Skills to develop
- Classify intermolecular forces as ionic, covalent,
London dispersion, dipole-dipole, or hydrogen bonding.
- Explain properties of material in terms of type of intermolecular forces.
- Predict the properties of a substance based on the dominate
Forces binding atoms in a molecule are due to chemical bonding.
The energy required to break a bond is called the bond-energy.
For example the average bond-energy for O-H bonds in water is 463 kJ/mol.
On average, 463 kJ is required to break 6.023x1023 O-H bonds,
or 926 kJ to convert 1.0 mole of water into 1.0 mol of O and 2.0 mol of
H atoms. A space filling model of water molecule is shown here.
The forces holding molecules together are generally called intermolecular
forces. The energy required to break molecules apart is much
smaller than a typical bond-energy, but intermolecular forces play
important roles in determining the properties of a substances.
Intermolecular forces are particularly important in terms
how molecules interact and form biological organisms or even life.
This link gives an excellent introduction to the interactions between
Classifying Intermolecular Forces
In general, intermolecular forces can be divided into several categories.
The four prominent types are:
Strong ionic attraction
Recall lattice energy and its relations to properties of solid.
The more ionic, the higher the lattice energy. Examine the following list and
see if you can explain the observed values by way of ionic attraction:
LiF, 1036; LiI, 737; KF, 821; MgF2, 2957 kJ/mol.
Intermediate dipole-dipole forces
Substances whose molecules have dipole moment have higher
melting point or boiling point than those of similar molecular
mass, but their molecules have no dipole moment.
- Weak London dispersion forces or van der Waal's force
These forces alway operate in any substance. The force arisen from induced
dipole and the interaction is weaker than the dipole-dipole interaction.
In general, the heavier the molecule, the stronger the van der Waal's
force of interaction. For example, the boiling points of inert gases
increase as their atomic masses increases due to stronger Landon dispersion
- Hydrogen bond
Certain substances such as H2O, HF, NH3 form hydrogen
bonds, and the formation of which affects properties (mp, bp, solubility)
of substance. Other compounds containing OH and NH2 groups
also form hydrogen bonds. Molecules of many organic compounds such as
alcohols, acids, amines, and aminoacids contain these groups, and thus
hydrogen bonding plays a important role in biological science.
- Covalent bonding
Covalent is really intramolecular force rather than intermolecular
force. It is mentioned here, because some solids are formed due to
covalent bonding. For example, in diamond, silicon, quartz etc.,
the all atoms in the entire crystal are linked together by
covalent bonding. These solids are hard, brittle, and have high
melting points. Covalent bonding holds atoms tighter than ionic
- Metallic bonding
Forces between atom in metallic solids belong to another category.
Valence electrons in metals are rampant. They are not restricted
to certain atoms or bonds. Rather they run freely in the entire
solid, providing good conductivity for heat and electric energy.
These behaviour of electrons give special properties such as
ductility and mechanical strength to metals.
The division into types is for convenience in their discussion.
Of course all types can be present simultaneously for many substances.
Usually, intermolecular forces are discussed together with
The States of Matter, which is linked to a well illustrated web-site.
Intermolecular forces also play important roles in solutions, a discussion
of which is given in
Hydration, solvation in water.
A summary of the interactions is illustrated in the following diagram:
See if you can answer the following questions.
- What are dipoles?
- What are dipole moments?
- How do dipoles interact?
- Why do molecules attract one another?
- How do London dispersion forces come about?
- What parameters cause an increase of the London dispersion forces?
- What is a hydrogen bond?
- What type of hydrogen bonds are strong?
- What chemical groups are hydrogen acceptors for hydrogen bonds?
If you are looking for specific information, your study will be
efficient. Some answers can be found in the Confidence Building Questions.
Consider carefully the purpose of each question, and figure out what is
there to be learned in it.
Confidence Building Questions
- Which of the following molecules have a permanent dipole moment?
CO2 CH4 and N2 are symmetric,
and hence they have no permanent dipole moments.
A molecule with polar bonds unsymmetrically arranged will possess
a permanent dipole.
- Which has the higher boiling point, Br2 or ICl?
They have similar molecular weights: Br2, 160; ICl, 162.
Their boiling points are 332 K and 370 K respectively.
- An atom or molecule can be temporarily polarized by a nearby
species. Polarization separates centers of charge giving
a. permanent dipole
b. temporary charges
c. hydrogen bonding
d. induced dipole
e. induced ions
Induced dipoles are responsible for the London dispersion forces.
The heavier the molecule, the larger the induced dipole will be.
Thus, London dispersion forces are strong for heavy molecules.
- Which has a higher boiling point, I2 or Br2?
Atomic weights for Br and I are 80 and 127 respectively. The higher the
molecular weight, the stronger the London dispersion forces.
- If only London dispersion forces are present, which should
have a lower boiling point, H2O or H2S?
The b.p. for H2O is 100 deg C, and that of H2S is -70 deg C.
Very strong hydrogen bonding is present in liquid H2O, but
no hydrogen bonding is present in liquid H2S.
- Contrary to most other substances, the density of water
decreases as temperature decreases between 4 and 0 deg C.
This is due to
a. dipole-dipole interaction
b. London dispersion
c. decreasing number of hydrogen bonds
d. increasing number of hydrogen bonds formed
As more hydrogen bonds form when the temperature decreases, the
volume expands, causing a decrease in density.
Above 4 deg C, the thermal expansion is more prominent than the effect
of hydrogen bonds.
- Ethanol (C2H5OH, molar mass 46)
boils at 351 K, but water (H2O, molar mass 18)
boils at higher temperature, 373 K. This is because:
a. water is denser
b. water has stronger London dispersion forces
c. water has stronger hydrogen bonds
d. water molecules contain no carbon
A hydrogen atom between two small, electronegative atoms (such as F, O, N)
causes a strong intermolecular interaction known as the hydrogen bond.
The strength of a hydrogen bond depends upon the electronegativities
and sizes of the two atoms.
- Ethanol C2H5OH and methyl ether
CH3OCH3 have the same molar
mass. Which has a higher boiling point?
R-OH group is both proton donor and acceptor for hydrogen bonding.
Methyl groups have very weak hydrogen bonding, if any.