liquids, solids, and intermolecular forces or why your ...€¦ · 1 liquids, solids, and...

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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


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


a) CH2FCH2F CH3CHF2

or

2 2 3 2

b)

C C

HH

H HF

F

C C

HH

H FH

F

or

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more polara) CH2FCH2F CH3CHF2

or

2 2 3 2

b)

C C

HH

H HF

F

C C

HH

H FH

F

orpolar nonpolar

Attractive Forces and Solubility• Solubility depends on the attractive forces of solute

and solvent moleculesLike dissolves Like i ibl li id ill l di l i h hmiscible liquids will always dissolve in each other

• polar substance dissolve in polar solventshydrophilic groups = OH, CHO, C=O, COOH, NH2,

Cl

• nonpolar molecules dissolve in nonpolar solventsh d h bi C H C Chydrophobic groups = C-H, C-C

• Many molecules have both hydrophilic and hydrophobic parts - solubility becomes competition between parts

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Immiscible Liquids

Polar Solvents

Dichloromethane

Water

(methylene chloride)

Ethanol(ethyl alcohol)

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Nonpolar Solvents

CH3

CH2

CH

CH2

CH

CH3CH CH

C

CH3

H2 H2

n-hexaneCH

CH

CH

toluene

Cl

CCl

ClCl

carbon tetrachloride

Hydrogen Bonding• When a very electronegative atom is bonded to

hydrogen, it strongly pulls the bonding electrons toward ittoward itO-H, N-H, or F-H

• Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded i th H texposing the H proton

• The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules

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H-Bonding

HF

H-Bonding in Water

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100

150

Relationship between H-bonding and Intermolecular Attraction BP, HX

BP, H2X

H O

-100

-50

0

50

100

1 2 3 4 5

Bo

ilin

Po

int,

°C

BP, H3X

BP, XH4

NH3

HF

H2O

SiH4GeH4

SnH4H2S

H2Se

H2Te

-200

-150

B

PeriodCH4

S 4

Practice – Choose the substance in each pair that is a liquid at room temperature (the other is a gas)

a) CH OH CH CHFa) CH3OH CH3CHF2

b) CH3-O-CH2CH3 CH3CH2CH2NH2

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Practice – Choose the substance in each pair that is a liquid at room temperature (the other is a gas)


b) CH3-O-CH2CH3 CH3CH2CH2NH2

can H-bond

can H-bond

Practice – Choose the substance in each pair that is more soluble in water


b) CH3CH2CH2CH3 CH3Cl

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Practice – Choose the substance in each pair that is more soluble in water


b) CH3CH2CH2CH3 CH3Cl

can H-bond with H2O

more polar

Ion-Dipole Attraction• in a mixture, ions from an ionic compound are

attracted to the dipole of polar molecules• the strength of the ion-dipole attraction is one of

the main factors that determines the solubility of ionic compounds in water

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Summary

• Dispersion forces are the weakest of theDispersion forces are the weakest of the intermolecular attractions.

• Dispersion forces are present in all molecules and atoms.

• The magnitude of the dispersion forces i ith lincreases with molar mass

• Polar molecules also have dipole-dipole attractive forces

Summary (cont’d)• Hydrogen bonds are the second strongest of the

intermolecular attractive forces b h a pure substance can have

• Hydrogen bonds will be present when a molecule has H directly bonded to either O , N, or F atoms only example of H bonded to F is HF

• Ion-dipole attractions are present in mixtures of ionic compounds with polar molecules.I di l i h i l l• Ion-dipole attractions are the strongest intermolecular attraction

• Ion-dipole attractions are especially important in aqueous solutions of ionic compounds

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Liquids

properties &

structure

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Surface Tension• the tendency of liquids to minimize their surface area

• liquids minimize their surface area → spherical as long as there is no gravity g g y

• molecules of the surface behave differently molecules of interior because the cohesive forces on the surface molecules have a

net pull into the liquid interior

• the surface layer acts like an elastic skin y

Surface Tension• surface molecules have fewer

neighbors to attract them g

• the surface is less stable than the interior have a higher potential energy

• surface tension is the energy required to increase the surface arearequired to increase the surface area a given amount at room temp, surface tension of H2O

= 72.8 mJ/m2

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Factors Affecting Surface Tension

• intermolecular attractive forces ↑ surface• intermolecular attractive forces ↑, surface tension ↑

• temperature ↑, surface tension ↓raising the temperature of the liquid increases the

average kinetic energy of the molecules

the increased molecular motion makes it easier to stretch the surface

Surface Tension of Water vs. Temperature

75

80

60

65

70

Su

rfac

e T

ensi

on, m

J/m

2

50

55

-20 0 20 40 60 80 100 120

Temperature, °C

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Viscosity• viscosity is the resistance of a liquid to flow1 poise = 1 P = 1 g/cm·soften given in centipoise, cPg p ,

• intermolecular attractions ↑ = viscosity ↑• temperature ↑ = viscosity↓

Viscosity of Water vs. Temperature

1

1.2

0.4

0.6

0.8

Vis

cosi

ty, c

P

0

0.2

0 20 40 60 80 100 120

Temperature, deg C

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Capillary Action• ability of a liquid to flow up a thin tube

against the influence of gravitythe narrower the tube the higher the liquid risesthe narrower the tube, the higher the liquid rises

• results from two forces working in conjunction, the cohesive and adhesive forces cohesive forces attract the molecules together

adhesive forces attract the molecules on the edge to the tube’s surface

Capillary Action

• adhesive forces pull the surface liquid up the side of the tube, while the cohesive forces pull , pthe interior liquid with it

• the liquid rises up the tube until the force of gravity counteracts the capillary action forces

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Meniscus• meniscus is due to the competition

between adhesive and cohesive forces

• the meniscus of water is concave in a glass tube because its adhesion to the glass is stronger than its cohesion for itself

• the meniscus of mercury is convex in a l t b b it h i f it lfglass tube because its cohesion for itself

is stronger than its adhesion for the glassmetallic bonds stronger than intermolecular

attractions

Vaporization• molecules are constantly in motion

• average KE α T

• some molecules have more kinetic energy than the average

• molecules at the surface may have enough energy to overcome the attractive forces to become a gas therefore – the larger the surface area,

the faster the rate of evaporation

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Distribution of Thermal Energy• only a small fraction of the molecules in a liquid have enough

energy to escape• As temperature ↑ the fraction of the molecules with “escape

energy” ↑energy ↑ the higher the temperature, the faster the evaporation

Condensation

• some molecules of the vapor will lose energy through molecular collisionsthrough molecular collisions

• → some of the molecules will get captured back into the liquid when they collide with it

• → some may stick and gather together to form droplets of liquid (e.g., rain, fog)p q ( g , , g)particularly on surrounding surfaces

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Evaporation vs. Condensation• opposite processes• in an open container the vapor molecules generally• in an open container, the vapor molecules generally

spread out faster than they can condense• the net result is that the rate of vaporization is greater

than the rate of condensation, and there is a net loss of liquid

• however, in a closed container, the vapor is not allowed pto spread out indefinitely

• → in a closed container at some time rate of vaporization = rate of condensation

Effect of Intermolecular Attraction on Evaporation and Condensation

• the weaker the attractive forces, the less energy they will need to vaporizethey will need to vaporize

• also, weaker attractive forces means that more energy will need to be removed from the vapor molecules before they can condense

• the net result will be more molecules in the vapor phase, and a liquid that evaporates faster – the weaker the attractive forces, the faster the rate of evaporation

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• liquids that evaporate easily are said to be volatile

e.g., gasoline, fingernail polish removerg , g , g p

• liquids that do not evaporate easily are called nonvolatile

e g motor oile.g., motor oil

Energetics of Vaporization• when the high energy molecules are lost from

the liquid, it lowers the average kinetic energy q , g gyof the liquid

• if energy is not drawn back into the liquid, its temperature will decrease – therefore, vaporization is an endothermic processand condensation is an exothermic processand condensation is an exothermic process

• vaporization requires input of energy to overcome the attractions between molecules

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