The document discusses the four main types of intermolecular forces: ion-ion interaction, dipole-dipole interaction, hydrogen bonding, and dispersion forces. It describes the characteristics of each type of interaction and provides examples. The document also explains how intermolecular forces influence various physical properties of substances, such as surface tension, viscosity, capillary action, and evaporation/vaporization. Stronger intermolecular forces generally result in higher melting and boiling points, as well as increased viscosity.
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Describe the general types of
intermolecular forces.
Explain the effects of
intermolecular forces on the
properties of substances.
IN THIS MODULE, YOU’LL BE ABLE TO:
3. INTERMOLECULAR FORCES
Intermolecular Forces are the forces of attraction or
repulsion which act between neighboring particles.
Particles such as:
● Atoms
● Molecules
● Ions
Intermolecular forces is also called Van der Waals
forces was named after the physicist Johannes Diderik
van der Waals, who in 1873 first postulated these
intermolecular forces in developing a theory to
account for the properties of real gases.
Image by macrovector on Freepik
4. INTERMOLECULAR FORCES
In 1930, Fritz London, a German-born physicist
(March 7, 1900 - March 30,1954) traced
intermolecular forces to electron motion within
molecules. He needed Quantum Mechanics(the
branch of physics that deals with the behavior of matter and
light on a subatomic and atomic level) in order to
correctly describe the forces. The investigation of
intermolecular forces starts from macroscopic
observations which indicate the existence and
action of forces at a molecular level.
By GFHund - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=11197534
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ION-ION INTERACTION DIPOLE-DIPOLE INTERACTION
HYDROGEN BONDING DISPERSION FORCES
FOUR TYPES OF INTERMOLECULAR FORCES:
7. ION-ION INTERACTION
Ion-ion Interaction exists between oppositely charged ions. It occurs between ionic
compounds. Most ion-ion interaction is strong and compounds which have them to have
high melting and boiling points. Ions of like charges repel while opposite charges attract.
The compound orients itself in such a way as to minimize repulsion. The strength of ion
interaction is inversely proportional to the square of distance between the ions.
● They are also referred to as
Ionic bonds and are the forces that hold
together ionic compounds.
Image by macrovector on Freepik
8. DIPOLE-DIPOLE INTERACTION
Dipole – Dipole Interaction occurs between polar molecules. This is due to the partial
positive pole and the partial negative pole of the molecule. Average dipole-dipole
interaction is relatively weak, around 4kJ/ mol. This interaction is effective over a very
short range. The strength of dipole-dipole interaction is inversely proportional to
distance raised to the fourth power (d4).
● Dipole-dipole forces are attractions between
polar molecules with opposite partial charges.
● They depend on the polarity, size, shape,
and distance of the molecules involved
https://www.breakingatom.com/learn-the-periodic-table/dipole-attraction
9. HYDROGEN BONDING
Hydrogen Bonding - is a very strong Dipole-dipole Interaction. Hydrogen bond occurs in
polar molecules containing H and any one of the highly electronegative elements, in
particular F, O, N. Hydrogen tends to be strongly positive due to the strong tendencies
of F, O, or N to attract the electron towards it. The highly electronegative elements
make hydrogen strongly positive.
https://upload.wikimedia.org/wikipedia/commons/b/b5/Hydrogen-bonding-in-water-2D.png
● Hydrogen bonding is responsible for the
unusually high boiling point and melting point
of water as compared to compounds of
similar molecular weight and geometry.
Typically, H-bond is in the range of 15-20
kJ/mol.
● Hydrogen bond occurs in polar molecules
containing H and any one of the highly
electronegative elements, in particular F, O,
N.
10. HYDROGEN BONDING
● The weak attractive force which binds the partially positively charged hydrogen
atom of one molecule, with the partially negatively charged atom of other
molecules of a similar or different type, or with some other negative center of
the same molecule.
Example: In the molecule of Ammonia, NH3,the N
atom is highly electronegative and acquires a partial
negative charge due to the pulling of the shared
pair. Therefore, in NH3, the H atom possesses a
partial positive charge. Due to the presence of
partial positive and negative charges, several
molecules of NH3 linked together through hydrogen
bonds.
11. DISPERSION FORCES
Dispersion forces -are the weakest kind of intermolecular attraction and occur between
molecules. They are thought to be caused by the motion of electrons. This is because they
are temporary attractive force that results when the electrons in two adjacent atoms
occupy a position that make the atoms form temporary dipoles. The strength of dispersion
forces increases as the number of electrons in a molecule increase.
12. Relative Strength of Intermolecular Forces of
attraction
Intermolecular Forces Occurs between Relative Strength
Ion-Ion Interaction Ionic Compounds Depends on the
electrostatic charge
density of the
interacting ions
Dipole-Dipole
Interaction
Partially oppositely
charged ions
Strong
Hydrogen Bonding H and O,N, F of atom Strongest of the dipole
attraction
London Dispersion
Force
Temporary or Induced
Dipoles
Weakest
14. SURFACE TENSION
● SURFACE TENSION caused by cohesive
forces (intermolecular forces)
between molecules allowing liquids to
create a thin film on its surface. This
causes liquids to acquire a certain
shape when put on a container or
dropped on surfaces.
15. FORMATION OF MENISCUS
● Concave Meniscus - occurs when there is stronger adhesive force between the
container and the liquid than the liquid’s molecules. The adhesive force overcomes
the cohesive force of the liquid. This causes the liquid to climb up the sides of the
container.
● Convex Meniscus - occurs when there is
stronger cohesive force between the
liquid’s molecules than the adhesive force
between the liquid and the container. This
causes the liquid to create a dome shape
on its surface.
16. VISCOSITY
● VISCOSITY - is a measure of a fluid's resistance to flow. The more viscous a liquid
is the thicker its consistency. In layman’s term it is the measure of the thickness
of a liquid.
➢ In general, stronger intermolecular forces
leads to higher viscosity. The liquid with
the longest time is the most viscous
while the one with the shortest time is
the least viscous. The most viscous
liquid has the stronger intermolecular
force and the least viscous liquid has the
weakest intermolecular force.
17. CAPILLARY ACTION
CAPILLARY ACTION- a
phenomenon wherein a liquid is
able to rise up on a narrow
tube. Adhesive forces between
the tube and the liquid allow
the liquid to exceed its weight.
The narrower the tube, the
higher the liquid will reach.
18. CAPILLARY ACTION
Evaporation / Vaporization - the process of turning liquid into gaseous form.
Weaker intermolecular forces equate to greater volatility.
● Vaporization is defined as the
phase transition of a compound or
an element that occurs during the
boiling or sublimation process.
● Evaporation is a type of
vaporisation which mostly occurs
at temperatures below the boiling
point.
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