Download - Lecture 7.1 - Inter Molecular Forces
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States Of Matter I:States Of Matter I:
Intermolecular ForcesIntermolecular Forces
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Geckos Hairy Feet
Nanostructures on thesoles of gecko feet.Thanks to about one
billion hierarchicallyorganized nanohairs, thegecko can go for a walkon walls and ceilings,
unlike people.
Image: Max Planck Institute for Metals Research
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Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
41 kJ to vaporize 1 mole of water (inter)
930 kJ to break all O-H bonds in 1 mole of water (intra)
Generally, intermolecular
forces are much weaker thanintramolecular forces.
Measure of intermolecular force
boiling point
melting point Hvap
Hfus
Hsub
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Intermolecular Forces Forces holding one
molecule to another ina substance.
van der Waals forces Dispersion forces
London Forces
Dipole-Dipole attraction
Hydrogen bonding
Ion-Dipole attraction
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Relative Magnitudes of Forces
The types of bonding forces vary in their strengthThe types of bonding forces vary in their strength
as measured by average bond energy.as measured by average bond energy.
Covalent bondsCovalent bonds(400 kcal/mol)
Hydrogen bondingHydrogen bonding(12-16 kcal/mol)
Dipole-dipole interactionsDipole-dipole interactions(2-0.5 kcal/mol)
LondonLondonforcesforces(less than 1 kcal/mol)
StrongestStrongest
WeakestWeakest
Strength of intermolecular forces determinesStrength of intermolecular forces determines
the phase of a substance at a giventhe phase of a substance at a given
temperature and pressuretemperature and pressure 7
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London DispersionLondon Dispersion
ForcesForces
The temporary separations ofThe temporary separations ofcharge that lead to the Londoncharge that lead to the London
force attractions are what attractforce attractions are what attract
oneone nonpolarnonpolarmolecule to itsmolecule to its
neighbors.neighbors.
Fritz LondonFritz London1900-19541900-1954
London forces increase with theLondon forces increase with the
size (polarizability) of thesize (polarizability) of the
molecules.molecules.
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London forces
Induced dipole induced dipoleor
Nonpolar - Nonpolar interactions
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Intermolecular ForcesDispersion Forces
Attractive forces that arise as a result oftemporary dipoles
induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
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PolarizabilityPolarizability
The ease with which a molecule/atoms electroncloud can be distorted, thereby inducing adipole moment.
Increasing the number of electrons increases
the polarizability of an atom or molecule.
-More electrons larger
distribution-Distribution more easily polarizedsince outermost electrons are less
tightly held by nucleus 13
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Boiling Point Increases w/ IMF Strength
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Strength of London Force Depends On
1.Polarizability (# of e- in molecules)
2. Surface area of interaction (morediffuse electron cloud)
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Melting Points of Similar Nonpolar
Compounds
Compound MeltingPoint (C)
CH4 -182.5
CF4
-150.0
CCl4 -23.0
CBr4
90.0
CI4
171.017
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Dipole Dipole forces
orPolar - Polar interactions
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Dipole Dipole Animation
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Dipole-Dipole Animation
http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIPOLED.html Next 20
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Why are these boiling points
different?
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CHCH22O has dipole-dipole forces!O has dipole-dipole forces!
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London Dispersion vs Dipole-DipoleLondon Dispersion vs Dipole-Dipole
Compound Boiling Point
(C)
CH3
F -78.4
CCl4 76.5
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CH3F is polar while CCl4 is not
CCl4 dispersion forces are stronger than the
dipole-dipole and dispersion forces of CH3F
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London Dispersion vs Dipole-DipoleLondon Dispersion vs Dipole-Dipole
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As molecular size increases, london
dispersion forces become more significant
than dipole-dipole attractions
-More electrons more polarizable-Larger size polar regions less
accessible/prominent
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Hydrogen Bonds
hydrogen bond: a special dipole-dipole interaction
between they hydrogen atom in a polar N-H, O-H, or
F-H bond and an electronegative O, N, or F atom.
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Hydrogen Bond
HCOOH and water
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Why are these boiling points
different?
Hyrdogen Bonding!Hyrdogen Bonding!28
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Why is the hydrogen bond considered aspecial dipole-dipole interaction?
Decreasing molar massDecreasing boiling point
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Hydrogen Bonding inHy
drogen Bonding in
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Hydrogen Bonding inHydrogen Bonding in
DNADNA
N O
OH
OP
O
OH
OHN
N
NNH2
NO
OH
OP
O
OH
OH
NH
O
O
CH3
TT AA
Thymine hydrogen bonds to Adenine
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Hydrogen Bonding in DNAHydrogen Bonding in DNA
CC GG
NO
OH
OP
O
OH
OH
N
NH2
O
N O
OH
OP
O
OH
OH
N
NH
N
NH2
O
Cytosine hydrogen bonds to Guanine
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I l l F
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Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
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S
OO
What type(s) of intermolecular forces exist
between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are alsodispersion forces between HBr molecules.
CH4CH4 is nonpolar: dispersion forces.
SO2SO2 is a polar molecule: dipole-dipole forces. There are also
dispersion forces between SO2 molecules.
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HomeworkHomework
p. 500 #36p. 500 #36
p. 501 # 38, 40 a-b, 44p. 501 # 38, 40 a-b, 44
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