Intermolecular Forces
Intermolecular Forces
• When molecules are attracted to other
molecules by a weaker force (when compared to
an ionic bond or a covalent bond holding a
molecule together)
• Intermolecular forces refers to an attraction
between molecules, intramolecular forces are
the forces within the molecule themselves
• when a substance evaporates, melts, or
sublimates (change of state), the molecules gain
kinetic energy from an outside source, this
allows them to overcome the intermolecular
forces holding them close together
• there are no intermolecular forces in ionic
compounds because ionic compounds do not
contain molecules!
• Ionic compounds are solids at room temperature
• In an ionic crystal ionic bonds hold all the ions
together, there is no difference between the bonds
holding the compounds together and those holding
the crystal together
• Ionic bonds are generally very strong and accounts
for the high melting point
Ionic crystal for NaCl
• Many molecular substances are gases at room
temperature, others are liquids with low boiling
points or solids that melt easily
• Forces between the molecules in molecular solids and
liquids are relatively weak, addition of thermal
energy can easily overcome these intermolecular
forces
• The strength of intermolecular forces determines the
physical properties of molecular compounds: state,
melting point, boiling point, surface tension,
hardness, texture and solubility
• As the intermolecular forces between molecules
increases, the compound’s m.p., b.p. and surface
tension also increase
Types of Intermolecular Forces
Dipole-dipole Forces
• Intermolecular force between oppositely
charged ends of a polar molecule
Ex: H-Cl
• These are generally strong intermolecular forces
• The more polar a substances is (refer to
electronegativity and shape), the stronger the
dipole-dipole force of attraction
London Dispersion Forces
• All gases will condense to from liquids (even noble gases) when cooled
enough which implies that there is some type of attraction between the
entities when they are cooled
• These attractions are caused by the temporary shifts in the electron
cloud in an atom or molecule
• This shift creates a temporary dipole for a fraction of a second that will
also cause a temporary dipole in an adjacent atom or molecule
• Very short lived as the attraction between them continually forms and
breaks, as a result these London forces are very weak
• The larger the molecule, the more electrons and protons there are
attracting one another, the stronger the forces, the higher the melting
point
• London dispersion forces exist between all molecules even polar
molecules
• In polar compounds the London forces are insignificant compared to the
much stronger dipole forces
• dipole-dipole forces and London forces are called Van der Waals forces in
honour of the scientist who proposed their existence
Hydrogen Bonds
• special category of very strong dipole-dipole
intermolecular forces
• a hydrogen bond is a particularly strong dipole-dipole
force that occurs between two molecules
• each consists of a hydrogen atom covalently bonded to a
highly electronegative atom of nitrogen, oxygen or
fluorine
• ex: H-F
• two reasons for hydrogen bonding are the large difference
in electronegativity and the small size of the hydrogen
atom (positive pole is highly concentrated and attracts the
negative pole of a nearby molecule)
...in biochemistry
• hydrogen bonds play a significant role in
determining the shape and function of large,
biologically important molecules (proteins
such as enzymes, hormones, antibodies,
DNA)
DNA
Protein
Hydrogen Bonding and Water
• one of most abundant compounds on Earth’s surface
• approx. 22% of Earth’s fresh water is in Canada
• in nature water contains many dissolved substances
necessary for life
• the water cycle, plants and marshes recycle and purify water
but cannot take out some pollutants which are dangerous to
our health and the health of the planet
• water is becoming increasingly contaminated by caffeine,
cosmetics, hormones, food additive, antibiotics, despite our
desperate need for clean water
• when compared to other similar compounds, water is
different…it has some very strange properties
a) unusually high melting and boiling points
• water is the only substance that commonly
exists in three states
• the mp and bp are much higher for water
than other similarly structured compounds
implying the forces holding water together
are very strong
• bent shape, polar bonds and two lone pairs
of electrons on the central oxygen atom
make water one of the most polar molecules
on Earth
b) low density of ice
• solid ice floats on water, the solid is less dense
• ice also acts as a “thermal blanket” holding in the heat
of the water below it (as in a frozen lake)
• at 4oC water is most dense and as it gets closer to
freezing it moves to the top of the lake and changes
state as it become less dense (leaving warmer water
below) because as water freezes the molecules lose
kinetic energy (slow down) and hydrogen bonds start to
arrange the molecules into a crystalline structure (think
snowflakes). This arrangement contains more spaces
between the molecules than just a random
arrangement of liquid water molecules so the solid
expands and is less dense.
c) Unusually high surface tension
• Attraction between molecules at the surface of
liquid water is very high
• Surface tension due to hydrogen bonds causes
beads of water to form on smooth surfaces,
raindrops to form in free fall and explains why
insects can “walk on water”
• Hydrogen bonds allow the water molecules to hold
on strongly to each other, pulling together and
resisting being broken apart so it shapes to the
smallest surface area (a droplet)
d) Unusually high Specific Heat Capacity
• Water holds thermal energy very well
without changing its temperature
• Takes a lot of energy to increase the
temperature of water by 1 oC
• This property allows large lakes and oceans
to influence climate and weather patterns
(think El Nino, El Nina)