Download Intermolecular Forces

Intermolecular Forces
Forces between molecules
Intermolecular Forces (IMFs)

Different molecules have different forces that act
between them.

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

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

The electrons in a compound spend more
time around the most electronegative
atoms than the other atoms.
Electronegativity

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.
 Does not have a full negative charge but a
partial negative charge (δ - , lower case
delta)

Hydrofluoric Acid (HF)
The red end (fluorine) has
a partial negative charge.
Polar Molecules

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?

How does this change
the way two
molecules interact?

The positive end of
the molecule is
attracted to the
negative end of a
different molecule.
How to spot dipole-dipole forces?

Look for molecular shapes that have
uneven placements of atoms.
Bent
 Trigonal pyramidal
 Anything that has more than one type of
atom around the outside

Is Carbon Dioxide Polar?
Is carbon tetrafluoride polar?
Is Water Polar?
Why is Polarity Important

Things that are polar or have charges
dissolve in things that are polar.

Things that are nonpolar dissolve in things
that are nonpolar.
“Like dissolves Like”
 Opposing types do not dissolve in each
other.

A Bio Reminder
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Hydrophilic – “water loving” – polar

Hydrophobic – “water fearing” - nonpolar
Dissolution Process

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

Special case of
dipole-dipole forces.

The difference in
electronegativity
between some atoms
and hydrogen is so
strong that it creates
a very strong dipole
What elements can do this?

Which elements have the strongest
electronegativity?

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

The more electrons you have in a
molecule, the more likely you are to have
momentary imbalances in charges.

The more electrons in an atom, the
stronger the London Dispersion Forces.

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

Crude oil is
refined based
on differences
in LDFs.

Longer carbon
chains have
higher boiling
points

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
 Draw a Lewis structure
 Figure out the molecular geometry
 Check to see what IMFs it has.
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Practice Problem

Explain why ammonia (-33.4ºC)has a higher
boiling point than phosphine, PH3 (-87.8ºC).
Justify your answer.
Figure out ammonia’s formula
 Draw Lewis structures for both compounds
 Figure out the molecular geometry for both
compounds
 Figure out what IMFs each compound has.
 Compare the two compounds.
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Practice Problems

Explain why ammonia (-33.4ºC)has a higher
boiling point than phosphine, PH3 (-87.8ºC).
Justify your answer.
Practice Problem

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.