence decreasing the compound's solubility.
Factors like the charge and size of the ions play a role in lattice energy. Higher charges and smaller ions generally lead to greater lattice energy because of the stronger attraction. Thus, when trying to dissolve a compound with a high lattice energy, a lot of energy is necessary to overcome these strong ionic bonds.

Polar Solvent

Solvents are crucial in determining how well an ionic compound will dissolve. The key is the polarity of the solvent. Polar solvents, such as water, are excellent at dissolving ionic compounds.
Why? Because polar molecules have a positive and a negative end, they can effectively surround and interact with the charged ions of an ionic compound.

• In polar solvents, the positive ends of solvent molecules attract anions.
• Negative ends approach cations.

This interaction helps pull the ions away from their solid structure and keeps them apart, thus increasing solubility. Nonpolar solvents, lacking the charged poles, do not have this ability, making them poorly suited for dissolving ionic substances. Therefore, when you want to dissolve an ionic compound effectively, choosing a polar solvent is key.

Enthalpy of Hydration

Enthalpy of hydration is the energy released when ions dissolve in water to form hydrated ions. This process plays a significant role in determining solubility.

A higher enthalpy of hydration implies more energy is released upon solvation, compensating the energy needed to overcome the lattice energy.

This process provides the drive for the ions to leave the lattice and disperse through the solvent, making the compound more soluble.

In a nutshell, when cations and anions have large enthalpies of hydration, the overall process of dissolving becomes energetically favorable, enhancing the solubility of the compound in polar solvents like water.