To understand why things dissolve at all, we will look at the solution
formation process from a
thermodynamic point of view. shows a thermodynamic
cycle that
represents the formation of a solution from the isolated solute and
solvent. From Hess's law we
know that we can add the energies of each step in the cycle to determine
the energy of the overall
process. Therefore, the energy of solution formation, the enthalpy of
solution, equals the sum
of the three steps--ΔHsoln = ΔH1 + ΔH2
+ ΔH3.
Figure %: The Solution Formation Process
ΔH1 and ΔH2
are both positive because it requires energy to pull molecules away from
each other. That energy cost
is due to the intermolecular forces present within any solute or solvent.
The forces acting between
molecules such as CH3Cl are largely van der Waals and
dipole-dipole interactions.
Some molecules that contain O-H, N-H, or F-H bonds can form hydrogen bonds
that are relatively
strong intermolecular forces. Ions of opposite charge, such as in a
crystal of NaCl, are attracted to
each other because of electrostatic forces. Each of those forces
increase with decreasing distance.
Therefore, it should make sense that it costs energy to pull molecules and
ions away from each other.
When the expanded form of the solvent and the solute are combined to form a
solution, energy is
released, causing ΔH3 to be negative.
This makes
sense because the solute and solvent can interact through the various types
of intermolecular forces.
What determines the enthalpy of solution is, therefore, the difference
between the energy required to
separate the solvent and solute and the energy released when the separated
solvent and solute form a
solution. To restate that in simpler terms, solutions will form only when
the energy of interaction
between the solvent and solute is greater than the sum of the
solvent-solvent and solute-solute
interactions. That situation can only occur when the solvent and solute
have similar properties. For
example, if a non-polar molecule, such as oil, is mixed with a
polar molecule like water,
no solution forms. Water's solvent-solvent intermolecular
interactions are mostly hydrogen
bonds and dipole-dipole while oil has only van der Waals. Water can
satisfy its hydrogen bonds and
become stabilized by dipole-dipole interactions only when near other water
molecules. Therefore,
water is destabilized when it forms a solution with oil. That is why such
a solution will never form
between oil and water. Therefore, the primary rule of solubility is that
like dissolves like. Only
when the solute and solvent molecules have several common structural
features such as their
polarities will a solution form.
Pressure and Temperature Effects
If you have not yet studied thermodynamics or you do not know what ΔG or
ΔS stands for, then please skip to the next
heading
in which the following discussion on temperature and pressure effects on
solubility is
summarized without the thermodynamics. The following discussion is a
slightly more advanced
treatment of the same phenomena.
The creation of disorder during the solution formation process is its
essential driving force. In fact,
most compounds that are soluble in water have positive enthalpies of
solution. The only reason why
those solutions form is due to the positive entropy of solution, ΔSsoln. As shows, both
the solvent
and the (solid or liquid) solute become less ordered upon solution
formation. Therefore, from the
equation ΔG = ΔH - TΔS we should predict that the solubility of every
compound should increase
with increasing temperature. That prediction turns out to be correct for
nearly every solvent and
solute. However, there are some exceptions, such as sodium sulfate in
water that actually become
less soluble at higher temperatures. That is usually due to their
negative entropies of
solution. The reason why some solutions have a negative entropy of
solution is beyond the scope of
this SparkNote. If you wish to pursue this topic further, search for the
hydrophobic effect.
Using the idea of the entropy of solution, we can predict other properties
of solutions. For example,
we should predict that all gasses should bcome less soluble in water
with increasing temperature
because they have a negative entropy of solution. Gasses have a negative
entropy of solution in
water because they are confined to a smaller volume when dissolved as
compared to their volumes as
gasses. As we should predict, all gasses become less soluble in water with
increasing temperature.
