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Forces Between Molecules
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Bonding model for covalent
molecular substances
Bonding for covalent molecular substances
falls into two categories
1. The strong forces of attraction which holds
atoms together within molecules
2. The weak forces of attraction between
molecules
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Forces between molecules
(intermolecular forces)
So far we have spoken about the forces orbonds between atoms e.g. covalent, ionic and
metallic
Now we will learn about the forces betweenmolecules or compounds are called
intermolecular forces
Inter means between or among Internet, interstate, international
What would Interstellar travel be?
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Intramolecular forces
What would intramolecular forces be?
Forces within molecules e.g covalent,
metallic or ionic
intra means within Intrastate, intranet, intracellular
Most of the intermolecular forces we look
at occur between covalently bondedmolecules or covalent molecular
substances
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Overview
All matter is held together by force.
The force between atoms within amolecule is a chemical orintramolecular
force. The force between molecules are a
physical orintermolecular force.
These physical forces are what weovercome when a chemical changes itsstate (e.g. gas liquid).
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What causes intermolecular forces?
Molecules are made up of charged particles:
positive nuclei and negative electrons.
When one molecule approaches another there
is a multitude of forces between the particles in
the two molecules. Each electron in one molecule is attracted to
the nuclei in the other molecule but also
repelled by the electronsin the other molecule.
The same applies for nuclei
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Types of Intermolecular forces
The three main types of intermolecular
forces are:
1. Dipole-dipole attraction occur only btw polar
molecules
2. H bonding only with Hydrogen and Oxygen,
Fluorine and Nitrogen)
3. Dispersion forces
1 Di l Di l tt ti Di l
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1. Dipole Dipole attraction or Dipole
forces Dipole forces occur between two polar
molecules
The dipole force is the electrostatic attraction
between the partial positive on one hydrogen
on one molecule and the partial negativecharge on the sulfur of another molecule.
One type of intermolecular force is dipole dipole
+ +
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Draw the molecule and determine its shape
using VSPER
Determine polarity within molecule
Determine how the molecules would
interact
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2. Hydrogen bonding
Hydrogen bonds are a special case of dipoleforces
Hydrogen bonding occurs betweenmolecules in which hydrogen is bonded to
nitrogen, oxygen or fluorine It occurs because N, F and O are very
electronegative
This makes the molecule formed verypolarised
The partial positive charge on hydrogen and
the lone pair of electrons on N,F and O
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Hydrogen bonds are stronger than dipole
dipole attractions
We know this because of the difference in
melting points of molecules with dipole
dipole attractions and hydrogen bonding
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3. Dispersion forces Dispersion forces, or London forces, are the
weakest of the intermolecular forces.
Dispersion forces occur between polar and
non-polar molecules. They are generally
weak and only considered in the absence ofstronger intermolecular forces.
The greater the number of electrons within
a molecule, the more significant thedispersion forces btw the molecule.
They occur between non-polar molecules.
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Essentially they are the same as dipole forces- an
electrostatic attraction between a partial positive
charge and a partial negative charge.
However, the partial charges are more rare innonpolar molecules. It is perhaps better to think of
these interactions as those between a "temporary
dipole" and an "induced dipole".
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Stages for Dispersion Forces
1. There are two molecules
2. One molecule has a temporary dipole
3. The other molecule has an induced dipole
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http://www.youtube.com/watch?v=IgcGuE
wHHKY&feature=related
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Strength of intermolecular forces
1. Hydrogen bonds are the strongest
intermolecular force
2. Dipole Dipole Interactions
3. Disperson Forces are the weakest
intermolecular force
dispersion forces < dipole-dipole interactions
< hydrogen bonds
H d i l l f
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How do intermolecular forces
affect melting point?
When heat is applied to a solid, the moleculesbegin to vibrate more and more. The energy
possessed by the molecules increases to the
point that some intermolecular attractions are
overcome.
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How to measure the strength
of a chemical bond? One measure of the strength of a chemical
bond is the energy required to break one
of those bonds.
http://www.youtube.com/watch?v=IgcGuE
wHHKY
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Overview Intermolecular forces (dispersion forces,
dipole-dipole interactions and hydrogenbonds) are much weaker than
intramolecular forces (covalent bonds,
ionic bonds or metallic bonds) dispersion forces are the weakest
intermolecular force (one hundredth-one
thousandth the strength of a covalentbond), hydrogen bonds are the strongest
intermolecular force (about one-tenth the
strength of a covalent bond).
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Quiz
1. Which attractions are stronger:intermolecular or intramolecular?
2. Which is stronger a covalent bond or a
dipole-dipole attraction?3. Suggest some ways that the dipoles in
London forces are different from the dipolesin dipole-dipole attractions.
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1. Intramolecular are stronger.2. A covalent bond is approximately100x stronger.3. London forces
Are present in all compounds
Are transient in nature (dipole-dipole are morepermanent).
London forces are weaker