AP Chemistry 3.10 Solubility Study Notes - New Syllabus 2024-2025

Solubility and Intermolecular Interactions

The solubility of a substance in a given solvent depends on the similarity and relative strength of the intermolecular interactions between the solute and solvent particles. The general rule governing solubility is expressed as: “Like dissolves like.” This means that substances with similar types of intermolecular forces tend to be soluble or miscible in each other.

1. Types of Intermolecular Interactions Affecting Solubility:

Ionic–Dipole Interactions: Occur when ionic compounds (such as salts) dissolve in polar solvents like water.

• • The cations (\(\mathrm{Na^+}\), \(\mathrm{K^+}\)) are attracted to the partially negative oxygen end (δ⁻) of water molecules.
  • The anions (\(\mathrm{Cl^-}\), \(\mathrm{SO_4^{2-}}\)) are attracted to the partially positive hydrogen ends (δ⁺) of water molecules.
  • These ion–dipole attractions overcome the ionic lattice forces, leading to dissociation and solvation (hydration).

Dipole–Dipole and Hydrogen Bonding Interactions: Occur among polar molecular compounds.

• • Polar molecules dissolve best in polar solvents because both experience dipole–dipole attractions or hydrogen bonding.
  • For example, ethanol (\(\mathrm{C_2H_5OH}\)) dissolves readily in water because both can form hydrogen bonds.

London Dispersion Forces: Dominant in nonpolar substances.

• • Nonpolar molecules (e.g., hexane, benzene, iodine) dissolve in nonpolar solvents because both interact through temporary induced dipoles.
  • Since there are no strong dipole or ion–dipole interactions to disrupt, solubility depends mainly on molecular size and polarizability.

2. Solubility Patterns and Examples:

Ionic Compounds in Polar Solvents:

• Ionic solids such as \(\mathrm{NaCl}\) dissolve in water because the strong ion–dipole attractions between ions and water molecules compensate for the lattice energy of the salt. Conversely, \(\mathrm{NaCl}\) is insoluble in hexane (a nonpolar solvent) because there are no strong attractive forces between ions and hexane molecules.

Molecular Compounds in Polar Solvents:

• Polar molecular solutes (like \(\mathrm{NH_3}\), \(\mathrm{CH_3OH}\)) dissolve readily in water via hydrogen bonding or dipole–dipole attractions.

Nonpolar Compounds in Nonpolar Solvents:

• Nonpolar solutes such as \(\mathrm{I_2}\) or \(\mathrm{CCl_4}\) dissolve in nonpolar solvents (like benzene or hexane) due to similar dispersion forces.

Immiscibility of Polar and Nonpolar Substances:

• Polar and nonpolar liquids (e.g., water and oil) do not mix because the strong hydrogen bonding network in water cannot be disrupted by weak London forces from oil molecules.

3. Microscopic Explanation of Solubility:

When a solute dissolves, three types of interactions are considered:

• Solute–solute attractions (e.g., ionic lattice or hydrogen bonds within solute)
• Solvent–solvent attractions (e.g., hydrogen bonds between water molecules)
• Solute–solvent attractions (new interactions formed during dissolution)

For dissolution to occur:

• The energy released by forming solute–solvent interactions must be comparable to or greater than the energy required to break solute–solute and solvent–solvent interactions.
• If solute–solvent forces are weaker, the solute will not dissolve appreciably.