Chapter 11 –focus points:
Intermolecular forces and their effect on properties of liquids
Intermolecular (or interparticle ) forces are weak interactions between particles. They
decrease as you go from solid Æ liquid Æ gas.
Remember that in a gas the particles have the highest degree of freedom of movement
and negligible or weak intermolecular forces.
As the intermolecular attraction increases,
• The vapor pressure ( the pressure of the vapor that is in equilibrium with its
liquid) decreases
• The boiling point ( the temperature at which the vapor pressure becomes equal to
the pressure exerted on the surface of the liquid) increases
• Surface tension ( the resistance of a liquid to spread out and increase its surface
area) increases
• Viscosity ( the resistance of a liquid to flow) increases.
Higher the intermolecular forces between the liquid particles, harder it is for it to escape
into the vapor phase, ie., you need more energy to convert it from liquid to the vapor
phase, in other words, higher its boiling point.
If it is harder for a liquid to escape into vapor, because it is held back into the liquid by
the neighboring particles attraction, you have less vapor and hence low vapor pressure.
Three types of intermolecular forces exist between neutral molecules which are known
as Van der Waals forces. They are:
Dipole-dipole between polar
molecules
ex: HCl weak
Hydrogen bonding between molecules
containing H bound
to small
electronegative
atoms such as O, N
and F – strong
Ex: H2O strong
London Dispersion
forces
between any two
particles
-tend to increase
with increasing size
and the molar mass
resulting from the
momentary
nonsymmetrical electron
distribution that produces
temporary dipoles
weak
Exercises:
1. Identify the intermolecular forces in each of the following substances:
a) HCl b) CH3CH3 c) CH3NH2 d) Kr
2. Write the following in the increasing order of boiling points:
a) H2S b) CH3CH3 c) CH3OH d) Ar
3. Between CO2 and SO2 , only _____ will have dipole-dipole interaction because _____________________.
