Lecture outline: Chapter 11 Intermolecular attractive...

S. Ensign, intermolecular forces1

Lecture outline: Chapter 11Intermolecular attractive forces

•Intermolecular forces

•Phase changes

•Vapor pressure

•Phase diagrams

•Types of solids


Gases• The volume occupied by gas molecules is

much less than the volume in which they reside

• Molecules of gases are in continual random motion

• Attractive/repulsive forces between gas molecules are negligible

• The average kinetic energy of a gas molecule is proportional to the temperature

Which of these properties apply to liquids and solids as well??


Physical state Defined volume?

Defined shape?

Compress/ expand?

Gas

Liquid

Solid

• Intramolecular forces: attractive forces of atoms within compounds (think: rope knot, zipper)– Molecular compounds: covalent bonds, bond

energies– Ionic compounds: ionic bonds, lattice energies

• Intermolecular forces: attractive forces between different molecules (think: velcro, snap button)


Attractive forces

δ+δ-HCl

δ+δ-HCl

All attractive forces are electrostatic in nature


Magnitudes of attractive forces

• Bond energies

• Lattice energies

• Intermolecular attractions


Intermolecular forces

• Ion-dipole• Dipole-dipole• London dispersion• Hydrogen bonding

Attraction between an ion and a molecule

Attraction between molecules

All attractive forces are electrostatic in nature

Review electronegativity, polarity, and dipole moment

Increases from left to right in a period7

S. Ensign, intermolecular forces

8

H F

less EN

C OO

more EN

Review electronegativity, polarity, and dipole moments

OHH

δ+ δ-

δ+ δ-

δ+ δ+

δ-

Net dipole

Polar

Non-polar

PolarN

et d

ipol

e

δ-

No net dipole

S. Ensign, intermolecular forces


Ion-dipole force• The attraction between an ion and the partial

charge on an end of a polar neutral molecule


Ion-Dipole Forces

+ δ+δ-

δ+

δ-

δ+

δ-

δ+

δ-


Ion-Dipole Forces

-δ-δ+

δ-

δ+

δ-

δ+

δ-

δ+


Ion-Dipole Forces involving water

δ+ δ-

δ+

Na+

Cl-

Cl-

Cl-

δ+

δ+

δ+

δ+

δ+

δ+

δ-

δ-

δ-

Cl-


Dipole-dipole forcesAttraction between neutral, polar molecules

δ+δ-HCl

δ+δ-HCl


Dipole-Dipole Forces

H Cl

+ -

HCl

HCl

HCl

HCl

HCl

H

Cl

H Cl

HCl

+

+

+

+

++

--

-

-

-

-+

+-


London Dispersion forces•Attraction between neutral, nonpolarmolecules (or individual atoms)

•Since all attractions are electrostatic, how is this possible??

??δ0 δ0 δ0 δ0


London Dispersion forces•Attraction between neutral, nonpolarmolecules (or individual atoms)

•The motion of electrons in atoms and molecules can result in a short-lived instantaneous dipole

δ0 δ0

δ+δ-

δ+ δ-

δ0 δ0

e- density shifts to left

e- density shifts to right


London Dispersion forces•The motion of electrons in atoms and molecules can result in a short-lived instantaneous dipole

•An instantaneous dipole on one molecule (or atom) can induce an instantaneous dipole on an adjacent molecule (or atom) resulting in the dipole-dipole attraction

δ0 δ0δ+ δ- δ0 δ0 δ0 δ0 δ0 δ0δ+ δ- δ+ δ- δ+ δ-


London Dispersion forces•The motion of electrons in atoms and molecules can result in a short-lived instantaneous dipole

•An instantaneous dipole on one molecule (or atom) can induce an instantaneous dipole on an adjacent molecule (or atom) resulting in the dipole-dipole attraction

•A switch in the direction of the instantaneous dipole on one molecule results in a switch in the direction on adjacent molecules

δ+ δ- δ+ δ- δ+ δ- δ+ δ-δ+δ- δ+δ- δ+δ- δ+δ-


London Dispersion forces

δ+ δ- δ+ δ- δ+ δ- δ+ δ-

δ+δ- δ+δ- δ+δ- δ+δ-

δ+ δ- δ+ δ- δ+ δ- δ+ δ-


London Dispersion Forces can also operate in a collection of neutral atoms

δ0 δ0

δ- δ+

δ+ δ-

e- density shifts to left

e- density shifts to right

δ- δ+ δ- δ+ δ- δ+ δ- δ+ δ- δ+

δ-δ+ δ-δ+ δ-δ+ δ-δ+ δ-δ+


Magnitude of London dispersion forces and size……..


Boiling and melting points are indicators of the strength of an intermolecular attractive force

Δ Δmelting boiling


Boiling points of the Noble gases:He 4.2 KNe 27.1 KAr 87.3 KKr 115.8 KXe 161.7Rn 211.3 K


Hydrocarbon boiling points

H C

H

H

H H C C

H

H

H

H

H

H C C

H

H

H

H

C

H

H

C

H

H

H

H C C

H

H

H

H

C

H

H

H

H C C

H

H

H

H

C

H

H

C

H

H

C

H

H

H

A. B. C.

D. E.

bp = -161° C bp = -89° C bp = -44° C

bp = -0.5° C bp = 36° C


Effect of shape on hydrocarbon boiling points: molecules with formula C5H12

bp = 36.0°C

bp = 9.5°C

bp = 27.7°C


Hydrogen bonding•Special type of dipole-dipole force

•The intermolecular attraction between a H atom in a very polar bond and an unshared e-pair on a small electronegative atom

•The intermolecular attraction between a H atom attached to N, O, or F and a N, O, or F atom


Hydrogen bonding in water

δ+ δ-

δ+

δ-

δ-

δ-

δ+

δ+ δ+

δ+ δ+

δ+δ+

δ-

Between waters

Between ammonias

Between water and ammonia

Between water and ethylene glycol


Examples of hydrogen bonds


Hydrogen bonding in ice



ICES-hex saemod2.pse


Hydrogen bonding in liquid water• At 25° C, thermal energy closely matches H-

bond energy (20 kJ/mol) • Average lifetime of aqueous H-bond is 10-11 s• Disordered 3-dimensional network • Each water has an average of 3.4 H-bonds


More than one type of force can contribute to intermolecular attractions in a molecule

Consider:

(1) size of molecule

(2) shape of molecule

(3) polarity of molecule (∆EN)

Amount of accessible valence electron density to overlap another molecule


Boiling points of group 6A hydrides

Molar mass (g/mol)0 20 40 60 80 100 120 140

boili

ng p

oint

(°C

)

-80

-60

-40

-20

0

20

40

60

80

100

120H2O

H2SH2Se

H2Te


Consider:




Boiling points of hydrogen halides

Molar mass (g/mol)0 20 40 60 80 100 120 140

boilin

g po

int (

°C)

-100

-80

-60

-40

-20

0

20

40HF

HClHBr

HI



Consider:





Intra- and intermolecular attractive force strengths

• covalent bond (bond E): ~150-1000 kJ/mol• ionic bond (lattice E): ~500-4000 kJ/mol• ion-dipole ~15 kJ/mol• Hydrogen bond ~5-25 kJ/mol• dipole-dipole ~2-10 kJ/mol• London dispersion ~0-10 kJ/mol

Remember it always takes energy to break a bond, whether it is intra or intermolecular


Intra and intermolecular forces in water

463 kJ/mol

~19 kJ/mol

OH

H

O H

Hδ+δ+

δ+

δ− δ+δ−


Intra and intermolecular forces in water

~19 kJ/mol

463 kJ/mol


Intermolecular attractions in neutral moleculesIn general, the relative strengths are: H-bond > dipole-dipole > London-dispersion

Strengths of dipole-dipole and London-dispersion forces are related to the polarity, size, and shape of the molecule

Boiling points are good indicators of the strength of an intermolecular attractive force: the higher the bp, the stronger the attraction; the lower the bp, the weaker the attraction


To visualize intermolecular forces, reference everything to absolute zero, where molecular motion is at a minimum, and everything is solid. Then think about adding heat, and ask what happens


What is the major type of intermolecular attractive force between molecules of:

• HF

• F2

• PCl3

• BrF


Which substance in the following pairs would you predict has a higher boiling point?

• HF or HBr

• CH4 or C4H10

• NH3 or PH3

• H2O or H2S

• MgBr2 or PBr3


Important properties of liquids•Melting and boiling points

•Viscosity

•Surface tension

•Ability to form mixtures

•Vapor pressure


Viscosity: resistance of a liquid to flowHydrocarbon Formula Boiling

pointViscosity (cP)

Hexane 69 0.326

Heptane 98 0.409

Octane 126 0.542

Nonane 151 0.711

Decane 174 1.42


Surface tension of a liquidSurface molecules are in a different environment than interior molecules


Surface tension of a liquid

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Changes of state

solid

Ener

gy o

f sys

tem

liquid

gas


Ene

rgy

of s

yste

m

deposition

subl

imat

ion

solid

liquid

gas

Δ Δm

eltin

g (fu

sion

)

freezing

vapo

rizat

ion

condensation endo

ther

mic exotherm

ic


Energy changes associated with changes of state

How does evaporative cooling work?

endothermicΔ

exothermic

Δ

endothermicΔ

exothermic

Δ


Energy changes associated with changes of state

Why does your hand get burned when exposed to gaseous water (steam) but not when exposed to hot air at the same temperature?

heat added (kJ)0 10 20 30 40 50 60

Tem

pera

ture

(°C

)

-25

0

25

50

75

100

125

Heating curve for water

liquid → gas

iceice → liquid

liquid

gas


lines in red: raising temperature of (heating) a single phase

heat added (kJ)0 10 20 30 40 50 60

Tem

pera

ture

(°C

)

-25

0

25

50

75

100

125

Heating curve for water

liquid → gas

iceice → liquid

liquid

gas


lines in green: phase transitions: all heat goes into breaking bonds, so no T increase

S. Ensign, intermolecular forces54heat added (kJ)

0 10 20 30 40 50 60

Tem

pera

ture

(°C

)

-25

0

25

50

75

100

125

The heat input for each region of the curve is determined by specific heats and enthalpies of phase transitions

ice

liquid

gasΔHvap = 40.67 kJ/mol

SHice =2.09 J/(g•˚C)

SHliquid = 4.18 J/(g•˚C)

SHgas = 1.84 J/(g•˚C)

ΔHfusion = 6.00 kJ/mol


Use specific heat values, and ΔHvaporizationand ΔHfusion values, to calculate ΔH for processes involving change of state (see chapter 11 self test)


States of matter are dependent on both temperature and pressure

• Critical temperature: the highest temperature at which a substance can exist as a liquid

• Critical pressure: the pressure required to liquify a substance at it’s critical temperature


Substance Critical T (K) Critical P (atm)N2 126.1 33.5

Ar 150.9 48

O2 154.4 49.7

CH4 190 45

CO2 304.3 73.0

Propane (C3H8) 370.0 42.0

Ammonia 405.6 111.5

H2O 647.6 217.7

Which of the following substances can be liquefied at room temperature (298 K), given sufficient pressure?

Substance Critical T (K) Critical P (atm)N2 126.1 33.5

Ar 150.9 48

O2 154.4 49.7

CH4 190 45

CO2 304.3 73.0

Propane (C3H8) 370.0 42.0

Ammonia 405.6 111.5

H2O 647.6 217.7


Which of the following substances can be liquefied at room temperature (298 K), given sufficient pressure?

Compressed gas vs. liquified gas


Vapor pressure


Distribution of kinetic energies for molecules in a liquid

kinetic energy

fract

ion

of m

olec

ules

T1

Kinetic energy required to escape from liquid

T2


Vapor pressureThe pressure exerted by molecules in the gas phase above a liquid when the vapor and liquid are at equilibrium

“Equilibrium” has been reached when rateescape = ratereturn


What is the relation between the strength of intermolecular forces and the value of vapor

pressure at a given temperature?


What is the relation between temperature and the value of vapor pressure for a given liquid ?

Equilibrium at T1Increase T:rate escape >rate return

Equilibrium reestablished at T2


Boiling point: the temperature at which the vapor pressure of a liquid is equal to the external pressure

Temperature (°C)0 20 40 60 80 100 120

vapo

r pre

ssur

e (to

rr)

0

200

400

600

800

1000

Diethyl ether

Ethanol

Water

Ethylene glycol


Location Elevation (ft)

P (torr) P (atm) Boilingpoint (°C)

Summit of Mt. Everest 29,028 253 0.333 68.0

Summit of King’s peak, UT 13,528 468 0.616 86.0

USU Campus Logan, UT 4,521 650 0.855 95.5

Madison, WI 863 738 0.971 99.3

San Diego, CA 17 759 0.999 99.98

Surface of Pacific Ocean 0 760 1.00 100.0

Death Valley, CA -282 767 1.009 100.3

A medical autoclave N/A 1535 2.02 121

Relationships between altitude, atmospheric pressure, and boiling point


A phase diagram shows the relation between pressure, temperature, and physical state of a substance

Temperature

Pre

ssur

e

solidliquid

gas

Melting(fusion)

freezing

deposition

sublimation

vaporization

condensation

Criticalpoint

Triplepoint


Phase diagram for CO2 (axes not to scale)

Temperature (°C)

Pre

ssur

e (a

tm)

solid liquid

gas

Criticalpoint

Triplepoint

-56.4

5.11

-78.5

1.00

31.1

73

Temperature (°C)

Pre

ssur

e (a

tm)


Phase diagram for water (axes are not to scale)

0.0

0.006

1.0

374

218

100.0

Criticalpoint

Triplepoint

Temperature (°C)

Pres

sure

(atm

)

solid liquid

gas

Criticalpoint

Triplepoint

-56.4

5.11

-78.5

1.00

31.1

73

Temperature

Pres

sure

0.0

0.006

1.0

374

218

100.0

Criticalpoint

Triplepoint


Note the differences in direction of slope for the melting point curves for CO2 and H2O

Phase diagram for CO2Phase diagram for H2O


Water is one of only a few molecular compounds for which the solid is less dense than the liquid

water Most other molecular compounds


Skate sailing on Lake Mendota, Madison Wisconsin


Solids• Three types of motion for molecules:

– Translational (diffusion)– Rotational– vibrational

• Solids are more restricted in freedom of motion than a gas or liquid

Δ Δmelting boiling


Four types of solids• Molecular solid: a solid composed of atoms or

molecules held together by intermolecular attractions (London, dipole-dipole, or H-bonds)

• Covalent network solid: a solid where all atoms are connected through covalent bonds

• Ionic solid: a solid where cations and anions interact through ionic bonds (electrostatic attractions

• Metallic solid: a solid consisting of metal atoms bonded through “metallic bonds”c

Solids can also consist of polymeric units such as polyethylene and polyester, and complex biomolecules such as cellulose and lignin


Ice: a molecular solid

Other examples: sucrose (C12H22O11), paraffin, I2, dry ice (CO2)


The C allotropes diamond and graphite: Covalent network solids

Other examples: silica quartz and glass (SiO2)n

C atoms are sp3, tetrahedral geometry

C atoms are sp2, trigonal planar geometry


The C allotropes diamond and graphite: Covalent network solids


Buckyball (C60) is an allotrope of C that exists in molecular form


NaCl: an ionic solid

Na+Cl-


Bonding in metallic solids? Compare Na and Ne

Property Neon SodiumAtomic number 10 11Std state form Monatomic gas solidbp -246.1 °C 883 °Cmp -248.6 °C 97.72 °CElectrical conductor

No Yes

Reactivity No Readily oxidizedColor Colorless Silvery-whiteAtomic radius 38 pm 190 pm


Metallic solids: Positively charged metal ions “swimming” in a “sea of electrons”

+ -+

+-

+ + + + +

++ + + + +

++ + + +- - - -

- - - - -

- - - - -

- -


Metallic solids: Positively charged metal ions “swimming” in a “sea of electrons”

2+ -+ -

+2 +2 +2 +2 +2 +2

+2 +2 +2 +2 +2 +2

+2 +2 +2 +2 +2 +2

-

-

-

--

-

--

-

- -

--

-

-----

--

-

-

-

--

-

-- -

-

-

-

--

-


Properties of metals accounted for by the “electron sea” model

• Solids at room temperature• Good electrical conductors• high heat conductivity and heat capacity• malleable and ductile• Readily undergo oxidation

+2 +2 +2

+2 +2

+2 +2 +2

-

-

-

-

-

- -

--

-

-

-- -

-

-

--

+-

+ +

++ +

++- -

- - -

- - -

- -

Lecture outline: Chapter 11 Intermolecular attractive...

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