Intermolecular forces are the attractive forces between molecules. The main types are London dispersion forces, dipole-dipole forces, and hydrogen bonding. Properties like melting point, boiling point, solubility, and viscosity depend on the strength of intermolecular forces - stronger forces require more energy to overcome and result in higher melting and boiling points, like dissolving like, and slower flow. Hydrogen bonding is the strongest intermolecular force.

1. INTERMOLECULAR
FORCES OF
ATTRACTION

3. Explore
QUESTIONS:
1. What do you think this illustration
represents?
2. What happens when ends with
similar signs (- & -) (+ and +)
interact?
3. What happens when ends different
signs (- & +) interacts?
4. Is this scheme of interaction true to
all types of molecules? Explain

4. Intermolecular forces are the attractive forces present between
molecules.
Generally, they are called van der Waals forces, named after
the Dutch scientist Johannes van der Waals.
The different types of intermolecular forces of attraction (IMFA)
are:
• London dispersion forces (LDF)
• Dipole-dipole forces
• Hydrogen bonding

5. London dispersion forces
• Dispersion force is present in all molecules. It is the only
force present in nonpolar molecules.
• It is very weak and acts in very small distances. It is formed
due to the attraction between the positively charged nucleus
of an atom with the negatively charged electron cloud of a
nearby atom.
• This interaction creates an induced dipole.

7. 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.
Dipole-Dipole Forces

9. Hydrogen Bond Hydrogen bond
◦ 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.

12. Assignment:
A. Bring the following materials:
1. Small basin
2. Paper Clips (at least 6 pcs)
3. Water (100 ml)
4. Cooking Oil / Baby oil (100 ml)
5. 2 plastic cups
B. Perform this at home
1. Melt 1 tsp of sugar and 1 tsp of salt at the same time with
equal amount of heat.

13. Properties dependent on IMFA
Phase at room temperature
• When molecules have strong intermolecular forces of
attraction, they are packed close together.
• They often exist as condensed phase (solid or liquid) at
room temperature.
• When molecules have weak intermolecular forces of
attraction, they are far apart from each other.
• They often exist as gas at room temperature.

14. Solubility
◦ Solubility refers to the ability of a substance to dissolve in a
given amount of solvent at a specified temperature. Recall the
rule “like dissolves like.”
◦ When the solute and the solvent both exhibit the same
intermolecular forces of attraction, they form a solution.
Properties dependent on IMFA

17. Melting point
• Melting point is the temperature at which the
substance changes from solid to liquid.
• Stronger intermolecular forces means greater amount
of energy is needed to break the attractive forces
between molecules.
• Substances with stronger IMFA have higher melting
points compared to those with weaker IMFA.

18. Boiling point
• Boiling point is the temperature at which the substance changes from
liquid to gas.
• Similar to melting point, stronger intermolecular forces means greater
amount of energy is needed to break the attractive forces between
molecules.
• Substances with stronger IMFA have higher boiling points compared to
those with weaker IMFA.
Surface Tension Surface
• Tension is the tendency of a fluid to acquire the least possible surface
area.
• Molecules with stronger intermolecular forces will exert greater
cohesive forces and acquire less surface area (higher surface tension)
than those with weaker IMFA.

19. Viscosity
• Viscosity is the measure of a fluid’s resistance to flow.
• Molecules with stronger intermolecular forces have greater
resistance to flow, and thus, higher viscosity compared to those
with weaker IMFA. Vapor Pressure
Vapor Pressure
• is the pressure exerted by a vapor in equilibrium with its liquid
phase in a closed system.
• Molecules with stronger intermolecular forces have less tendency
to escape as gas, and thus lower vapor pressure compared to
those with weaker IMFA.

20. Strengths of Intermolecular Forces
When comparing the strengths of intermolecular forces, check the
difference in molecular weight.
If the difference is too large (>1000), then generally, the molecule with
greater molecular weight has stronger intermolecular forces. Otherwise,
you may use the following strategy:
1. Check which molecule exhibits hydrogen bonding. This molecule will
have stronger intermolecular forces.
2. Check which molecule is polar. Polar molecules have stronger
intermolecular forces.
3. Compare London dispersion forces. More massive molecules have
stronger intermolecular forces.