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CHE140 Lab Manual Lab 3
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CHE140 Lab Manual - Laboratory Three
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Page 25Laboratory 3 Part A Intermolecular Forces
Aims Learning Outcomes
To observe some physical 1. Describe the different types of intermolecular
properties that indicate the type forces
and relative strength of 2. Identify intermolecular forces that are possible
attractive forces between the between molecules of various substances
molecules making up 3. Explain physical properties of substances in terms
substances. of intermolecular forces
Intermolecular forces (IMFs) Dispersion forces are attractive forces between non-
are attractive forces between polar molecules. How these come about is explained
molecules. Not to be confused with the images below. Some points about dispersion
with intramolecular forces, forces:
which are chemical bonds. The weakest of the IMFs, but strength increases
You will learn about four types with the number of electrons (electron density)
of IMFs. present in a molecule
1. Dispersion (London) forces Dispersion forces make it possible for non-polar
2. Dipole-dipole attraction substances to exist as liquids and solids
3. Hydrogen bonding Dispersion forces are also called induced dipole
4. Ion-dipole attraction interactions
Non-polar molecules At any given instant a non-polar molecule
Instantaneous
are represented by can have an uneven distribution of electron
dipoles can be
spheres in the image density – this is called an instantaneous
attracted to
above. dipole and is shown in the image directly
each other, as
However, dispersion above.
shown in the
forces are applicable to The more electrons that are present in a
image on the
non-polar molecules of molecule, the more readily it forms an
far right above.
any shape. instantaneous dipole.
Non-polar substances that exist as
solids have stronger IMFs than
those that exist as liquids or gases.
This is because the stronger IMFs
hold the molecules that make up
the solid in fixed positions.
In liquids the molecules are
attracted to each other less
strongly (weaker IMFs) and they
move freely. In gases there are no
IMFs between the molecules.
, Page 26
Refer back to the information on the previous page as needed to complete the following
exercise.
Comparison of two non-polar substances – fill in the fields below
Bromine
Substance Iodine (I2)
(Br2)
Physical state – observe the samples of bromine and iodine
provided and note the physical state (solid, liquid or gas)
The molecules that make up the substances bromine (Br2) and iodine (I2) are non-polar. Write
a brief explanation of what makes these molecules non-polar.
Dispersion forces are the only type of IMF possible for non-
polar molecules. For which one of these non-polar
substances are the dispersion forces stronger?
Explain the difference in strength of dispersion forces between bromine (Br 2) and iodine (I2)
molecules.
Dipole-dipole attraction is another type of IMF. It
describes the attractive forces between polar
molecules. All polar molecules have distinct
regions of partial positive and partial negative
charge. When they are in proximity, oppositely
charged regions of nearby molecules are attracted
to each other. This is represented in the image on
the right. When comparing molecules of similar
molar mass, dipole-dipole attraction is stronger
than dispersion forces.
Hydrogen bonding is a particularly strong dipole-
dipole attraction. Only possible for molecules
that have a hydrogen atom covalently bonded to
a highly electronegative atom (N, O, F). The
hydrogen atom bonded to N, O or F is attracted to
an electronegative atom on a neighbouring
molecule. Shown for water molecules on the
right. When comparing molecules of similar
molar mass, hydrogen bonding is stronger than
dipole-dipole attraction.
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