Intermolecular
forces may be
attractive or
repulsive.
 Johannes D van der
Waals, Dutch, was
the first to postulate
intermolecular forces
in developing a
theory to account for
properties of real
gases.
Intermolecular Forces
The attractions between molecules
are not nearly as strong as the
intramolecular attractions that hold
compounds together.
Intermolecular Forces
They are, however, strong enough
to control physical properties such
as boiling and melting points,
vapor pressures, and viscosities.
Intermolecular Forces
These intermolecular forces as a
group are referred to as
Van der Waals forces.
Van der Waals Forces
• Van der Waals forces are also known as
London forces.
• They are weak interactions caused by
momentary changes in electron density in
a molecule.
• They are the only attractive forces present
in non polar compounds.
6
•Weakest intermolecular force; present in all
organic molecules
•The surface area of a molecule determines the
strength of the Van der Waals interactions
between molecules.
•The larger the surface area, the larger the
attractive force between two molecules, and
the stronger the intermolecular forces.
•Van der Waals forces are also affected by
polarizability.
Polarizability is a measure of how the
electron cloud around an atom responds to
changes in its electronic environment.
Dipole-Dipole Interactions
 Molecules that have
permanent dipoles are
attracted to each other.
– The positive end of one
is attracted to the
negative end of the other
& vice-versa.
– These forces are only
imp. when the molecules
are close to each other.
Dipole-Dipole Interactions
The more polar the molecule, the
higher is its boiling point.
Between two polar compounds
Hydrogen Bonding
The dipole-dipole
interactions
experienced when H is
bonded to N, O, or F
are usually strong.
We call these
interactions hydrogen
bonds.
Hydrogen Bonding
Hydrogen
bonding arises in
part from the high
electro negativity
of nitrogen,
oxygen, and
fluorine.
Also, when hydrogen is bonded to one
of those very electronegative elements,
the hydrogen nucleus is exposed.
Summarizing Intermolecular Forces
Boiling point – defined as the
temperature at which the vapor pressure
of the substance equals the pressure of
the atmosphere above it.
Boiling Point
• The boiling point of a compound is the
temperature at which liquid molecules are
converted into gas.
• In boiling, energy is needed to overcome
the attractive forces in the more ordered
liquid state.
• The stronger the intermolecular forces, the
higher the boiling point.
15
Consider the example below. Note that the
relative strength of the intermolecular forces
increases from pentane to butanal to 1butanol. The boiling points of these
compounds increase in the same order.
For two compounds with similar functional groups:
• The larger the surface area, the higher the boiling
point.
• The more polarizable the atoms, the higher the
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boiling point.
Data and results:
 N- butyl alcohol
-C4H9OH
-Has a boiling
point of 118 C.
(Literature boiling point)
Data and results:
Tert- Butyl
alcohol ; C4H10O
- Has a boiling
point of 82 C
Data and results:
N-amyl Alcohol
C5H11OH
- Has a boiling point
of 136-138 C
CONCLUSION
•Non volatile impurities
usually decrease the vapor
pressure of the liquid, causing
an increase in the boiling
point.
•Volatile impurities usually
decrease the boiling point due
to the increase in the vapor
pressure
•Compounds having strong
bonds need higher boiling point.
Weak bonds on the other hand
exhibit lower boiling points.
•More Carbon atoms mean
stronger bonds, increase in
boiling point
•Less carbon atoms mean weaker
bonds, decrease in boiling point
•If the compounds have
the same number of
carbon atoms, depend on
the structure:
*Straight chainedStrong bonds; needed
high boiling point
*Branched chainweaker bonds; low boiling
point
Reference:
http://www.slideshare.net/junita55/boiling-presentation
http://www.suu.edu/faculty/howard/courseinfo.html
http://www.google.com.ph/imgres?q=boiling+point+gif
http://www.slideshare.net/gadams61/intermolecularforces-3015692
http://www.slideshare.net/kwarne/intermolecular-forcesproperties
http://www.slideshare.net/gjr2323/lecture5-123101
Reference:
http://www.slideshare.net/opalair1020/intermolecularchemical-bonding
http://www.digitalofficepro.com/ppt/nature-environmentpowerpoint-templates.html
http://www.slideshare.net/kwarne/intermolecular-forcesproperties
http://www.slideshare.net/elmochem/chapter-11-lectureintermolecular-forces-liquidssolids?src=related_normal&rel=5787060
http://www.slideshare.net/search/slideshow?searchfrom=he
ader&q=Molecular+forces
Group Members:
Morana, Kevin
Caunan, Marisol
De Leon, Lourdez
Delos Reyes, Pam
Principe, Geene