Define: Choose the word from the left
• A homogeneous
• Phase
part
of
a
system
in
• Condensed
contact with other
phases
parts of the
• Intermolecular system, but
forces
separated from
• Intramolecular these other parts
by
well-defined
forces
boundaries.
1
Define: Choose the word from the left
•• Condensed
Phase phases
• Intramolecular forces
• Condensed
and intermolecular
forces
phases
• Intramolecular forces
• Intermolecular
forces
• Intramolecular
forces
Liquids and
solids
2
Define: Choose the word from the left
• Condensed phases
• Phase
• Intramolecular forces
intermolecular
• and
Condensed
forces
phases
• Intramolecular forces
• Intermolecular
forces
• Intramolecular
forces
• are attractive
forces between
molecules
3
Define: Choose the word from the left
• Condensed phases
• Phase
• Intramolecular forces
intermolecular
• and
Condensed
forces
phases
• Intramolecular forces
• Intermolecular
forces
• Intramolecular
forces
• hold atoms
together in a
molecule
4
Recall: Kinetic Molecular Theory
• All matter is made of tiny particles.
• These particles are in constant motion.
• The speed of particles is proportional to
temperature. Increased temperature means
greater speed.
• Solids, liquids, and gases differ in distances
between particles, in the freedom of motion
of particles,and in the extent to which the
particles interact.
5
Recall: Molecular Geometry, Determining
Polarity, Bond Dipole, Dipole Moment
Cl2
NH3
CH3Br
CH4
6
Recall: Molecular Geometry, Determining
Polarity, Bond Dipole, Dipole Moment
Cl2
NH3
CH3Br
CH4
7
Mr. Shields
Regents Chemistry
U07 L01
8
Forces of attraction
There exist between Molecules of
gases and liquids
Forces of attraction.
Some forces may be strong
other forces may be weak.
9
Forces of attraction
It’s these forces of attraction that ultimately
determine how easy or difficult it is gases to
coalesce into Liquids and liquids to solidify
into solids.
These forces of attraction will, for example,
determine a liquids boiling point.
10
Forces of Attraction
What are the forces of attraction we’re talking about?
For gases and liquids these forces are of two types:
intermolecular - between molecules
intramolecular - within the molecule
Intramolecular forces involve the formation of true
chemical bonds
11
Intermolecular Forces of Attraction
Intramolecular forces of attraction are much stronger
than intermolecular forces
- on the order of 30 – 400x stronger
Intermolecular forces of attraction are also known as
van der Waals forces
So how many types of intermolecular or van der
Waals forces of attraction exist?
Let’s take a look …
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Intermolecular Forces of Attraction
There are three types of intermolecular
forces:
1. London Dispersion force
(sometimes called induced dipole force*)
2. Dipole-Dipole force
3. Hydrogen bond
NYS Regents also refers (incorrectly) to
induced dipole IMF as “Van Der Waal’s”
13
Intermolecular Forces of Attraction
As we mentioned before Intermolecular forces are much
weaker than intramolecular (i.e. chemical bond) forces
And … Intermolecular forces also vary in strength from
one another:
Dispersion
Dipole-dipole
Hydrogen bond
Weakest
Stronger
The Strongest
(<1 Kcal)
(2-5 Kcal)
(12-16 Kcal)
(for comparison, a true covalent bond is about 400Kcal)
14
Intermolecular Forces of Attraction
Intermolecular forces hold molecules
together in the Solid & Liquid phase &
Allow gases to condense on cooling.
They also prevent instantaneous
evaporation from occurring.
Strong Attractive forces between
molecules also make Vaporization
(L  G) phase changes more
difficult.
Why?
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1. Dispersion Forces
Dispersion forces, the weakest force, is sometimes
called LONDON Dispersion Forces or induced dipole
forces
- They result when there is a momentary shift in
electron density within the molecules electron cloud
???
Non-polar molecules
Formation of
Temporary Dipoles
This intermolecular force is the predominant
intermolecular force for non-polar compounds
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Non-polar compounds are molecules that do
not have a permanent charge distribution
within the molecule.
How does this shift happen?
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δ -δ+ δ -δ+
δ- δ+ δ -δ+
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The larger the molecule the more easily δ charges develop 19
1. Dispersion Forces
As we said before Non-polar compounds are molecules
that do not have a permanent charge distribution within
The Molecule.
Molecules that have an even electron distribution
Are typically 1) single atoms, 2) molecules of the same
element, 3) hydrocarbons 4) symmetrical molecules
For example: Ne, Kr, H2, O2, N2, S8, C3H8, CCl4, CO2
20
Dispersion forces
Induced Dipole forces become stronger the larger the
Atom or Molecule.
- larger electron clouds are easier to deform
One might expect the boiling pt. of compounds to
be low for compounds that interact by this type of
force.
Why?
The minimal force holding these Molecules together
Allow them to move readily from the liquid
Phase into the gas phase. Or even from the solid phase
to gas phase - like I2
21
CH4
Smaller molecule
C8H18
Larger molecule
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Dispersion forces
If Molecules have High BP’s they will have Low VP
(and conversely Low Bp’s mean High VP)
Since dispersion forces increase with increasing
Molecular weight the Trend for non-polar compounds is…
increasing BP with increasing MW .
(this trend is also true with melting pts/freezing pts; as
MW inc so does mp/fp)
Low MW
Higher MW
Low BP
Higher BP
Low dispersion force
Let’s look at some bp data…
higher dispersion force
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Dispersion forces
Which member of each pair should have the higher
B.P. (and thus the stronger Induced dipole)
N2, O2
O2 – Higher MW
larger molecule
B.P: O2 -183.0 C / N2 -195.6 C
Ne, Kr
Kr - Higher MW
B.P: Kr -152.9 C / Ne -245.9 C
Larger Atom
F2, Cl2
Cl2 – Higher MW
B.P: Cl2 -34.6 C / F2 -187 C
Larger Molecule
C3H8 (propane), C4H10 (butane)
Butane (larger)
B.P: C4H10 -0.5 C / C3H8 -42.1 C
M.P:
-134.4 C
-187.7 C
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2. Dipole- Dipole Forces
Molecules that have permanent dipoles are said to be
Polar.
What does it mean to be a polar molecule?
One area is always positive and another part of the
Molecule is always negative
An example of a
Polar molecule is HCl
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Dipole-Dipole Forces
Dipole- Dipole forces are stronger than London
dispersion forces (<1 Kcal vs. 2-5 Kcal)
- In this intermolecular force
Dipoles are always present
as opposed to constantly
forming and reforming
dipoles in weaker Dispersion
force molecules
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Hydrogen Bond Forces
The hydrogen bond is a special form of the
Dipole-Dipole force, its not really a true chemical bond.
- It is the strongest intermolecular force
- much stronger than either Dipole-Dipole
Or Dispersion forces.
However It can ONLY exist when a Hydrogen atom
is present in the molecule AND it’s bonded to …
- Either a N, O, or F atom also present in the
same molecule.
27
3. Hydrogen bonds
In all compounds that form H-bonds the
Hydrogen atom is ALWAYS the Positive Dipole.
O, F, or N is ALWAYS the negative
Dipole
As one molecule approaches
another the positive Hydrogen
aligns with the negative O, F, or N.
+
H
+
F
-
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3. Hydrogen bonds
All Hydrogen bonds are stronger than either
Dispersion or dipole-dipole interactions.
However, among molecules with
Hydrogen bonds there are some that
Are stronger than others.
How do we determine what H-bonds
Are the strongest?
Strength of H-bond: F>O>>N
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3. The Hydrogen Bond
Examples of compounds that can form H-bonds are:
Water, Methyl or Ethyl Alcohol, HF, NH3
30
Water Alignment during formation of the
H-Bond
Positive aligns to negative
Water molecules in ice align to form a repeating hexagon
31
Hydrogen Bonds
Compounds that form Hydrogen bonds take a lot
more energy to move molecules from the liquid phase
Into the gas phase.
Molecules of similar molecular weight without the
Ability to form H-bonds have higher vapor pressures
And thus lower B.P.
Compound
Molar Mass
B.P. deg C
H-bond
H20
18
+ 100
H-bond
NH3
17
- 33
Non-polar
CH4
16
- 164
32
These compounds form H-Bonds
B.P
-75
Theoretical B.P
Of Water
dec. MW
If F forms stronger H bonds than O why is the BP of
Water higher than HF? (Hint: Consider the Mol. Structure)
33