Transcript Chapter 12
Chapter 12: Solutions
Solutions are homogeneous mixtures consisting of a
solute and solvent.
Not all solutions are liquids!
A solution can be a solid, liquid, or a gas.
Types of Solutions
Solution Process
The intermolecular
forces between solute
and solvent particles
must be strong enough
to compete with those
between solute particles
and those between
solvent particles.
Solution Process
As an ionic compound dissolves in water, the ions
are surrounded by six water molecules.
Solution Process
Solution Process
Energy of solution formation for ionic compounds has
two factors.
Factor One: Lattice energy of ionic compound.
Factor Two: Ion-dipole force formed between water and
ions.
Which one requires energy?
Which one produces energy?
NH4NO3(s) and MgSO4(s)
Ion-Dipole Force
Intermolecular force between water and ion is called
the Ion-Dipole force.
Hydration energy = combined interactions of all six
waters with the ion.
Energy of Ionic Solutes
DHsolution = DHhydration – DHlattice energy
Insoluble Ionic Compounds
Compounds with very LARGE lattice energies are
insoluble.
Compounds with smaller lattice energies are highly
soluble.
Ex) KBr(s) vs. PbS(s)
Ex) Al2O3(s) vs. KNO3(s)
Energy of Solution
1. Energy to overcome solute-solute interaction (requires energy =
endothermic)
2. Energy to overcome solvent-solvent interaction (requires energy
= endothermic)
3. Energy of solute-solvent interaction (produces energy =
exothermic)
Energy of Solution
•If 1 + 2 – 3 > 0, then net
process is endothermic.
•If 1 + 2 – 3 < 0, then net
process is exothermic.
•If 1 + 2 – 3 >> 0, then the
two substances will NOT
mix.
Spontaneity
Enthalpy is not the sole factor in why things dissolve.
Natural tendency of the universe is to go from order to
disorder.
A solution is going from order to disorder when mixing
occurs.
The amount of disorder is called the entropy of the
system.
Spontaneity
Both CCl4 and C6H14 are
non-polar molecules
with similar boiling
points.
Adding the two together
will produce a solution
increasing the entropy
of the system.
Non-polar and Polar
“Like dissolves like”
Polar substances
dissolve in polar
solvents.
Non-polar substances
dissolve in non-polar
solvents.
Non-polar substances
do NOT dissolve in
polar solvents!
Intermolecular Forces
Types of Solvents
Molecular Compounds
Molecular compounds will dissolve in water IF
they are polar or have hydrogen bonding.
CH3CH2OH, CH3CHO
Size, though matters!
CH3CH2CH2CH2CH2CH2OH
W14, #1
Vitamins
Solubility of Alcohols
Saturated Solutions
When more solid solute is added to a liquid and
the solid does NOT dissolve, the solution is said to
be saturated.
Solubility limit = maximum amount solute that
can be dissolved in a given quantity of solvent.
Ex) @25oC, KNO3 = 36g / 100mL.
Super-saturated solution.
Sodium Acetate Solution
Temperature Effects
The solubility
limits of most
solid solutes
INCREASES with
increasing
temperature.
Temperature Effects
The solubility of ALL
gases decreases with
an increase in
temperature.
Can have an adverse
reaction in ponds and
lakes in the hot
summer months.
Concentrations
1.
There are many methods for expressing
concentration.
Percent
a.
b.
c.
mass / mass
volume / volume
mass / volume
Concentrations
2.
ppm (106) or ppb (109)
3.
Mole fraction = moles A / total moles
4.
Molarity (M) – Chapter 4
5.
Molality (m)
Colligative Properties
Why do we put “salt” on our roads in the winter?
Why do we add “antifreeze” to the radiator in our cars?
Can we reduce the vapor pressure of “volatile” liquids?
Yes!
Colligative Properties
Any property that depends only on the quantity of
solute particles and not their identity.
Vapor Pressure lowering
Freezing-pt depression
Boiling-pt elevation
Osmotic Pressure
Vapor Pressure
A non-volatile solute (solid) can be added to a volatile
solvent and LOWER its vapor pressure.
Pa = ca Pao
Mole fraction is for the solvent and Pao is the vapor
pressure for the pure solvent (dependent on T).
Addition of a nonvolatile
solute reduces the rate of
vaporization, decreasing the
amount of vapor
Vapor Pressure
Two volatile liquids, of similar intermolecular forces,
will both contribute a partial pressure.
Raoult’s Law
Ptotal = caPao + cbPbo
Graphical interpretation
Fractional distillation
Ideal vs. Non-ideal Behavior
BP and FP
When a solute is added to a solvent:
the freezing-pt is lowered.
the boiling-pt is raised.
Formulas are similar:
DT = Kf cm
DT = Kb cm
Osmosis
In biological systems, the cellular walls are made up of
cellulose.
Cellulose allows water molecules to pass in and out of
the cells.
Larger molecules or ions are generally “blocked” from
entering.
Note: glucose is transported into the cells via a
complex process.
Osmosis
When two solutions of different concentrations are
separated by a semi-permeable membrane, solvent
molecules will flow from lower solute concentration to
higher solute concentration.
Osmosis
This is important in maintaining water in our cells.
Osmotic Pressure
The osmotic pressure is measured in atmospheres.
p = MRT
M is the molarity of the solution
R = 0.08206 L atm/mol K
T = temperature, K
Molar Masses
Many experiments involving colligative properties can
find a molar mass of a solute.
Generally, this requires us to work backwards through
several formulas.
Ionic Solutes
Because ionic solutes break apart into ions, an
adjustment to our formulas must be made.
i = van’t Hoff factor.
Equals the number of ions per formula unit.
True factor depends on the concentration. As the
concentration increases, i becomes less than
predicted.
Why?
Ionic Solutes
Ionic Solutes
Assume “ideal” values for each electrolyte.
Becomes multiplier in all formulas.
What is i for:
CaCl2
Na2CO3
Al(NO3)3
NH3
Colloids
When larger solute
particles remain
suspended in a
solution, they form
a colloid.
Colloids will exhibit
the Tyndall Effect
due to the larger
particles.
Colloids
Tyndall effect in nature.
Colloids
There are several types of colloids.
Aerosol = a liquid or solid in a gas.
fog, smoke
Foam = a gas in a liquid or solid.
whipped cream, marshmallow
Emulsion = a liquid in a liquid or a solid in a solid.
milk, butter, mayo
Sol = a solid in a liquid or solid.
cement, paint, gems