Transcript Slide 1

Physical Properties
of Solutions
Chapter 12
Definitions
A solution is a homogenous mixture of 2 or
more substances
The solute is(are) the substance(s) present in the
smaller amount(s)
The solvent is the substance present in the larger
amount
A saturated solution contains the maximum amount of a
solute that will dissolve in a given solvent at a specific
temperature.
An unsaturated solution contains less solute than the
solvent has the capacity to dissolve at a specific
temperature.
A supersaturated solution contains more solute than is
present in a saturated solution at a specific temperature.
Sodium acetate crystals rapidly form when a seed crystal is
added to a supersaturated solution of sodium acetate.
Rapid Crystallization
Enthalpy of solution
(or enthalpy of dissolution)
 The
heat of solution (DHsol) is defined
as the sum of the energy absorbed
(endothermic), or released
(exothermic) as the solute dissolves
in a solvent at constant pressure.
 The
value of the overall enthalpy
change is the sum of the individual
enthalpy changes of each step.
Examples
 Dissolving
ammonium nitrate in
water is endothermic (solvation does
not weigh up against energy spent in
breaking down the crystal lattice)
 Adding potassium hydroxide is
exothermic, the solute-solvent
attractions are stronger than the
other steps.
 Solutions with negative heats of
solution have lower vapor pressures.
“like dissolves like”
Two substances with similar intermolecular forces are likely
to be soluble in each other.
•
non-polar molecules are soluble in non-polar solvents
CCl4 in C6H6
•
polar molecules are soluble in polar solvents
C2H5OH in H2O
•
ionic compounds are more soluble in polar solvents
NaCl in H2O or NH3 (l)
The Cleansing Action of Soap
Fat Soluble and Water Soluble
Vitamins
Vitamin A is soluble in nonpolar
compounds (like fats).
 Vitamin C is soluble in water.

Temperature and Solubility
Solid solubility and temperature
Generally, solubility
increases with increasing
temperature
Some substances,
solubility decreases
with increasing
temperature (gives off
heat as they dissolve)
Temperature and Solubility
Gas solubility and temperature
solubility usually
decreases with
increasing temperature
Pressure and Solubility of Gases
The solubility of a gas in a liquid is proportional to the
pressure of the gas over the solution (Henry’s law).
c is the concentration (M) of the dissolved gas
c = kP
P is the pressure of the gas over the solution
k is a constant (mol/L•atm) that depends only
on temperature
low P
high P
low c
high c
Concentration Units
The concentration of a solution is the amount of solute
present in a given quantity of solvent or solution.
Percent by Mass
mass of solute
x 100%
% by mass =
mass of solute + mass of solvent
mass of solute x 100%
=
mass of solution
Mole Fraction (X)
moles of A
XA =
sum of moles of all components
12.3
Concentration Units Continued
Molarity (M)
M =
moles of solute
liters of solution
Molality (m)
m =
moles of solute
mass of solvent (kg)
What is the molality of a 5.86 M ethanol (C2H5OH)
solution whose density is 0.927 g/mL?
moles of solute
moles of solute
m =
M =
mass of solvent (kg)
liters of solution
Assume 1 L of solution:
5.86 moles x MM ethanol = 270 g ethanol (solute)
1000 mL x 0.927 g/mL = 927 g of solution
mass of solvent = mass of solution – mass of solute
= 927 g – 270 g = 657 g = 0.657 kg
moles of solute
m =
mass of solvent (kg)
=
5.86 moles C2H5OH
0.657 kg solvent
= 8.92 m
Convert % mass to Molarity
 What
is the Molarity of a 95%
acetic acid solution? (density =
1.049 g/mL)
1000 mLx 1.049g/ml = the mass of solution = 1049 g
95% of the solution is acetic acid
1049 g solution x 0.95 = 997 g solute
997 g X 1 mol/60.05 g Acetic acid = 16.6 mol solute
Since we assumed 1 L, that’s 16.6 mol / 1 L or
16.6 M
Colligative Properties of Solutions
Colligative properties are properties
that depend only on the number of
solute particles in solution and not on
the nature of the solute particles.
 Vapor
Pressure Lowering
 Boiling
Point Elevation
 Freezing
Point Depression
 Osmotic
Pressure
Raoult’s law
Vapor-Pressure Lowering; The vapor pressure of a
Solution is lower than pure solute.
P1 = X1 P
0
1
P 10 = vapor pressure of pure solvent
X1 = mole fraction of the solvent
DP1 = X2P01
X2 = mole fraction of the
solute
Lowering the Pressure Above the Solution
(by opening bottle) Decreases Gas
Solubility
Boiling-Point Elevation of Nonelectrolyte Solutions
DTb = Tb – T 0b
0
Tb
boiling point of
the pure solvent
T b boiling point of
the 0solution
DTb = Kb m
m is the molality of the solution
Kb is the boiling-point
elevation constant (0C/m)
Osmotic Pressure


Osmotic pressure is the “funky” colligative
property, but it is very important
biologically
Osmotic pressure is the pressure required
to prevent osmosis.
P= M RT
where P is osmotic pressure
van’t Hoff factor i
 Colligative
properties depend on
the concentration of particles
 Strong electrolytes, like NaCl,
should produce (nearly) two moles
of solute particles for mole of NaCl
that dissolves
 The van’t Hoff factor i scales the
solute molatity to the correct
number of particles
van’t Hoff factor i
If 100 %
Salt
MolalConc
Dissociat ed
0.1 0.01 0.001 Occured
NaCl
KCl
1.87 1.94 1.97
1.85 1.94 1.98
2.00
2.00
K 2SO 4
2.23 2.70 2.84
3.00
van’t Hoff factor i
Δ
T f= i K fm
D
Tb = i Kf m
 P1
P
= X1P01
= iMRT