Ch. 14: Mixtures & Solutions
Sec. 14.3: Factors Affecting Solvation
Objectives
• Relate intermolecular forces and the
process of solvation.
• Define solubility and identify factors
affecting it.
Solvation
• Why are some substances insoluble &
others are soluble in water?
• “Like dissolves like” is a rule that usually
indicates whether a substance will dissolve
in a specific solvent.
Solvation
• Defined as the process of surrounding
solute particles with solvent particles to
form a solution.
• Solvation in water is also called
“hydration.”
Solvation
• To form a solution:
– solute particles have to separate from one
another
– solvent particles have to separate from on
another
– solute particles have to become surrounded by
solvent particles
Example: Solvation of NaCl
in Water
Hydrogen bonding in water (H2O)
solvent
solute
Steps in Solvation
• This means:
1 - attractive forces between solute particles must
be broken. (requires energy - is endothermic)
2 - attractive forces between solvent particles
must be broken. (requires energy - is
endothermic)
3 - attractive forces between solute and solvent
particles must form and be stronger than the
other attractive forces that exist. (releases
energy - is exothermic)
When hydration of ionic compounds (like NaCl) occurs, the
solvent-solute attractions have to overcome the solventsolvent attractions and the solute-solute attractions.
Heat of Solution
• The breaking of solute to solute and solvent to
solvent attractions is endothermic; creating
solute to solvent attractions is exothermic.
• The overall energy change that occurs during the
solution formation process is called the heat of
solution.
– ΔHsoln can be exothermic or endothermic (depends on
. . . ?)
– temperature change upon solvation indicates if process
is exothermic or endothermic.
Aqueous Solutions of Polar
Molecular Compounds
•Solvation occurs in
much the same way.
Polar sugar molecules
are attracted to polar
water molecules.
•However, aqueous
solutions contain
dissolved polar
molecules and do not
conduct electricity.
•Only solutions with hydrated ions are electrolytes.
Oil & Water
• Aqueous solutions of nonpolar molecules
do not form.
• Oil does not form a solution with water
because there is little attraction between
polar water molecules and nonpolar oil
molecules.
Rate of Solvation
• To increase collisions between solute &
solvent particles (& increase the rate or
speed of solvation):
1. Agitate the mixture (stir or shake) - this
moves dissolved particles away from
undissolved particles, allowing more
collisions to occur.
Rate of Solvation
2. Increase the surface area of the solute
(break the solute into small pieces) - this
allows more area of the solute to be
exposed to water molecules, so more
collisions will occur.
3. Increase the temperature of the solvent this increases the KE of the solvent
particles so more frequent & energetic
collisions occur.
Solubility
•Solubility is defined as the maximum
amount of solute that can dissolve in a given
amount of solvent at a particular temperature
and pressure.
•As we have seen, solubility depends on the
nature of the solute and solvent.
•Table 4, pg. 494, shows the solubilities of
various solutes in water. Some are slightly
soluble, others are very soluble.
Solubility Curves
•Solubility is
usually expressed
in grams of solute
per 100 g of solvent
at a specified
temperature.
•A graph of
solubility vs.
temperature is
called a solubility
curve.
Factors Affecting Solubility
(Amount NOT Rate)
•Notice that solubility is affected by
temperature.
• Generally, increasing
the temperature of the
solvent causes the
kinetic energy of the
particles to increase,
so more collisions
(more solvation)
occurs.
Factors Affecting Solubility
(Amount NOT Rate)
GASES
SOLIDS
Factors Affecting Solubility
• For solids in water, generally solubility
________ with temperature.
• For gases in water, solubility _________
with temperature.
Saturated Solutions
•A saturated solution contains the maximum
amount of dissolved solute for a given amount
of solvent at a specific temperature and
pressure.
• Solubility curves, therefore, relate information
about saturated solutions.
–In a saturated solution, as long as the conditions
remain constant, the overall amount of dissolved
solute remains constant. If more solute is added to a
saturated solution, it will . . .
–In a saturated solution, if the temperature of the
solvent is increased, generally more solute will . . .
Unsaturated Solutions
• An unsaturated solution is one that
contains less dissolved solute for a given
temperature and pressure than a saturated
solution.
• If more solute is added to an unsaturated
solution, it will . . .
Formation of a Saturated
Solution
Which solutions are saturated?
Which are unsaturated?
Supersaturated Solutions
• A supersaturated solution contains more
dissolved solute than a saturated solution at the
same temperature.
• Such solutions are made by dissolving excess
amounts of solid solute at high temperatures and
then slowly cooling the solution. If the excess
solute remains dissolved, the solution is
supersaturated.
• Such solutions are unstable. If the solution is
disturbed or “seeded”, the excess solute will
precipitate out.
Example: Glucose
Supersaturated Solutions
Other Factors Affecting Solubility
• For gases in liquids, solubility _________
with pressure.
Factors that Affect Solubility
Henry’s Law
• Henry’s Law states that at a given
temperature, the solubility (S) of a gas in a
liquid is directly proportional to the
pressure (P) of the gas above the liquid.
• Mathematically,
S1 = S2
P1 P2
Solubility is generally expressed as g/L.
Henry’s Law
• If 0.85 g of a gas at 405.2 kPa of pressure
dissolves in 1.0 L of water at 25 degrees C,
how much will dissolve in 1.0 L of water at
101.3 kPa of pressure and the same
temperature?
• A gas has a solubility of 0.66 g/L at 10.0
atm. What is the pressure on a 1.0 L
sample that contains 1.5 g of gas?