Intermolecular%20Forces%20and

1
Intermolecular Forces and Liquids and Solids
Chapter 12
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Midterm II

• Any conflicts with March 20? If yes, let me know
  ASAP. The original date was March 22.

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Buckyball (C60)
4

• Diethyl ether is a volatile, highly flammable
  organic liquid that is used mainly as a solvent.
  The vapor pressure of diethyl ether is 401 mmHg
  at 18oC. Calculate its vapor pressure at 32oC.
• Convert temperatures from Celsius to Kelvin
  because the gas constant has units of J mol-1
  K-1.
• Plug and chug.

P2 657 mmHg
5
Phase Diagram of Water

• Note the high critical temperature and critical
  pressure
• These are due to the strong van der Waals forces
  between water molecules.

• The slope of the solidliquid line is negative.
• This means that increasing the pressure above 1
  atm will raise the boiling point and lower the
  melting point.
• Lower the melting point?

6
Phase Diagram of Carbon Dioxide

• Carbon dioxide cannot exist in the liquid state
  at pressures below 5.11 atm CO2 sublimes at
  normal pressures.

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Phase Diagram of Carbon Dioxide

• Carbon dioxide cannot exist in the liquid state
  at pressures below 5.11 atm CO2 sublimes at
  normal pressures.

At 1 atm, solid CO2 does not melt at any
temperature. Instead, it sublimes to form CO2
vapor. Why might it be useful as a refrigerant?
8
Phase Diagram of Carbon Dioxide

• Carbon dioxide cannot exist in the liquid state
  at pressures below 5.11 atm CO2 sublimes at
  normal pressures.

If you want to send something frozen across the
country, you can pack it in dry ice. It will be
frozen when it reaches its destination, and
there will be no messy liquid left over like you
would have with normal ice.
9
The slope of the curve between solid and liquid
is positive for CO2 as well as almost all other
substances. Why does water differ?
10
Freeze-drying
Normal (right) and freeze-dried (left) spaghetti

• Completely remove water from some material, such
  as food, while leaving the basic structure and
  composition of the material intact
• Two reasons
• Keeps food from spoiling for a long period of
  time
• Significantly reduces the total weight of the
  food
• How?
• Freeze the material
• Lower the pressure (lt0.006 atm)
• Increase the temperature slightly

11
Freeze-drying
Normal (right) and freeze-dried (left) spaghetti

• How?
• Freeze the material
• Lower the pressure
• Increase the temperature slightly

12
Physical Properties of Solutions
Chapter 13
13
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
13.1
14
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.
13.1
15
Solutions

• The intermolecular forces between solute and
  solvent particles must be strong enough to
  compete with those between solute particles and
  those between solvent particles.

16
How Does a Solution Form?

• As a solution forms, the solvent pulls solute
  particles apart and surrounds, or solvates, them.

17
How Does a Solution Form

• If an ionic salt is soluble in water, it is
  because the ion-dipole interactions are strong
  enough to overcome the lattice energy of the salt
  crystal.

18
Energy Changes in Solution

• Simply, three processes affect the energetics of
  the process
• Separation of solute particles
• Separation of solvent particles
• New interactions between solute and solvent

19
Energy Changes in Solution

• The enthalpy change of the overall process
  depends on ?H for each of these steps.

20

• Three types of interactions in the solution
  process
• solvent-solvent interaction
• solute-solute interaction
• solvent-solute interaction

DHsoln DH1 DH2 DH3
13.2
21
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)

13.2
22
Concentration Units
The concentration of a solution is the amount of
solute present in a given quantity of solvent or
solution.
Percent by Mass
by mass
Mole Fraction (X)
13.3
23
Concentration Units Continued
Molarity (M)
Molality (m)
13.3
24
What is the molality of a 5.86 M ethanol (C2H5OH)
solution whose density is 0.927 g/mL?
Strategy Find mass of solvent Know mass of
solute mass of solvent mass of solution If
mass of solution and mass of solute known, can
calculate mass of solvent Can calculate mass of
solute from moles of solute Can calculate mass of
solution from density and volume of the
solution Solve
13.3
25
What is the molality of a 5.86 M ethanol (C2H5OH)
solution whose density is 0.927 g/mL?
0.586 moles of solute per 1 L of solution 5.86
moles ethanol 270 g ethanol 927 g of solution
(1000 mL x 0.927 g/mL)
mass of solvent mass of solution mass of
solute
927 g 270 g 657 g 0.657 kg
8.92 m
13.3
26
Temperature and Solubility
Solid solubility and temperature
No clear correlation between ?Hsoln and the
variation of solubility with temperature
13.4
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Fractional crystallization is the separation of a
mixture of substances into pure components on the
basis of their differing solubilities.
Suppose you have 90 g KNO3 contaminated with 10 g
NaCl.

• Fractional crystallization
• Dissolve sample in 100 mL of water at 600C
• Cool solution to 00C
• All NaCl will stay in solution (s 34.2g/100g)
• 78 g of PURE KNO3 will precipitate (s 12
  g/100g). 90 g 12 g 78 g

13.4
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Temperature and Solubility
Gas solubility and temperature
solubility usually decreases with increasing
temperature
13.4
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Pressure and Solubility of Gases
The solubility of a gas in a liquid is
proportional to the pressure of the gas over the
solution (Henrys 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/Latm) that depends only on
temperature
13.5
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Colligative Properties of Nonelectrolyte 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
X1 mole fraction of the solvent
Raoults law
If the solution contains only one solute
X1 1 X2
X2 mole fraction of the solute
13.6
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PT PA PB
13.6
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PT is greater than predicted by Raoultss law
PT is less than predicted by Raoultss law
13.6
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Fractional Distillation Apparatus
13.6
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Boiling-Point Elevation
T b is the boiling point of the solution
DTb gt 0
m is the molality of the solution
Kb is the molal boiling-point elevation
constant (0C/m)
13.6
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Freezing-Point Depression
T f is the freezing point of the solution
DTf gt 0
m is the molality of the solution
Kf is the molal freezing-point depression
constant (0C/m)
13.6
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13.6
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What is the freezing point of a solution
containing 478 g of ethylene glycol (antifreeze)
in 3202 g of water? The molar mass of ethylene
glycol is 62.01 g.
DTf Kf m
Kf water 1.86 0C/m
2.41 m
DTf Kf m
1.86 0C/m x 2.41 m 4.48 0C
0.00 0C 4.48 0C -4.48 0C
13.6
38
Osmotic Pressure (p)
Osmosis is the selective passage of solvent
molecules through a porous membrane from a dilute
solution to a more concentrated one.
A semipermeable membrane allows the passage of
solvent molecules but blocks the passage of
solute molecules.
Osmotic pressure (p) is the pressure required to
stop osmosis.
more concentrated
dilute
13.6
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Osmotic Pressure (p)
High P
Low P
p MRT
M is the molarity of the solution
R is the gas constant
T is the temperature (in K)
13.6
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A cell in an
isotonic solution
hypotonic solution
hypertonic solution
13.6
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Colligative Properties of Nonelectrolyte 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.
13.6
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Colligative Properties of Electrolyte Solutions
0.1 m NaCl solution
0.1 m Na ions 0.1 m Cl- ions
Colligative properties are properties that depend
only on the number of solute particles in
solution and not on the nature of the solute
particles.
0.1 m NaCl solution
0.2 m ions in solution
i should be
nonelectrolytes
1
NaCl
2
CaCl2
3
13.7
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Colligative Properties of Electrolyte Solutions
13.7
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A colloid is a dispersion of particles of one
substance throughout a dispersing medium of
another substance.

• Colloid versus solution
• collodial particles are much larger than solute
  molecules
• collodial suspension is not as homogeneous as a
  solution

13.8
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The Cleansing Action of Soap
13.8
46
Chemistry In Action Desalination