Inter Molecular Forces
-Forces that exist between molecules
1. Ion-dipole intermolecular force
2. Dipole-Dipole intermolecular force
Van der waals forces
3.London Dispersion intermolecular force
4. Hydrogen bonding intermolecular force
1. Ion-dipole Force
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Exists between an ion and the partially charged end of a polar
molecule
Polar molecules are dipole – they have a positive end and a negative
end
Positive end of the molecule (water) is attracted to the negative end of the water
molecule. Is attached to the positive ion
2. Dipole – Dipole force
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Exists between polar molecules
Occurs when positive end of one molecule is attracted to negative end
of another molecule
Effective only when molecules are close to each other (weaker then
ion-dipole force)
The strength of a dipole-dipole intermolecular bond depends on the
strength of the polarity in molecules concerned (assuming the
molecules are a similar size)
3. London dispersion Forces – This force results from attraction
between induced dipoles
Dispersion forces tend to increase in strength with increasing molecular mass (The
larger electron cloud, the more easily it can be temporarily distorted and create a
momentary dipole
Dispersion forces operate in all molecules weather they are polar or non-polar
Dispersion forces increase with increasing molecular mass so it is stronger in larger
molecules
Comparing strength of double – dipole and dispersion forces
a) for molecules with comparable MW + Shape dispersion forces are going
to be equal so the degree of attraction is due to the strength of the dipole
– dipole attractions – most polar = strongest attraction
b) When molecules have different molecular masses dispersion forces tend
to dominate
- The most massive molecule have the strongest attractions (more
electrons shift, Therefore stronger induced dipole)
Hydrogen Bonds
-result from the intermolecular transfer between a hydrogen atom and a very
electronegative atom
Hydrogen bonds are considered unique because fluorine nitrogen and oxygen are so
electronegative the covalent bond between hydrogen and any of the three O N F is
very polar
Plus the hydrogen atom has no inner core electrons so it has an exposed proton and
this plus the small size of hydrogen atom allows the hydrogen to approach the
electronegative atom very closely. This establishes a very strong attraction between
molecules
Water
Gas Phase H2O
Water Phase H2O
H2O
H2O
Solid Phase H2O H2O
Terms
Cohesive Forces – Intermolecular forces that exist between similar
molecules. To one another
Adhesive Forces – The intermolecular forces that bind one substance to
another different substance i.e. water to glass
Phase Changes
Three changes of state
1) heat of fusion - heat required to melt a solid
2) heat of vaporization - Heat required to vaporize a liquid
3) Sublimation – heat required to go to a solid from a gas directly
Note – More energy is required for heat of vaporization why than for heat of
fusion why?
Less energy is required to stretch the hydrogen bond which is melting then
actually break the hydrogen bond which is (vaporization)