14.1 Molecular Substances
Intermolecular Forces
► Forces
between molecules determine the
melting and boiling point of substances.
► Ionic compounds have high melting and
boiling points due to their electrostatic
attraction.
► Molecular substances are held together by
one of three forces.
 Dispersion
 Dipole
 Hydrogen Bond
Dispersion Forces
► Dispersion
forces, also called London
Forces, are a result of the temporary
location arrangement of the electrons.
► Electrons have a probability of being located
on one side of the atom.
► This side of the atom becomes temporarily
charged negative while the other side
becomes positive.
► This is a temporary situation
► This is the weakest of the forces
Temporary Dipole
Dipole Forces
► Polar
molecules have a permanent
asymmetrical distribution of electrons.
► Although the total molecule is neutral, it has
a positive and negative side.
► The opposite attraction between +/- sides
of the molecule is permanent and thus will
cause polar substances to have a higher
melting and boiling point than non-polar
substances.
► Dipole forces are the next strongest.
Forces Between Polar Molecules
Hydrogen Bonding
► The
strongest intermolecular force is a
subset of dipole forces.
► If hydrogen is bonded to oxygen, nitrogen,
or fluorine the molecule has the strongest of
polarities.
► This is called a hydrogen bond.
► The hydrogen is highly positive and the
negative side is similarly negative.
► These substances have the highest melting
and boiling points.
Hydrogen Bonding in Water
Unique Properties of Water
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Survey
Water is unique in that it is the only natural substance that is found in all three
states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally
found on Earth.
Water freezes at 32o Fahrenheit (F) and boils at 212o F. Water is unusual in
that the solid form, ice, is less dense than the liquid form, which is why ice
floats.
Water has a high specific heat index. This means that water can absorb a lot
of heat before it begins to get hot. The high specific heat index of water also
helps regulate the rate at which air changes temperature, which is why the
temperature change between seasons is gradual rather than sudden, especially
near the oceans.
Water has a very high surface tension. In other words, water is sticky and
elastic, and tends to clump together in drops rather than spread out in a thin
film. Surface tension is responsible for capillary action, which allows water
(and its dissolved substances) to move through the roots of plants and through
the tiny blood vessels in our bodies.
Homework
► Terms
Chapter 14
► Come tomorrow with a unique water fact
► Pg 514 [1-3, 7,8,10-13]
Heating Curves
►A
heating Curve is a graphical representation
of the phase change of a substance.
Heating Curves
►A
to B is heating
the solid
► B to C is melting
the solid
► C to D is heating
the liquid
► D to E is vaporizing
the liquid
► E to F is heating
the gas
Heating Curves
► The
amount of
energy from B to C
and from D to E is
dependent on the
intermolecular forces
between atoms or
molecules.
► The slope of AB or
CD or EF depends on
the specific heat of
the substance.
Evaporation & Vapor Pressure
► Evaporation
is a natural process in which a
liquid converts to a gas in an endothermic
process.
► H2O(l) → H2O(g) ∆H = +
► Evaporation is a cooling process
► The energy is supplied from the environment.
► Fun Fact! Lake Tahoe evaporates 335 million
of water daily
► If
we can contain evaporation by enclosing our liquid
source (think of placing a lid on a pot) a pressure
would begin to build within the air space.
► Eventually the pressure will cause condensation at
the same rate as evaporation.
► The pressure this happens at is called the vapor
pressure of water.
► At vapor pressure, Vp, the system is at dynamic
equilibrium.
► This pressure is above atmospheric pressure
► Total Pressure = Vp + 1 atm.
Boiling Point
► The
first bubbles to form are the air
molecules (N2, O2) coming out of the
solution.
► Boiling occurs when the pressure formed
within an air bubble is great enough to
sustain the inward pressure of the water.
► This occurs when the vapor pressure of the
bubble is equal to atmospheric pressure.
Boiling Points
► B.P.
decreases
with altitude
because atm
pressure
decreases.
► You have to
cook food
different at
altitude!
Homework
► Pg
515 [24,25,26,28,29,31,32]