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Intermolecular Forces - Winston

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Intermolecular Forces
Forces between molecules
Intermolecular Forces (IMFs)
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Different molecules have different forces that act
between them.
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These forces attracting the separate molecules
together control many physical properties
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Boiling Point
Melting Point
Solubility
Viscosity
Surface Tension
Three Main IMFs
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Dipole-dipole forces
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Hydrogen bonding
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London dispersion forces (LDFs)
Dipole-Dipole Forces
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Electronegativity – the ability of an atom in
a compound to attract electrons to itself.
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Fluorine has the highest electronegativity
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The electrons in a compound spend more
time around the most electronegative
atoms than the other atoms.
Electronegativity
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If the electrons are spending more time
around the one particular atom, how will
that atom be different from the other
atoms?
It will have a more negative charge than
the other atoms in the compound.
пЃ¬ Does not have a full negative charge but a
partial negative charge (Оґ - , lower case
delta)
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Hydrofluoric Acid (HF)
The red end (fluorine) has
a partial negative charge.
Polar Molecules
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One end of the
molecule is positive
while the other end of
the molecule is
negative.
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This difference in
charge is called a
“dipole”
What effect does this have?
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How does this change
the way two
molecules interact?
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The positive end of
the molecule is
attracted to the
negative end of a
different molecule.
How to spot dipole-dipole forces?
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Look for molecular shapes that have
uneven placements of atoms.
Bent
пЃ¬ Trigonal pyramidal
пЃ¬ Anything that has more than one type of
atom around the outside
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Is Carbon Dioxide Polar?
Is carbon tetrafluoride polar?
Is Water Polar?
Why is Polarity Important
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Things that are polar or have charges
dissolve in things that are polar.
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Things that are nonpolar dissolve in things
that are nonpolar.
“Like dissolves Like”
пЃ¬ Opposing types do not dissolve in each
other.
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A Bio Reminder
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Hydrophilic – “water loving” – polar
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Hydrophobic – “water fearing” - nonpolar
Dissolution Process
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How are strong ionic bonds broken in
water?
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Dissolving_NaCl-Electrolyte.exe
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The polar nature of water creates
attractions between the water and ionic
compound.
Hydrogen Bonding
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Special case of
dipole-dipole forces.
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The difference in
electronegativity
between some atoms
and hydrogen is so
strong that it creates
a very strong dipole
What elements can do this?
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Which elements have the strongest
electronegativity?
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Anything with an N-H bond, O-H bond, or
F-H bond will have hydrogen bonding.
Hydrogen Bonding is Very Important
It is the
reason
why ice
floats.
Hydrogen Bonding is Very
Important
DNA Base Pairs
London Dispersion Forces (LDFs)
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Often called “induced dipoles”
A momentary change in where the electrons are
in one molecule, “induces” a dipole in another
molecule.
LDFs
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The more electrons you have in a
molecule, the more likely you are to have
momentary imbalances in charges.
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The more electrons in an atom, the
stronger the London Dispersion Forces.
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Any molecule can have London Dispersion
Forces.
LDFs
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This explains why the
boiling point goes up
as you move down a
column.
Hydrogen telluride
has more electrons
than hydrogen sulfide
Hydrogen telluride
has stronger LDFs
Hydrogen telluride
has a higher boiling
point
Crude Oil
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Crude oil is
refined based
on differences
in LDFs.
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Longer carbon
chains have
higher boiling
points
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Have larger
number of
electrons
IMF Comparison
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LDFs are the weakest
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Dipole-dipole are in the middle
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Hydrogen bonding is the strongest.
Practice Problems
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List all of the intermolecular forces acting
on two phosphorus trichloride molecules
Figure out the formula
пЃ¬ Draw a Lewis structure
пЃ¬ Figure out the molecular geometry
пЃ¬ Check to see what IMFs it has.
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Practice Problem
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Explain why ammonia (-33.4ВєC)has a higher
boiling point than phosphine, PH3 (-87.8ВєC).
Justify your answer.
Figure out ammonia’s formula
пЃ¬ Draw Lewis structures for both compounds
пЃ¬ Figure out the molecular geometry for both
compounds
пЃ¬ Figure out what IMFs each compound has.
пЃ¬ Compare the two compounds.
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Practice Problems
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Explain why ammonia (-33.4ВєC)has a higher
boiling point than phosphine, PH3 (-87.8ВєC).
Justify your answer.
Practice Problem
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Hexane (C6H14) is a liquid at room
temperature. Its Lewis structure has each
carbon connected to another in a long
chain. Will sodium chloride dissolve in
hexane? Justify your answer.
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