Chem 400 Intermolecular Forces
•From our discussion on bonding, you should realize that CO2 is a nonpolar
molecule.
•Now is CO2 naturally a solid at room temp?
•No, at room temperature it is a gas, and you can see the solid CO2 turning into
CO2 gas as I speak.
•So why is CO2 a gas at room temperature, while H2O is a liquid at room
temperature, and sucrose (C12H22O11) is a solid at room temperature?
•What determines what phase or state a compound will exist in at room
temperature?
•To answer this, let’s first review the 3 states of matter.
Gas
•No definite shape or
volume
•Very low densities
•Density varies with T, P
•Rapid, random motion
•High kinetic energy
Liquid
•Fixed volume
•No fixed shape
•High densities
•Not compressible
•Fluid, motion
•Some kinetic energy
Solid
•Fixed shape
•Fixed Volume
•Rigid
•High densities
•Not compressible
•No or little motion
•So what determines whether a compound is a solid, liquid, or a gas at room
temperature?
•The strength of the attractions between molecules: the stronger the attraction,
the more likely it will be a solid, while the weaker the attractions, the more likely
it will be a gas.
•These attractive forces “glue” solids or liquids together.
What’s the force or “glue” that holds molecules
together as liquids or solids?
• Intermolecular forces or van der Waal forces
• The stronger these forces, the more likely it is that
a compound will be a liquid or solid at room
temperature
• These forces are responsible for many physical
properties, including boiling and melting points
There are 3 types of Intermolecular Forces:
• Ion-Dipole (talk about later)
• Dipole-Dipole
• London Dispersion
Dipole-Dipole Intermolecular Force
• Attractive force between POLAR molecules (have a
dipole moment)
• Electrostatic attraction of partial positive end of
molecule to partial negative end of another molecule
• In liquid or solid,molecules align themselves so are
attracted to several other molecules
• These dipole-dipole forces are much weaker than a
real covalent bond, about 3-4kJ/mol.
• So they may be broken with a low amount of
energy, so the solid melts, and the liquid
evaporates!
431 kJ/mol
16 kJ/mol
Factors in Strength of Dipole-Dipole
• Distance between molecules: the greater the distance, the lower
the dipole-dipole force
• Polarity of molecules: the more polar the molecule, the stronger
the dipole-dipole force
Hydrogen Bonding:
A Special Case of Dipole-Dipole Forces
• Occurs when a molecule is small and very polar
• Occurs when have a N-H, O-H, or H-F bond
• The molecules have a very large dipole moment, and they can get
very close to one another due to the small size of H, N, O, F
• H-bonding is usually about 10-40kJ/mol
• H-Bonding is responsible for the very high melting and boiling point
of water
• H-Bonding responsible for shape of proteins and DNA
Hydrogen Bonding in Water
H2O
HF
NH3
H2Te
H2S
H2Se
SbH3
HI
SnH4
CH4
Hydrogen Bonding in Proteins
• H-Bonding responsible for very high
melting and boiling point of water
• H-Bonding responsible for shape of
proteins and DNA
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But what about CO2?
It’s nonpolar, so it can’t have dipole-dipole forces!
But it can exist as a solid, so there must be another intermolecular
force!
This force is called London Dispersion Force
They are the only intermolecular force which acts between nonpolar
molecules like CO2
How does it operate with no dipole moment?
By creating what we call instantaneous or induced dipoles!
London Dispersion Forces
• London Forces are usually about 1-10kJ/mol
• Even though London forces seem weak, they can be very important
• They are 2 factors in the strength of London forces:
• Size and mass of molecule
• Shape of molecule
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Effect of Size/Mass on London Forces
The larger the size or mass of a molecule, the more electrons it
has
These electrons are also generally further away from the nucleus
So it is easier to push or pull these electrons from one end of the
molecule to the other to create a dipole
Thus, larger molecules or atoms are more polarizable, their
electron cloud may be distorted (pushed or pulled) more readily.
So larger atoms like I or Br are much more polarizable than
smaller atoms like F or Cl.
And it it’s more polarizable, it will have stronger London Forces.
Effect of Shape on London Forces
• The closer the molecules can align themselves, the stronger the
London force, and the higher the melting and boiling points.
Pentane Series
Molecular Formula:
Molar Mass:
C5H12
72
Summary of Intermolecular Forces
• London forces act upon all molecules
but are most important for larger,
heavier molecules
• Dipole-dipole and Hydrogen-bonding
are found in polar molecules and
depend on the size and polarity of the
molecules
• Although they are weak compared to
covalent bonds, these intermolecular
forces greatly affect the boiling
and melting points of a compound
• The stronger the forces, the higher
the melting and boiling points
• The stronger the forces, the more
likely a compound will be a solid or
liquid