Download L1-Intermolecular Forces of Attraction

Intermolecular
Forces
When studying the gas laws, we
learned the ideal gas law describes ideal
gases as having no intermolecular forces of
attraction between them.
This is
why real
gases deviate
from the
ideal gas law.
In gases, intermolecular forces of
attraction have very little effect on the
individual molecules because of the
high amount of kinetic energy.
However, if you decrease the
amount of available kinetic energy, the
intermolecular forces have a greater
effect on the molecules present,
resulting in the liquid and solid phases
The strength of the intermolecular
forces between species affect:
• Physical states
• Melting point (m.p.)
• Boiling point (b.p.)
• Volume
• Order of arrangement (structure)
Intramolecular
forces
Forces of
attraction within
molecules (i.e.
covalent bonds,
dipole moments)
Intermolecular
forces
Forces of
attraction between
molecules and ions
The intermolecular forces:
1.
Ion-ion force
2.
Ion-dipole force
3.
Hydrogen bonding
4.
Dipole-dipole force
5.
Induced Dipoles
(a.k.a. Dispersion forces)
Strongest
Strong
Weak
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Ion-ion Force
Dipole-dipole interactions
The electrostatic force
of attraction between ions of
opposite charge (ionic bond)
Dissociation Energy of Alkali Halides
D Hdissoc (kJ/mol)
These are the
strongest
forces.
Lead to solids
with high
melting
temperatures.
• Force of attraction between the
partial charges of polar molecules
1100
δ+H
– Cl δ-
δ+
H – Cl δ-
F
900
Cl
700
Br
I
500
Li
Na
K
Rb
Cs
Ion-Dipole Force
Force of attraction between an ion and
the partial charge on the end of a polar
molecule.
Attraction Between
Ions and Permanent
Dipoles
••
-
••
water
dipole
H
H +
O
Water is highly polar and
can interact with
positive ions to give
hydrated ions in water.
The Dissolution of
Ionic Solids Into
Solution (Hydration)
• Many metal ions are
hydrated. This is the reason
metal salts dissolve in water.
Induced Dipoles
London Dispersion Forces
Dispersion forces are the forces of
attraction created by the formation of
instanteous dipoles created from electron
movement
As the molecular weight increases, the
dispersion forces increase between the
molecules
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FORCES INVOLVING
INDUCED DIPOLES
28
How can nonnon-polar molecules such as O2 and I2
dissolve in water?
FORCES INVOLVING
INDUCED DIPOLES
29
Solubility increases with mass the gas
Dipole-induced
Dipole
Dipole-induced
dipole
dipole
The water dipole INDUCES a dipole
in the O2 electric cloud.
FORCES INVOLVING
INDUCED DIPOLES
Formation of a dipole in two nonpolar I2
molecules.
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Hydrogen Bonding
The force of attraction between the
hydrogen of one molecule and a highly
electronegative atom (F, O, N) in a
different molecule
Induced
dipole
Induced dipoledipole-induced
induced dipole
dipole
H-bonding is a special form of
dipole-dipole attraction, which
enhances dipole-dipole attractions.
H-bonding is
strongest when X
and Y are N, O, or
Hydrogen Bonding in H22O
15
H-bonding is especially
strong in water
because
• the O—
O—H bond is very
polar
• there are 2 lone pairs
on the O atom
Accounts for many of
water’
water’s unique
properties.
F
3
Hydrogen
Hydrogen Bonding
Bonding in
in H
H22O
O
• H bonds --->
---> abnormally
high specific heat capacity
of water (4.184 J/g
J/g•K).
– This is the reason water is
used to put out fires, it is the
reason lakes/oceans control
climate, and is the reason
thunderstorms release huge
energy.
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The intermolecular forces between
species are directly related to the
available kinetic energy of a substance.
For example, all three phases of
water have the same acting
intermolecular forces, but different
amounts of kinetic energy
• Ice has open latticelattice-like
structure.
• Ice density is < liquid and
so solid floats on water.
What is the relationship between
kinetic energy, intermolecular forces
and the phases?
Effect of IMF’s on Phase
Gases-
Effect of IMF’s on Phase
Liquids-
• Kinetic energy completely overcomes
the intermolecular forces
• Kinetic energy partially overcomes
intermolecular forces
• Assume volume and shape of
container
• Takes the shape of their container
(no inherent structure)
• Virtually incompressible
• Compressible
• Slow diffusion
• Rapid diffusion/flows easily
Effect of IMF’s on Phase
Physical Properties of Alkanes
Solids• Kinetic energy overcome by
intermolecular forces
gases
• Does not flow
liquids
• Retains own shape
• Virtually incompressible
• No to little diffusion
Heptadecane (17)
•
•
•
•
London forces are the only I.M.F’s of attraction.
Boiling point increases with length of chain.
Insoluble in water due to being non-polar (hydrophobic).
All are less dense than water
4
Physical properties of halocarbons:
Higher boiling points than hydrocarbons of
similar molecular weight but still relatively low.
•Due to polarity of carbon/halogen bond
Relatively soluble
Physical Properties of Alcohols
The –OH group is polar and capable of
hydrogen bonding.
This makes low molecular weight alcohols
highly soluble in water.
Hydrogen bonding in a water-methanol solution:
•Slightly soluble due to being slightly polar.
Some common halocarbons include:
• (CFC’S) i.e. Freon
CClF2CClF2
• (HFC’S)
CF3CHF2
• PVC
-[-CH2-CHCl-]-
Physical Properties of Alcohols, cont.
•Solubilities
in water of
selected
primary (1o)
alcohols.
Physical Properties of Alcohols, cont.
Larger alcohols have greater hydrophobic
regions and are less soluble or insoluble in
water.
Water interacts only with the hydrophilic –OH
group of 1-heptanol:
•Notice as
chain length
increases,
solubility
decreases
Physical Properties of Alcohols, cont.
The –OH group can hydrogen bond between
alcohol molecules leading to relatively high
boiling points.
Hydrogen bonding in pure ethanol:
Physical Properties of Alcohols, cont.
•Boiling points of
selected primary
(1o) alcohols
compared to
alkanes and
ethers of similar
molecular
weights
•Notice B.P.’s
increase with
increasing M.W.
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Physical Properties of Carbonyls
The carbonyl group is moderately polar,
but it doesn’t have any hydrogens
attached, so it cannot hydrogen bond
between molecules.
Physical Properties of Carbonyls, cont.
The C=O group can hydrogen bond with water
molecules.
This makes low molecular weight aldehydes
and ketones water soluble (they have small
hydrophobic sections).
Notice,
The boiling
points of
carboxylic
acids are
higher that
that of
alcohols
Physical Properties of Carbonyls, cont.
Because of the polarity of the C=O group,
these groups can interact, but the
attraction is not as
strong as hydrogen
bonding.
This makes the
boiling point of
aldehydes and
ketones higher than
alkanes, but lower
than alcohols.
Physical Properties of Carboxylic Acids
The carboxylate group is very polar.
Hydrogen bonding between carboxylates
creates dimers (two identical molecules
bonded together).
This gives carboxylic acids high boiling
points (greater than alcohols).
Physical Properties of Carboxylic Acids, cont.
Carboxylate groups can hydrogen bond
with water. If the hydrophobic –R group
is not too large, carboxylic acids are very
water soluble.
Long chained carboxylic acids were first
extracted from fats and are called “fatty
acids”
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Physical Properties of Esters
Esters are slightly polar, however have
lower b.p.’s than alcohols or aldehydes.
Low solubility
Many have very pleasant odors
Physical Properties
of Amines
Effects of hydrogen
bonding in primary
alcohols vs. primary
amines
Physical Properties of Esters, cont.
Primary and secondary amines can form
hydrogen bonds between molecules.
The N-H bond is not quite as polar as
the O-H bond.
The hydrogen bonds are not as strong
as those of alcohols, so amine boiling
points are somewhat lower than those of
alcohols.
Physical Properties of Amines, cont.
Amines can hydrogen bond with water,
making smaller amine molecules usually
water soluble.
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