Why do solutes dissolve in solvents?

Apr 25, 2014

The major factor that determines whether solutes dissolve in solvents is entropy.

To form a solution we must:
1. Separate the particles of the solvent.
2. Separate the particles of the solute.
3. Mix the particles of solvent and solute.

ΔH_("soln") = ΔH_1 + ΔH_2 + ΔH_3

ΔH_1 and ΔH_2 are both positive because it requires energy to pull molecules away from each other. ΔH_3 is negative because intermolecular attractions are forming.

For the solution process to be favourable, ΔH_3 should at least equal ΔH_1 + ΔH_2.

Nonpolar Solvent – Nonpolar solute

If both solvent and solute are nonpolar, all the ΔH values are small. The major factor then is the increase in entropy (disorder) that occurs when a solution forms. This is a favourable process.

Polar Solvent – Polar Solute

If both solvent and solute are polar, all the ΔH values are large but similar in size. The major factor again is the increase in entropy.

LIKE DISSOLVES LIKE.

Polar Solvent – Nonpolar Solute
If a nonpolar solute such as oil mixes with a polar solvent like water, ΔH_1 is large and positive. This outweighs ΔH_3. A solution does not form.