KEY2CHEM

Solubility of Ionic Compounds

Ionic compounds are held together by strong ionic bonds. When an ionic compound interacts with solvent, it will be soluble if the cumulative ion-solvent interparticle forces are stronger than the individual interparticle forces holding each of the solution components together. Ion-dipole interparticle forces are relatively strong interactions (still weaker than covalent bonds) that hold ions together with the permanent dipole moment in polar molecules.

Example 1.

Based on Coulomb’s law, which ionic compound is expected to be least soluble in water?

A. $$\require{mhchem}\ce{LiCl}$$

B. $$\require{mhchem}\ce{LiF}$$

C. $$\require{mhchem}\ce{LiBr}$$

Solution

B. $$\require{mhchem}\ce{LiF}$$

The cation is the same in each compound but the anion differs. $$\require{mhchem}\ce{F-}$$ is the smallest anion and $$\require{mhchem}\ce{Br-}$$ is the largest anion. The charge on the anions are all $$-1$$. Coulomb’s law states that the force of attraction increases with increasing charge and decreasing ion size. The compound with the smallest ions with have the strongest electrostatic attraction of the ions to one another. As such, more energy is required to separate the ions and allow them to experience ion-dipole interactions with water molecules.

Example 2.

What is the strongest type of interparticle force between solute and solvent when sodium chloride is dissolved in water?

A. hydrogen bonding

B. ion-dipole forces

C. dispersion forces

Solution

B. ion-dipole forces

$$\require{mhchem}\ce{NaCl}$$ is an ionic compound and water is a polar molecule. The strongest type of interparticle force between these species is ion-dipole forces. The cumulative ion-dipole forces are stronger than the individual ionic bonds holding $$\require{mhchem}\ce{NaCl}$$ together and the hydrogen bonds holding neighboring water molecules together. As such, $$\require{mhchem}\ce{NaCl}$$ is soluble in water.

Example 3.

Which ion is expected to be most strongly attracted to the oxygen atom in a water molecule?

A.

B.

C.

Solution

A.

Water is a polar molecule, with the oxygen atom bearing a partial negative charge. Positively charged cations will be most attracted to the partial negative oxygen atom. Based on Coulomb’s law, the ion with the largest positive charge will experience the strongest attraction to the partial negative charge.