Polarity and Intermolecular

Forces
Review
Electronegativity (EN) is an atom's tendency
to attract electrons in chemical bonds.
EN increases to the right and up on the periodic
table, excluding the noble gases.
We know how to predict the 3-D geometry of
these molecules and ions, if we apply the
VSEPR Theory.
We know how to determine if a molecule is
polar or nonpolar.
Polarity of Diatomic Molecules
Diatomic Molecules -
molecules made of only two
atoms.
If atoms are the same, molecule is
nonpolar.
If atoms are diff., molecule is polar.
NOTE: Polar does not mean
charged.
Is Cl2 polar or nonpolar?
Molecules With 3 or More
Atoms
Generally, a molecule with 3 or more
atoms is:
Polar if its central atom has lone
pairs OR
If the outer atoms are not all the
same.
Nonpolar if its central atom has no
lone pairs AND
All the outer atoms are identical.
CO2 vs. H2O
Consider the Lewis structure of CO2:

This molecule is nonpolar.

CO2 vs. H2O
Consider the Lewis structure of H2O:

This molecule is polar.

CH4 vs. CH3Cl
Neither CH4 nor CH3Cl has any lone pairs
on the central carbon atom.
Is CH4 polar or nonpolar?
Is CH3Cl polar or nonpolar?
 EXPERIMENT # 1

How many drops of water fit on the head of a penny?

How many drops of isopropanol fit on the head of a

penny?
 EXPERIMENT # 1
Why did more water drops fit on the surface of the
penny than isopropanol drops?

 This is due to surface tension caused

by the cohesive property of water.
 Isopropanol does not have surface
tension in the same way that water
does because it does not feature
hydrogen bonding between molecules
due to the lack of polarity
 EXPERIMENT # 2

Which evaporates faster, water or isopropanol?

 EXPERIMENT # 2
Why does isopropanol evaporate faster than water?
 Isopropanol does not have hydrogen bonding between
molecules due to the lack of polarity. This results in
the molecules not “sticking together” more, as
compared to water. Isopropanol turns liquid to gas at
82.5C. Since the vaporization of isopropanol occurs at a
lower temperature than water, this means it takes less
energy to turn isopropanol into a gas, and therefore it
will evaporate faster than water.
 Isopropanol has weaker intermolecular forces holding
its molecules together, so it takes less energy (a lower
temperature) to separate the molecules to enter the
 EXPERIMENT 2
Why does isopropanol evaporate faster than water?

 In water, the strong force due to H-bonding

(molecules are more stuck together) must be
overcome for liquid water to become a gas. It
takes a lot of energy to overcome the force of
hydrogen bonding.
Properties of water:
1. Polarity
 The hydrogen atoms and one
oxygen atom form polar
covalent bond - the shared
electrons spend more time
associated with the oxygen
atom than they do with
hydrogen atoms.
 The result is that there is a
slight positive charge on each
hydrogen atom and a slight
Properties of water:

2. Cohesion
 In water, the attraction between hydrogen and
oxygen is called hydrogen bonding.
 Hydrogen bonding is one of the strongest
intermolecular forces, which makes neighboring
water molecules strongly attracted to one
another. This is called cohesion.
 The property of cohesion describes the ability of
water molecules to be attracted to other water
molecules.
Properties of water:
3. Surface tension
 Water molecules at the surface of liquid water
have fewer neighbors, they will have an even
stronger attraction to the few water molecules
that are nearby. This enhanced attraction is
called surface tension.
 It makes the surface of the liquid water slightly
more difficult to break through than the interior
of the water.
 Cohesion gives rise to surface tension, the
capacity of a substance to withstand rupture
Properties of water:
4. Solvent
 Water is a solvent—a substance capable of
dissolving another substance. Because of water’s
polarity and structure, ionic compounds and
polar molecules can readily dissolve in it.
 The charged particles in water molecules will
form hydrogen bonds with a surrounding layer of
water molecules. When a substance readily
forms hydrogen bonds with water, it can dissolve
in water. Since water can dissolve so many it is
important in its many roles in living systems.
Properties of water:
5. Evaporation
 The hydrogen bonds in water allow it to absorb
and release heat energy more slowly than
many other substances. Water can absorb a
great deal of energy before its temperature
rises.
 Hydrogen bonding causes water molecules to
“stick” together in liquid form. This force must
be overcome for liquid water to become a gas. It
takes a lot of energy to overcome the force
of hydrogen bonding.
INTERMOLECULA
R FORCES OF
ATTRACTION
Which
attachment is
weaker?
Intramolecular forces are the forces that
hold atoms together within a molecule
(chemical bond)
Intermolecular forces are forces that
exist between molecules
 These forces of attraction are responsible for the
existence of the different phases of matter
 IMFA, in combination with the particles’ kinetic
energy, determine the properties of each phase
and the possible phase changes
 Strong IMFA tend to yield liquids and solids, while
Types of intermolecular forces
of attraction (IMFA)
Arranged from strongest to
weakest:
1. Ion-ion interaction
2. Ion-dipole interaction
3. Hydrogen-bonding
4. Dipole-dipole interaction
5. Dipole-induced dipole interaction
6. London dispersion forces (or induced
dipole – induced dipole)
 ION - ION INTERACTION
 Exist between ions
 This IMFA is based on Coulomb’s law, which
suggests that the force of attraction
between two oppositely charged ions is
directly proportional to the magnitude of
the charges of the ions but is inversely
proportional to the distance between them.
 Two oppositely-charged
particles will be
attracted toward each
other, and the force
becomes stronger and
stronger as they
approach until
eventually they will
stick together and a
considerable amount of
energy will be required
to separate them.

 They form an ion-pair, a

new particle which has
 Examples of Ion-ion interaction
 Sodium chloride: When a saturated solution
of sodium chloride (NaCl) has too many
ions, the sodium cations (Na) attract the
chloride anions (Cl) to form solid salt
crystals.
 Ionic solids: The strong bonds that hold
ionic solids together are the result of ion
interactions.
 Potassium chloride (KCl)
 Potassium fluoride (KF)
 Potassium bromide (KBr)
 Potassium iodide (KI)
 Sodium fluoride (NaF)
Types of intermolecular
forces
ION-DIPOLE
 Ion-dipoleINTERACTIONS
interaction: results from the
attraction between a cation (+) or anion
(-) with polar molecules.
 The ions will be attracted to the
opposite charges present in the dipole
resulting to this type of attraction
 HYDROGEN BONDING
• Exist between polar molecules containing hydrogen
atoms bonded to very electronegative atoms such as
fluorine (F), nitrogen (N), and oxygen (O).
• A hydrogen atom with a partial positive charge is
attached to a N, O, or F with a partial negative charge.
Types of intermolecular
forces
 H-bonding is a relatively strong force of
attraction between molecules, and
considerable energy is required to
break hydrogen bonds.
 This explains the exceptionally high
boiling points and melting points of
compounds like H2O and HF.
 Examples of Hydrogen bonds:
1. Water (H2O): Hydrogen bonding in water
results in the crystal structure of the ice,
making it less dense than water and able to
float.
 Water striders use the high surface
tension of water to help them stay
above water

The H-bonds formed between water

molecules in water droplets are
stronger than the other
intermolecular forces between the
water and the leaf, contributing to
high surface tension and distinct
 The base pairs of guanine with
cytosine and adenine with thymine
connect to one another using
hydrogen bonds. These H-bonds
are what keep the two strands of a
DNA helix together
Types of intermolecular forces: van
der Waals

DIPOLE – DIPOLE INTERACTIONS

 Exist among polar molecules
 Occur when the partially positively
charged part of a molecule interacts
with the partially negatively charged
part of the neighboring molecule.
 Dipole−Dipole Interactions
Polar molecules are attracted to each other
by dipole−dipole attractions when the
positive end of one dipole is attracted to the
negative end of a second dipole, such as the
attractive forces between two molecules of H
—Cl.
 DIPOLE – INDUCED DIPOLE
INTERACTIONS
 Exists between a polar covalent molecule and
a nonpolar covalent molecule
 Partial charges present in dipole cause the
polarization or distortion of electron
distribution in the other molecule. This gives
rise to regions of partial positive and partial
negative poles.
 The other molecule becomes an induced
dipole, which will be attracted to the original
Van der Waals (Dipole-induced dipole interaction)
 London dispersion forces
(Induced dipole – induced dipole
interaction)

 Weakest of the IMFA and exist between all

types of molecules, whether ionic or covalent –
polar or nonpolar.
 Nonpolar molecules have only London
dispersion forces
 Depends on the molar mass.
Bigger Molar Mass → More Electrons
(Molecules or atoms with a higher molar mass have
 London dispersion forces
(Induced dipole – induced dipole
interaction)

 The more electrons a molecule has, the

stronger the London dispersion forces are.
 For example, Br2 has more electrons than Cl2,
so bromine will have stronger London
dispersion forces than chlorine, resulting in a
higher boiling point for bromine.
 The breaking of London dispersion forces doesn’t
require that much energy, which explains why
nonpolar covalent compounds like methane (CH4),
oxygen, and nitrogen – which only have London
dispersion forces of attraction between the molecules
– freeze at very low temperatures.
 Types of Intermolecular Forces of
Type of IMFA Attraction (IMFA)
Interacting Examples
Substances
Ion-dipole Ion (cation or NaCl dissolved in
anion) and a polar H2O
molecule
Hydrogen bonding Polar molecules H2O, NH3, CH3OH
containing H
bonded to N, O,
and F
Dipole-dipole
London dispersion AllPolar molecules
substances and OCH 3F and H2S, HCl
2, N2, He gas, Br2
Dipole-induced
forces Polar
solelyand nonpolar
for nonpolar HI and CH4
dipole molecules
molecules and
Bonding and Attractive Forces
EFFECTS OF IMFA
ON PROPERTIES OF
SUBSTANCES
Properties of Substances

1. Solubility: the ability of a solid

substance to be dissolved in a
given amount of solvent
 A solid substance is said to be
soluble in a liquid if it can be fully
dissolved in the liquid.
 ‘like dissolves like’
“Like Dissolves Like”
Polar molecule dissolves in a
polar molecule.
Nonpolar molecule dissolves in a
nonpolar molecule.
Polar and nonpolar molecules do
not dissolve each other.
Properties of Substances

2. Miscibility: the ability of the two

liquids to combine or mix in all
proportions, creating a homogeneous
mixture
 A liquid substance is said to be miscible
with another liquid if the two liquids mix
completely without leaving a boundary
between them.
“Like Mixes With Like”
Polar molecules mix with each
other.
Nonpolar molecules mix with
each other.
Polar and nonpolar molecules do
not easily mix.
Examples of Miscible Liquids
Acetic acid and water
Alcohol and water
Grease and Water Don't Mix!
Amphipathic Molecules
Amphipathic - has a hydrophobic
region and a hydrophilic region.
Hydrophobic - “water-fearing”
Nonpolar
Hydrophilic - “water-loving”
Polar or charged
Surfactants such as dish detergents
contain amphipathic molecules.
Why?
Amphipathic Molecules

Hydrophilic head

Hydrophobic tails
Amphipathic Molecules
Properties of Substances

3. Boiling point: the temperature at which

the vapor pressure and atmospheric pressure
of a liquid substance are equal
: the temperature at which a liquid turns
into a gas.

 Molecules with strong intermolecular forces (like

hydrogen bonds or dipole-dipole attractions) hold
onto each other tightly, so it takes more heat to
make them boil.
 Molecules with weak intermolecular forces (like
Properties of Substances

4. Melting point: temperature at which solid

becomes liquid. At this point, the solid and
liquid phases exist in equilibrium
Molecules in solids stick together because of
intermolecular forces (IMFA):
 If the forces are strong, the molecules hold
tightly, and the solid won’t melt until it gets
really hot.
 If the forces are weak, the molecules come
apart easily, and the solid melts at a lower
temperature.
Properties of Substances

5. Viscosity: the resistance of the liquid to

flow
• A liquid with high viscosity is thick and
flows slowly, like honey or syrup.
• A liquid with low viscosity is thin and flows
quickly, like water or juice.

 Strong IMFA: Molecules stick tightly to each

other, making it harder for the liquid to flow.
This makes the liquid thicker and increases
viscosity.
Properties of Substances

6. Vapor pressure:
Vapor pressure is a measure of how much a
liquid wants to turn into a gas.

 Even though the water is a liquid, some of the

water molecules at the surface can escape
into the air and become water vapor (gas).
The pressure created by these escaping
molecules is called vapor pressure.
•Strong IMFA (like in water) hold the molecules
tightly together, so it’s harder for them to
escape and turn into gas. This means the vapor
pressure will be low because fewer molecules
are able to escape.

•Weak intermolecular forces (like in alcohol)

make it easier for molecules to escape from the
liquid and turn into gas. This means the vapor
pressure will be high because more
molecules are able to escape into the air.
Properties of Substances

7. Volatility
Volatility is how easily a substance turns from
a liquid into a gas.
• A highly volatile substance evaporates
quickly, while a low volatility substance
evaporates slowly.

 For example, if you spill some rubbing alcohol

(which is highly volatile), it will disappear into the
air quickly. But if you spill some syrup, it will stay
liquid for a long time because it’s not very volatile.
• Weak IMFA(like in alcohol or gasoline) make
it easier for the molecules to break free from
the liquid and turn into a gas. This means the
substance is highly volatile and evaporates
quickly.
• Strong IMFA (like in water or syrup) hold the
molecules more tightly together, so it's harder
for them to escape into the air. This means the
substance is low in volatility and evaporates
slowly.
High vapor pressure = High volatility
(evaporates quickly) — like perfume,
rubbing alcohol

Low vapor pressure = Low volatility

(evaporates slowly) — like syrup or
honey
Activity:
Put a check (/) to the properties applicable
for polar molecule.

/
____1. high boiling point ____7. high vapor
pressure /
/
____2. low boiling point / ____8. low vapor
pressure
____3. high melting point/ ____9. high surface
/
tension
____4. low melting point/ ___10. low surface
tension
Activity:
Put a check (/) to the properties applicable
for polar molecule.

/
____1. high boiling point ____7. high vapor
pressure /
/
____2. low boiling point / ____8. low vapor
pressure
____3. high melting point/ ____9. high surface
/
tension
____4. low melting point/ ___10. low surface
tension
 Polar molecules Nonpolar molecules
IMFA: H-bonding & dipole- IMFA: London dispersion
dipole
Exist as solids or liquids at Exist as gases at room
room temperature temperature
High boiling point Low boiling point
High melting point Low melting point
High surface tension Low surface tension
Low vapor pressure High vapor pressure
Low volatility High volatility
Soluble in water Insoluble in water
 CCl4 CCl4 CCl4 CCl4
Both are nonpolar molecules CH4
CH4 but CCl4 has stronger London
dispersion forces since it has
higher molar mass.
H2O H2O H2O H2O
H2O is a polar molecule and H2S
IMFA is H-bonding which is
stronger than dipole-dipole
Ethanol, CH3CH2OH
Methanol
Both have H-bonding but ,
ethanol has higher molar
mass than methanol CH3OH
Acetic acid, CH3COOH Aceton
Acetic acid has H-bonding,
e,
acetone has dipole-dipole CH3O
interaction CH3
Activity:
1. Distinguish intramolecular forces of
attraction and intermolecular forces of
attraction.

• Intramolecular forces of attraction

exist within a molecule. Ionic, covalent and
metallic bonds constitute the
intramolecular forces of attraction.
• Intermolecular forces of attraction are
forces of attraction between molecules.
Ion-dipole, H-bond, dipole-dipole and
2. In your own words, explain the
Solubility Rule for polar and non polar
substances.

Solubility rule states that “like dissolves like.”

This means that polar solutes are soluble
only to polar solvents while nonpolar solutes
are only soluble to nonpolar solvents.
3. What relationship can you derive
between the strength of IMFA and the
physical properties of molecules?

• Boiling point, melting point, viscosity and

surface tension increase with increasing
strength of IMFA.
• Vapor pressure increases with decreasing
strength of IMFA.
• Boiling point, melting point, viscosity and
surface tension are directly proportional to the
strength of IMFA while vapor pressure is
KEY CONCEPTS:
1. There are several types of IMFA and below
they are arranged from STRONGEST to
WEAKEST.
Ion-dipole → H-bonding→ dipole-dipole →
dipole-induced dipole→ London forces of
attraction

2. The strength of IMFA greatly affects the

physical properties of substances such as
boiling point, melting point, vapor pressure,
3. The boiling point, melting point, viscosity
and surface tension increase as the strength
of intermolecular forces increases. On the
other hand, vapor pressure and volatility
decrease with increasing strength of IMFA.
London dispersion forces increase as the
molecular mass of a substance increases.