MEMBERS:

Lagbang, Marjun
Dayupay, Princess
Logronio, Reinhart
Franco, Abbie
Bautista, Angelo
Tillano, Kristine Eve
Yap, Marc Adrian
Vasaylaje, Princess Marie
INTERMOLECULAR
FORCES
What are intermolecular forces?

• These are the forces which mediate

interaction between molecules, including
forces of attraction or repulsion which act
between molecules and other types of
neighboring particles.
• It exist between molecules and influence the
chemical properties.
 Two factors determine whether a substance is a solid,
a liquid, or a gas

• The kinetic energies of the

particles (atoms, molecules, or
ions) that make up a substance.
It tends to keep the particles
moving apart.
• The attractive intermolecular
forces between particles that
Types of attractive forces

There are several types of attractive intermolecular

forces:
• Dipole-dipole forces,
• London dispersion forces,
• Hydrogen bonding, and
• Induced-dipole forces
The first three forces are also collectively called van
der Waals forces
Types of attractive forces

• All molecular and intermolecular attractive forces

are electrostatic in nature. That is, they involve
attractions between positive and negative species.
• All of the intermolecular forces that hold a liquid
together are called cohesive forces.
How Intermolecular and Intramolecular Force
differ

• The intermolecular forces are weaker and exists

between molecules to hold the molecules together,
while the intramolecular forces are stronger and
exist within molecules to hold the atoms together.
• Intermolecular forces (IMF) are the forces which
cause real gases to deviate from ideal gas
behavior. They are also responsible for the
formation of the condensed phases, solids and
liquids.
• The IMF govern the motion of molecules as well.
The Intermolecular force is also responsible for transition
of phases:
• As temperature increases, the heat increases the
kinetic energy of each molecules begins to overcome
and break the intermolecular forces of each other
molecules.
• If the average kinetic energy is greater than the
attractive forces between the particles, a substance will
not condense to form a liquid or a solid. If the kinetic
energy is less than the attractive forces, a liquid or
solid will form.
• The average kinetic energy of the particles in a gas is
great enough to overcome the forces of attraction
between them. The molecules of a gas move apart
when they collide.
In the solid phase, the allowed motions are in
vibrational movements within the molecules.
In the case of water, the bonds are stretching and bending.
The bond lengths and angles are oscillating around the
predicted values.
When the Kinetic energy breaks the Intermolecular forces, the
molecules become loose and starts moving freely apart.
Heating curve

• In this Heating Curve, we

are starting with ice at -
20oC.
• When the temperature
reaches 0oC, the melting
point of ice, further addition
Heating curve
• At the phase transition
temperature, 0oC, all of the ice
will be converted to liquid
water. As soon as the phase
change is complete, addition
of heat will then lead to an
increase in temperature of the
liquid water.
Heating curve
• There is a second phase
transition at 100oC. At this
temperature, the water, at
100oC, is converted to steam
(gas).
Intermolecular Force Strength

• The heat of fusion (heat required to melt a solid)

and heat of vaporization (heat required to
vaporize a liquid) are determined by the strength
of the Intermolecular Forces.
• Substances with high IMF will have higher melting
and boiling points. It will require more energy to
break the IMF.
• Since all IMF are electrostatic in nature, the interaction
of positive and negative charges. The strength of the
IMF will, then, depend on the magnitude of the charges.
 Ion Bonding

• The strongest IMF is ionic bonding.

• Substances with high IMF will have higher melting
and boiling points. It will require more energy to
break the IMF.
• In salts, there are full positive charges on the
cations, which have lost electrons, and full
negative charges on the anions, which have
• One of the defining features of salts is their extremely
high melting points. A large amount of energy is
required to separate the positive and negative ions
from their positions in the crystalline lattice.
 Ion-dipole attraction

• The next strongest IMF is ion-dipole.

• This involves the interaction between the full
charge of an ion and the partial charge of a polar
compound its dipole moment.
• The partial charge on the polar compound is
smaller than a full positive or negative charge on
ions, so the interaction will not be as strong.
 dipole-dipole attraction

• These are the interactions that

exist between neutral, but polar
substances, or molecules which
have permanent dipoles.
• These interactions tend to align
the molecules to increase
attraction (reducing potential
• They involve the attraction of partial positive and
partial negative charges present in polar compounds.
 London Dispersion Forces
• These are the IMF that exist between non-polar
atoms or molecules. They are due to attractions
between opposite charges.
• The attractive forces are due to instantaneous dipole
moments
• The picture below represents a "snapshot" of two
helium atoms.
• At this particular instant, both valence electrons on
the atom on the right are on the same side of the
nucleus.
• This will create an instantaneous dipole moment in
that helium atom. There will be an attraction
between this area of negative charge and the
nucleus of an adjacent helium atom.
• In addition, the electrons on the second atom will be
repelled by the electrons on the first, and will also
form an "induced" dipole in the second atom.
• The forces are very weak, and only operate over very
short distances and its strength depends on how
easily the electron cloud of an atom or molecule can
be distorted or polarized.
• The further from the nucleus that an electron exists,
the more loosely it is held and the more polarizable it
will be, leading to a stronger LDF.
 Hydrogen Bonding
• This is a special case of dipole-dipole
interactions. The partial positive charge
comes from a hydrogen atom bonded to
Fluorine, Oxygen or Nitrogen.
• The partial negative charge comes from a
lone pair on O, N or F.
• These interactions tend to align the
molecules to increase attraction (reducing
• Hydrogen bonds are strong intermolecular forces created

when a hydrogen atom bonded to an electronegative atom

approaches a nearby electronegative atom.

• Greater electronegativity of the hydrogen bond acceptor will

lead to an increase in hydrogen-bond strength.

• The hydrogen bond is one of the strongest intermolecular

attractions, but weaker than a covalent or an ionic bond.

• When Hydrogen is bonded to one of those very
electronegative elements, the hydrogen is exposed.

• Hydrogen bonds are responsible for holding together

DNA,

proteins, and other macromolecules.

• The electromagnetic attraction created between a

partially positively charged hydrogen atom attached to
a highly electronegative atom and another nearby
electronegative atom.
 Hydrogen Bond Donor

• A hydrogen atom attached to a relatively

electronegative atom is a hydrogen bond donor.

• This electronegative atom is usually fluorine, oxygen, or

nitrogen.

• The electronegative atom attracts the electron cloud

from around the hydrogen nucleus and, by
decentralizing the cloud, leaves the hydrogen atom with
a positive partial charge.
• Because of the small size of hydrogen relative to other
atoms and molecules, the resulting charge, though only
partial, is stronger.

• In the molecule ethanol, there is one hydrogen atom

bonded to an oxygen atom, which is very
electronegative. This hydrogen atom is a hydrogen bond
donor
 Hydrogen Bond Acceptor

• A hydrogen bond results when this strong partial

positive charge attracts a lone pair of electrons on
another atom, which becomes the hydrogen bond
acceptor.

• An electronegative atom such as fluorine, oxygen, or

nitrogen is a hydrogen bond acceptor, regardless of
whether it is bonded to a hydrogen atom or not.
• Greater electronegativity of the hydrogen bond acceptor
will create a stronger hydrogen bond. The diethyl ether
molecule contains an oxygen atom that is not bonded to
a hydrogen atom, making it a hydrogen bond acceptor.
 Hydrogen bond donor and hydrogen
bond acceptor

• Ethanol contains a hydrogen atom that is a hydrogen

bond donor because it is bonded to an electronegative
oxygen atom, which is very electronegative, so the
hydrogen atom is slightly positive.

• Diethyl ether contains an oxygen atom that is a

hydrogen bond acceptor because it is not bonded to a
hydrogen atom and so is slightly negative.
1. These interactions tend to align the molecules

to increase attraction (reducing potential energy).

a) Ionic Bonds

b) Covalent Bonds

c) Hydrogen Bonds
2. The partial positive charge comes from a

hydrogen ______ bonded to Fluorine, Oxygen or

Nitrogen.

a) Atom

b) Charge
3. __________ electronegativity of the hydrogen

bond acceptor will lead to an increase in

hydrogen-bond strength.

a) Greater

b) Lesser
4. When a hydrogen atom attached to a relatively

electronegative atom is a __________________.

a) Hydrogen Bond Donor

b) Hydrogen Bond Acceptor

c) Hydrogen bond donor and hydrogen bond

acceptor
5. It contains Ethanol which is a hydrogen atom

that is a hydrogen bond donor because it is

bonded to an electronegative oxygen atom?

a) Hydrogen Bond Donor

b) Hydrogen Bond Acceptor

c) Hydrogen bond donor and hydrogen bond

• ANSWERS!

C
A
A
A
C