Lecture 7.1 - Inter Molecular Forces

8/14/2019 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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Lecture 7.1 - Inter Molecular Forces