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Intermolecular forces holding molecules together.
Intermolecular Forces
- Surface Tension -
Cohesive forces:
interactions
between like
particles.
Adhesive forces:
interactions
between unlike
particles.
Intermolecular Forces
- Cohesive vs Adhesive Forces -
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Two liquids that mix completely together are said to
be MISCIBLE - otherwise they are IMMISCIBLE.
Nonpolar CCl4
CuSO4(aq) Nonpolar octane (C8H18)
Polarity and Solubility
Mixing of Polar Liquids
CH3CH2 - O - H
CH3CH2 - O - H
CH3CH2 - O - H
• •
• •
• •
• •
• •
• •
- +
O H
H -
+
+
O H
H -
+
+
O H
H -
+
+
- +
- +
Polar molecules
can break up
groups of other
polar molecules
through dipole-
dipole
interactions,
resulting in a
thorough mixing
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Mixing of Polar and Nonpolar Liquids
H H
O
+ +
-
H H
O
+ +
-
H H
O
+ +
-
H - C - C - C - H
H H H
H H H
H - C - C - C - H
H H H
H H H
The weak London forces present in the CH3CH2CH3 molecules are
not strong enough to break the hydrogen bonds, so the two liquids
are immiscible.
Mixing Nonpolar Liquids
H - C - C - C - C - C - C -H
H H H
H H H
H H H
H H H
C Cl
Cl
Cl
Cl
London forces are the only interactions, but the strength
of these forces between CCl4 molecules is comparable to
those between C6H14 molecules, so the two liquids are
miscible with each other.
General Observation:
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More Solubility Examples
More than one intermolecular force may need to
be considered when examining solubility.
Solubility decreases
as relative energy of
H-bonding decreases
and dispersion
increases.
Combination of Forces
5
Extensive dispersion forces limit solubility of octanol in water.
Octanol
Combination of Forces – cont’d
Hydrophobic (“water-fearing”)
• Interaction that repels water, diminishes water solubility.
Hydrophilic (“water-loving”)
• Interaction that attracts water, promotes water solubility.
Hydrophobic Hydrophilic
Solubility Behavior
6
Sample Exercise 10.5
Which of these compounds should be very soluble in water and which should have limited solubility in water: carbon tetrachloride (CCl4), ammonia (NH3), hydrogen fluoride (HF), oxygen (O2)?
Chemistry In Action: The Killer Lake
Lake Nyos, West Africa
8/21/86
CO2 Cloud Released
1700 Casualties
Trigger?
• earthquake
• landslide
• strong Winds
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Henry’s law - the solubility of a gas in a liquid is proportional to
the pressure of the gas over the solution ().
c = kP
c is the concentration (M) of the dissolved gas
P is the pressure of the gas over the solution
k is a constant (mol/L•atm) that depends only
on temperature
low P
low c
high P
high c
Solubility of Gases in Water
8
Sample Exercise 10.6
Calculate the solubility of oxygen in water in moles per liter at 1.00 atm pressure and 20°C. The mole fraction of O2 in air is 0.209. (Remember that the sum of the mole fractions of all the gases in a mixture equals 1.) The Henry’s law constant for oxygen at 20°C is 1.3 × 10-3 mol/L . atm.
States of matter as a function of temperature and pressure
Pre
ssu
re
Temperature
solid
liquid
gas
Phase Diagrams
9
Triple Point
Critical Point
Critical Temperature
Critical Pressure
Supercritical Fluid
Equilibrium Lines
Phase Diagram of Water
On the left is water; water is unusual because the solid is
less dense than the liquid. For most substances, the solid is
more dense than the liquid like for benzene (on the right).
Some Remarkable Properties of Water
10
Unique Property of Water
- density decreases when it freezes -
H-bonding results in cage-like
structure in solid state; less
dense than liquid state.
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Water and Aquatic Life
Importance of Density:
• Lakes/rivers freeze from top down, allowing fish and
aquatic life to survive below.
• As surface waters warm or cool, nutrient-rich bottom
waters cycle to the surface; oxygen-rich surface
waters cycle to the bottom.
Phase Diagram for CO2
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1. Positive slope for the solid-liquid interface (normal)
2. Sublimation occurs at room temperature under 1
atm of pressure.
3. There is a “critical point” at 73 atm and 31 oC
(varies from substance to substance).
4. Above the critical point the substance is known as
a “supercritical fluid” (not a liquid; not a gas).
Increasing the pressure normally would convert a
gas to a liquid but doesn’t happen.
5. Supercritical fluids have enhanced solvation
abilities, i.e. the fluid will dissolve greater amounts
of solute than normal.
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