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S. Ensign, intermolecular forces1
Lecture outline: Chapter 11Intermolecular attractive forces
•Intermolecular forces
•Phase changes
•Vapor pressure
•Phase diagrams
•Types of solids
S. Ensign, intermolecular forces2
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??
S. Ensign, intermolecular forces3
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)
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Attractive forces
δ+δ-HCl
δ+δ-HCl
All attractive forces are electrostatic in nature
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Magnitudes of attractive forces
• Bond energies
• Lattice energies
• Intermolecular attractions
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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
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S. Ensign, intermolecular forces9
Ion-dipole force• The attraction between an ion and the partial
charge on an end of a polar neutral molecule
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Ion-Dipole Forces
+ δ+δ-
δ+
δ-
δ+
δ-
δ+
δ-
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Ion-Dipole Forces
-δ-δ+
δ-
δ+
δ-
δ+
δ-
δ+
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Ion-Dipole Forces involving water
δ+ δ-
δ+
Na+
Cl-
Cl-
Cl-
δ+
δ+
δ+
δ+
δ+
δ+
δ-
δ-
δ-
Cl-
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Dipole-dipole forcesAttraction between neutral, polar molecules
δ+δ-HCl
δ+δ-HCl
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Dipole-Dipole Forces
H Cl
+ -
HCl
HCl
HCl
HCl
HCl
H
Cl
H Cl
HCl
+
+
+
+
++
--
-
-
-
-+
+-
S. Ensign, intermolecular forces15
London Dispersion forces•Attraction between neutral, nonpolarmolecules (or individual atoms)
•Since all attractions are electrostatic, how is this possible??
??δ0 δ0 δ0 δ0
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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
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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-
S. Ensign, intermolecular forces18
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-
S. Ensign, intermolecular forces19
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-
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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+
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Magnitude of London dispersion forces and size……..
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Boiling and melting points are indicators of the strength of an intermolecular attractive force
Δ Δmelting boiling
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Boiling points of the Noble gases:He 4.2 KNe 27.1 KAr 87.3 KKr 115.8 KXe 161.7Rn 211.3 K
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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
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Effect of shape on hydrocarbon boiling points: molecules with formula C5H12
bp = 36.0°C
bp = 9.5°C
bp = 27.7°C
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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
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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
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Examples of hydrogen bonds
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Hydrogen bonding in ice
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Hydrogen bonding in ice
ICES-hex saemod2.pse
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Hydrogen bonding in ice
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Hydrogen bonding in ice
ICES-hex saemod2.pse
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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
S. Ensign, intermolecular forces34
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
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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
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)
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
S. Ensign, intermolecular forces36
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)
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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
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Intra and intermolecular forces in water
463 kJ/mol
~19 kJ/mol
OH
H
O H
Hδ+δ+
δ+
δ− δ+δ−
S. Ensign, intermolecular forces39
Intra and intermolecular forces in water
~19 kJ/mol
463 kJ/mol
S. Ensign, intermolecular forces40
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
S. Ensign, intermolecular forces41
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
S. Ensign, intermolecular forces42
What is the major type of intermolecular attractive force between molecules of:
• HF
• F2
• PCl3
• BrF
S. Ensign, intermolecular forces43
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
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Important properties of liquids•Melting and boiling points
•Viscosity
•Surface tension
•Ability to form mixtures
•Vapor pressure
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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
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Surface tension of a liquidSurface molecules are in a different environment than interior molecules
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Surface tension of a liquid
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S. Ensign, intermolecular forces48
Changes of state
solid
Ener
gy o
f sys
tem
liquid
gas
S. Ensign, intermolecular forces49
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
S. Ensign, intermolecular forces50
Energy changes associated with changes of state
How does evaporative cooling work?
endothermicΔ
exothermic
Δ
endothermicΔ
exothermic
Δ
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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
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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
S. Ensign, intermolecular forces53
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
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Use specific heat values, and ΔHvaporizationand ΔHfusion values, to calculate ΔH for processes involving change of state (see chapter 11 self test)
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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
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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
S. Ensign, intermolecular forces58
Which of the following substances can be liquefied at room temperature (298 K), given sufficient pressure?
Compressed gas vs. liquified gas
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Vapor pressure
S. Ensign, intermolecular forces60
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
S. Ensign, intermolecular forces61
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
S. Ensign, intermolecular forces62
What is the relation between the strength of intermolecular forces and the value of vapor
pressure at a given temperature?
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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
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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
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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
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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
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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)
S. Ensign, intermolecular forces68
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
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Note the differences in direction of slope for the melting point curves for CO2 and H2O
Phase diagram for CO2Phase diagram for H2O
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Water is one of only a few molecular compounds for which the solid is less dense than the liquid
water Most other molecular compounds
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Skate sailing on Lake Mendota, Madison Wisconsin
S. Ensign, intermolecular forces72
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
S. Ensign, intermolecular forces73
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
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Ice: a molecular solid
Other examples: sucrose (C12H22O11), paraffin, I2, dry ice (CO2)
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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
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The C allotropes diamond and graphite: Covalent network solids
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Buckyball (C60) is an allotrope of C that exists in molecular form
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NaCl: an ionic solid
Na+Cl-
S. Ensign, intermolecular forces79
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
S. Ensign, intermolecular forces80
Metallic solids: Positively charged metal ions “swimming” in a “sea of electrons”
+ -+
+-
+ + + + +
++ + + + +
++ + + +- - - -
- - - - -
- - - - -
- -
S. Ensign, intermolecular forces81
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
-
-
-
--
-
--
-
- -
--
-
-----
--
-
-
-
--
-
-- -
-
-
-
--
-
S. Ensign, intermolecular forces82
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
-
-
-
-
-
- -
--
-
-
-- -
-
-
--
+-
+ +
++ +
++- -
- - -
- - -
- -