Chapter 12

Report
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

similar documents