Download Intermolecular Forces

Intermolecular Forces
What binds molecules
together?
INTRAMOLECULAR VS
INTERMOLECULAR
• Intramolecular force - Atoms
within a molecule are attracted
to one another by the sharing of
electrons.
• Intermolecular forces – Forces
that hold molecules together in
solids and liquids. These are
much weaker then
Intramolecular forces.
EXAMPLES OF
INTRAMOLECULAR FORCES
• Ionic Bonds – The attraction
between ions to form
compounds based on charge.
• Covalent Bonds – The attraction
between the nucleus of one
atom and the electrons of
another atom. The electrons
are shared between atoms.
MORE ON COVALENT
BONDS
• Polar Covalent Bonds – The
electrons in a covalent bond are
shared unequally, giving one atom a
slightly positive charge and the
other a slightly negative charge.
• Nonpolar Covalent Bonds – The
electrons in a covalent bond are
shared equally, resulting in both
atoms having a neutral charge.
The States of Matter and
Intermolecular Forces
The Nature of
Intermolecular Forces
• A very strong type of force that is
responsible for much of chemistry is
electrostatics.
• The attraction of a positive charge with a
negative charge is the force that allows for
the structure of the atom,
• causes atoms to stick together to form
molecules, both ionic and covalent,
• and is responsible for the formation of
liquids, solids and solutions.
HOW INTERMOLECULAR
FORCES WORK
Trends in the Forces
• The intramolecular forces keep the
atoms in a molecule together and
are the basis for the chemical
properties,
• The intermolecular forces are those
that keep the molecules themselves
together and are virtually
responsible for all the physical
properties of a material.
INTRAMOLECULAR VERSUS
INTERMOLECULER FORCES
Covalent
Bonds >
Hydrogen
Bonding>
DipoleLondon
Dipole
Forces
Attractions>
400 kcal >
12-16 kcal
>
2-0.5 kcal >
Less than
1 kcal
Clearly normal covalent bonds are almost 40 times
the strength of hydrogen bonds. Covalent bonds are
almost 200times the strength of dipole-dipole forces,
and more than 400 times the size of London
dispersion forces.
Dipole-Dipole
Attractions
• Dipole-dipole attractions exist
between molecules that are polar.
• This requires the presence of polar
bonds and an unsymmetrical
molecule.
• The slightly positive atom in one
molecule is attracted to the slightly
negative atom in another molecule.
EXAMPLE
ANOTHER EXAMPLE
London Dispersion
Forces
• London dispersion forces exist in nonpolar
molecules.
• These forces result from temporary charge
imbalances. The temporary charges exist
because the electrons in a molecule or ion
move randomly in the structure.
• The nucleus of one atom attracts electrons
from the neighboring atom.
• At the same time, the electrons in one
particle repel the electrons in the neighbor
and create a short lived charge imbalance.
EXAMPLE
These temporary charges in one molecule or atom attract opposite
charges in nearby molecules or atoms. A local slight positive
charge d+ in one molecule will be attracted to a temporary slight dnegative charge in a neighboring molecule.
Hydrogen Bonding
• Hydrogen bonding is a unique type of
intermolecular attraction.
• There are two requirements:
• The first is a covalent bond between an
H atom and either F, O, or N these are
the three most electronegative
elements.
• The second is an interaction of the H
atom in this kind of polar bond with a
lone pair of electrons on a nearby atom
like F, O, or N.
WATER AND HYDROGEN
BONDING
• The normal boiling point for water is
100ºC.
• The observed boiling point is high
compared to the expected value.
• The predicted boiling point from the
trend of boiling points for H2Te,
H2Se, H2S and H2O is very low.
• If the trend continued the predicted
boiling point would be below -62ºC.
• The “anomalous” boiling point for
water is the result of hydrogen
bonding between water molecules.
WATER AND FREEZING
• Hydrogen bonding is responsible for
the expansion of water when it
freezes.
• The water molecules in the solid
have tetrahedral arrangement for the
two lone pairs and two single bonds
radiating out from the oxygen.
• The lone pairs on the “O” atom can
attracted to nearby water molecules
through hydrogen bonds.
• A cage like structure results.
EXAMPLE OF ICE