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Lecture 32

CHEM10003 Lecture Notes - Lecture 32: London Dispersion Force, Intermolecular Force, Covalent Bond


Department
Chemistry
Course Code
CHEM10003
Professor
Guy Jameson
Lecture
32

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Lecture 32: Intermolecular forces & solubility
Intermolecular forces
Significant in solids and liquids
Comparatively weak interactions
between
molecules
Compare to bonding
within
molecules
The covalent forces that hold the molecule together are
intramolecular
(strong)
The forces that cause aggregation of molecules are
intermolecular
(weak)
Intermolecular forces range from dipole-dipole forces London dispersion forces
These intermolecular forces are known as van der Waals forces
Intermolecular forces may be weak but are very important

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London Dispersion Falls
These occur between all molecules, both polar and non-polar
Relatively weak forces
Energy approx. 2kJ/mol
Originate because instantaneous dipoles arise from fluctuations in electron
distribution
These forces are often very weak but can sometimes be relatively strong.
Origin
How do 2 non-polar molecules attract each other?
Spherical distribution of electron density not always maintained
o The more electrons in an atom, the greater the chance that there won’t be
an even distribution
This leads to instantaneous dipole which can induce dipoles in neighbouring
molecules/atoms

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Polarizability
Dispersion forces are stronger for atoms that are more polarized
They have the ability to change shape of electron cloud (the larger the
orbitals/atoms, the more easily polarised they are)
Polarizability relates to the ability of a molecule or atom to undergo distortion of its
electron “clouds”
Molecules with extended electron clouds like I2 can be polarized to a greater
extent than molecules like H2
Induced dipole interaction tends to be a very weak because
o Polarizabilities of most species are not very large
o Effective only at very short distances
High polarizability is often associated with large numbers of electrons which in turn leads
to stronger dispersion forces
Consider boiling points of group 18 elements (increase as going down periodic
table)
Shape
Shape also affects the strength of dispersion forces
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