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Liquids, Solids, and Intermolecular Forces

or

Why your Water Evaporates and your

Cheerios Don’t

Why are molecules attracted to each other?

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• Intermolecular attractions determine how tightly liquids and solids pack

• The strength of intermolecular attractions• The strength of intermolecular attractions determine many physical properties:

• vapor pressure• viscosity• surface tension• density• melting point

4

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5

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Freedom of Motion• the molecules in a gas have complete freedom of

motion so kinetic energy overcomes the attractive forcesforces

• the molecules in a solid are locked in place, they cannot move around though they do vibrate

• the molecules in a liquid have limited freedom they• the molecules in a liquid have limited freedom – they can move around a little within the structure - have enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other

Properties of the 3 Phases of Matter

State Shape Volume Compressible Flow Strength of l lIntermolecular

Attractions

Solid Fixed Fixed No No very strong

Liquid Indef. Fixed No Yes moderate

Gas Indef. Indef. Yes Yes very weak

•Fixed = keeps shape when placed in a container

•Indefinite = takes the shape of the container

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Kinetic - Molecular Theory

• the properties explained based on the kinetic energy of the molecules and the attractive forces b t thbetween them

• kinetic energy is proportional freedom of motiondegrees of freedom = translational, rotational,

vibrational

• attractive forces keep the molecules togetherattractive forces keep the molecules together

• kinetic energy depends only on the temperatureKE = 1.5 kT

Gas Structure

Gas molecules generally don’t stick to each other.

Why not?

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Explaining the Properties of Solids• solid particles packed close together and fixed

in position

• retain their shape and volume when placed in a new container; and prevents the particles from flowing

• incompressible• incompressible

Solids• crystalline solids salt and diamonds salt and diamonds

• amorphous solids rubber and window glass

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Explaining the Properties of Liquids• close contact → higher densities than

gases

• limited freedom of movement →indefinite shape but definite volume (no escape)(no escape)

Compressibility

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Phase Changes

• intermolecular attractions are due to attractive forces between opposite charges i i+ ion to - ion+ end of polar molecule to - end of polar moleculeH-bonding especially strong

even nonpolar molecules will have temporary charges• larger the charge = stronger attraction• longer the distance = weaker attraction

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th tt ti f ll l ti tthese attractive forces are small relative to the ionic, covalent and metallic bonding forces between atoms

•generally smaller charges•generally over much larger distances

• Dipole-dipole attractionsPolar molecules tend to align their partial

charges

The attractive force is about 1% of a covalent bond and drops off as 1/d3 (d=distance between dipoles)

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Hydrogen bonds (5-10% of covalent bond)Very strong dipole-dipole attraction when H is

l l b d d hi hl l icovalently bonded to to a highly electronegative atom (F, O, or N)

Typically about ten times stronger than other dipole-dipole attractions

Are responsible for the expansion of water as it freezes

Partly responsible for twisting of proteins in a helix (DNA)

Hydrogen bonds

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• London or dispersion forcesThe (very) weak attractions between nonpolar

molecules

Arise from the interactions of instantaneous dipoles on neighboring molecules

London forces depend on the number of atoms in the molecule

The boiling point of hydrocarbons demonstrates this trend

42 1HC

68.7 HC 88.6- HC

36.1 HC 161.5- CH

C)( atm 1at BP Formula C)( atm 1at BP Formula

14662

1254

oo

327 HC 0.5- HC

42.1- HC

4622104

83

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Trends in the Strength of Intermolecular Attraction

• the stronger the attractions between the atoms orthe stronger the attractions between the atoms or molecules, the more energy it will take to separate them

• boiling a liquid requires we add enough energy to overcome the attractions between the molecules or atomsatoms

• the higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces

Attractive Forces+ - + - + - + -

+++

+

____

+ + + + + + +

- - - - - - -

++ + +

+

--

- --

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Dispersion Forces• fluctuations in the electron distribution in atoms and

molecules result in a temporary dipolep y p region with excess electron density has partial (─) charge region with depleted electron density has partial (+) charge

• the attractive forces caused by these temporary dipoles are called dispersion forces aka London Forces

ll l l d ill h h• all molecules and atoms will have them• as a temporary dipole is established in one molecule, it

induces a dipole in all the surrounding molecules

Dispersion Force

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Size of the Induced Dipole• the magnitude of the induced dipole depends on

several factors

• polarizability of the electrons volume of the electron cloud

larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractionsg

• shape of the molecule

more surface-to-surface contact = larger induced dipole = stronger attraction

Effect of Molecular Sizeon Size of Dispersion Force

Noble Gases are allAs the molar massNoble Gases are all nonpolar atomic elements.

As the molar mass increases, the number of electrons increase. Therefore the strength of the dispersion forces increases.The stronger theThe stronger the attractive forces between the molecules, the higher the boiling point will be.

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150

200

250

Relationship between Induced Dipole and Molecular Size

BP Noble Gas

-150

-100

-50

0

50

100

1 2 3 4 5 6

Bo

ilin

g P

oin

t, °

C

BP, Noble Gas

BP, Halogens

BP, XH4

-300

-250

-200

150

Period

Name Molar Mass BP, °C MP, °C Density, g/mLMethane 16 -162 -183 0.47Ethane 30 -89 -183 0.57Propane 44 -42 -188 0.5B t 58 0 138 0 58

Properties of Straight Chain AlkanesNon-Polar Molecules

Butane 58 0 -138 0.58Pentane 72 36 -130 0.56Hexane 86 69 -95 0.66Heptane 100 98 -91 0.68Octane 114 126 -57 0.7Nonane 128 151 -54 0.72Decane 142 174 -30 0.74Undecane 156 196 -26 0 75Undecane 156 196 -26 0.75Dodecane 170 216 -10 0.76Tridecane 184 235 -5 0.76Tetradecane 198 254 6 0.77Pentadecane 212 271 10 0.79Hexadecane 226 287 18 0.77

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Boiling Points of n-Alkanes

300

400

500 n-Alkane Boiling & Melting Points

-100

0

100

200

Tem

pera

ture

, °C

BP, n-alkane

-300

-200

0 50 100 150 200 250 300 350 400 450 500Molar Mass

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Effect of Molecular Shapeon Size of Dispersion Force

140 Alkane Boiling Points

Alkane Boiling Points

• branched chains

40

60

80

100

120

140

era

ture

, °C

Alkane Boiling Points

n

iso-

have lower BPs than straight chains

• the straight chain isomers have

-20

0

20

58 72 86 100 114

Tem

pe

Molar Mass

more surface-to-surface contact

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Practice – Choose the Substance in Each Pair with the Highest Boiling Point

a) CH CH CH CH CHa) CH4 CH3CH2CH2CH3

b) CH3CH2CH=CHCH2CH3 cyclohexane

CC

CC

H

H H

HH

H H

HH

HC

H

HHH

CC

CC

H

H

H

HH

H

CC

H

HH

H

HH

H

H HH

H

HC

CH

H

HC

CH C

H

HC

Practice – Choose the Substance in Each Pair with the Highest Boiling Point

a) CH CH CH CH CHboth molecules are nonpolara) CH4 CH3CH2CH2CH3

b) CH3CH2CH=CHCH2CH3 cyclohexane

are nonpolarlarger molar mass

CC

CC

H

H H

HH

H H

HH

HC

H

HHH

both molecules are nonpolarflat molecule larger surface-to-surface contact

CC

CC

H

H

H

HH

H

CC

H

HH

H

HH

H

H HH

H

HC

CH

H

HC

CH C

H

HC

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Dipole-Dipole Attractions• polar molecules have a permanent dipole because of bond polarity and shape dipole moment dipole moment as well as the always present induced dipole

• the permanent dipole adds to the attractive forces between the molecules raising the boiling and melting points relative to nonpolar

molecules of similar size and shape

Effect of Dipole-Dipole Attraction on Boiling and Melting Points

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Molar Mass

BoilingPoint

DipoleSize

CH3CH2CH3 44.09 -42°C 0.08 D

CH3-O-CH3 46.07 -24°C 1.30 D

CH3 - CH=O 44.05 20.2°C 2.69 D

CH3-CN 41.05 81.6°C 3.92 D

Practice – Choose the Substance in Each Pair with the Highest Boiling Point

a) CH2FCH2F CH3CHF2

or

2 2 3 2

b)

C C

HH

H HF

F

C C

HH

H FH

F

or