1 chapter 11 fritz london 1900-1954. studied intermolecular induced-dipole interactions. johannes d....
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Chapter 11Chapter 11
Fritz London 1900-1954.Studied intermolecularinduced-dipole interactions.
Johannes D. van der Waals1837-1923.*Studied intermolecular forcesin VPT relationships inliquids and gases.
Liquids, Solids, and MaterialsLiquids, Solids, and Materials
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Kinetic Energy
Attractive Intermolecular Forces
High temperaturesLow temperatures
Gases, Liquids and SolidsGases, Liquids and Solids
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Ionic ForcesIonic Forces
++
-- -
-+
++
Ion-Ion
e.g. NaCl(s)
Ion-Dipole
e.g. NaCl(aq)
Ions form strong intermolecular forces with the polar molecule water.
Ion-ion forces are very strong and produce high boiling points and melting points.
NaCl dissolved in water
From now on we will concentrate on covalent molecules
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Intermolecular InteractionsIntermolecular InteractionsNon-Polar Molecules:
Polar Molecules:
Molecules withF-H, O-H, N-HBonds:
1. Dispersion (Disp) Forces
1. Dispersion (Disp) Forces2. Dipole-Dipole (DD) Forces
1. Dispersion (Disp) Forces2. Dipole-Dipole (DD) Forces3. Hydrogen Bonding (HB) Forces
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London Dispersion Forces London Dispersion Forces (aka van der Waals Forces)(aka van der Waals Forces)
The electrons on one atom are attracted tothe nucleus on a neighboring atom.
This creates an “instantaneous” (i.e. temporary)dipole on the first atom.
The instantaneous dipole on the first atomthen induces an instantaneous dipole on thesecond atom.
The two induced dipoles attract each other.
London Dispersion Forces are proportional to a molecule’s polarizability,which is the ease with which the electron cloud can be deformed.
The polarizability, is approximately proportional to the number of electronsin the molecule.
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Element MW # e- Tbp
[amu] [K]
He 4.0 2 4
Ne 20.2 10 27
Ar 39.9 18 87
Kr 83.9 36 121
Xe 131.3 54 166
The noble gases are spherical, non-polar atoms.
As the molecule (atom) increases in size, the boiling point increases (vapor pressure decreases).
Noble GasesNoble Gases
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London Dispersion ForcesLondon Dispersion ForcesLondon Dispersion Force # e-
London Dispersion Force MW
Tbp is a good measure of the strength of intermolecular forces.
A higher Tbp indicates stronger intermolecular forces.
Compd. MW # e- Tbp
[amu] [oC ]
F2 38 18 -188
Cl2 71 34 -34
Br2 160 70 +59
I2 254 106 +184
Compd. MW # e- Tbp
[amu] [oC ]
CH4 16 10 -161
C2H6 30 18 -88
C3H8 44 26 -42
n-C4H10 58 34 0
Disp
ersion
Fo
rces
Increasin
g
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Dispersion forces also depend upon the molecular shape.Pentane: C5H12 - MW = 72 amu
H3C
CH2
H2C
CH2
CH3
n-pentane
Tbp = 36oC
neopentane
Tbp = 9oC
H3C
C
CH3
CH3H3C
LargeContact Area(Surface Area)
SmallContact Area(Surface Area)
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Dipole-Dipole ForcesDipole-Dipole Forces
In liquids of polar molecules, oppositelycharged ends of the molecules tend toattract each other, causing partial alignment.
For molecules of roughly equal MW’s(i.e. with similar Dispersion Forces),the molecule with the higher dipole momentwill have a higher boiling point due to greaterDipole-Dipole forces.
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C
C
CH
HH
H H
H
H H
Propane
CH3CH2CH3
Acetonitrile
CH3CN
MW = 44 amu
Tbp = -42 oC
MW = 41 amu
Tbp = +82 oC
C C
H
H
H
N
0No Dipole Moment
= 3.9 Debye (D)
Large Dipole Moment
Why is the boiling point of acetonitrile so muchhigher than the boiling point of propane???
Dipole-Dipole Forces
Consider this….
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Hydrogen Bonding
F-H Bonds:
H-F
O-H Bonds:
e.g. H2O , CH3OH
N-H Bonds:
e.g. NH3, CH3NH2
O
H H
O
H H
N
HH
H
N
HH
H
H F H F
H bonding:between H and a veryelectronegativeatom
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Compd. MW # e- [amu] [D]
F-F 38 18 0
H-Cl 36 18 1.1
Tbp
[oC ]
-188
-85
H-F 20 10 1.8 +20 Why so high??
Hydrogen Bond
H F
+ -
H F
+ -
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Hydrogen BondingHydrogen BondingCompd. MW # e- [amu]
H2S 34 18
H2O 18 10
Tbp
[oC ]
-60
+100
PH3 34 18
NH3 17 10
-88
-33
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CH3CH2OH HOCH2CH2OH CH3OCH3 CH3CH2CH3
MW=74 MW=62 MW=46 MW=44
Disp Disp Disp Disp
DD DD DD
HB
LowestBP
HighestBP
Ethylene glycol -a viscous liquid;
keeps radiator fluidfrom boiling over
in your car(198°)
Propane -a gas used
for charcoalGrills(- 42°)
Ether -a veryvolatileLiquid(35°)
Ethyl alcohol -a liquid with slightly lowerbp than water
(78°)
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Vapor PressureVapor Pressure
Vap. Press. (H2O)
liquid water
H2OH2O
H2O
H2O
H2OH2O
H2O
H2O
H2O
H2OH2OH2O
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Vapor Pressure Rises with Temperature
Temperature
Va
por
Pre
ssur
e
At higher temperatures, more molecules have sufficient energy to escape from the liquid.
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The Boiling PointThe Boiling PointThe boiling point of a liquid is the temperature at whichits vapor pressure is equal to the external pressure.
The “normal” boiling point is the temperature at whichthe vapor pressure of the liquid is equal to 760 torr (1 atm.)
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The Critical TemperatureThe Critical TemperatureH2O
T Vap. Press.[oC] [atm]
25 0.03
100 1
150 5
250 40
300 100
374 218
375
Tc = Critical Temperature
Highest temperature at which substance can be liquified.
Pc = Critical Pressure
Pressure required to liquifysubstance at Tc
Tc = 374 oCPc = 218 atm
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Substance Tc Pc
Helium -268 oC 2.3 atm
Oxygen -119 50
Ethane 32 48
Propane 97 42
Freon (CCl2F2) 112 40
Water 374 218
The Critical TemperatureThe Critical Temperature
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Phase DiagramsPhase Diagrams1. Why can’t you ice skate in Red Lake, Minnesota in February?
2. Why does carbon dioxide sublime, whereas water first melts and then vaporizes?
toSt. Paul
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Phase Diagram of HPhase Diagram of H22OO
Temperature (oC)
Pre
ssu
re (
atm
)
1
0 100
Solid Liquid Vapor
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A
D
B
C
Temperature (oC)
Pre
ssu
re (
atm
) Solid Liquid Vapor
AB: Liquid-Vapor Equilibrium
AC: Solid-Liquid Equilibrium
AD: Solid-Vapor Equilibrium
A: Triple Point Solid-Liquid-Vapor
B: Critical Point
0.01
4.6torr
Phase Diagram of HPhase Diagram of H22OO
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Pressure dependence of TPressure dependence of Tbpbp
Temperature (oC)
Pre
ssu
re (
atm
)
Liquid Vapor
100
1
2
120
Liquid Vapor
d(liq) >> d(vap)
V(liq) << V(vap)
Increased pressure shiftsequilibrium in directionof lower volume.
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Pressure dependence of TPressure dependence of Tmpmp
in Hin H22OO
Temperature (oC)
Pre
ssu
re (
atm
)
0
1
120
-20
Solid (ice) Liquid
d(ice) < d(liq)
V(ice) > V(liq)
Increased pressure shiftsequilibrium in directionof lower volume.
Solid Liquid
Application toice skating
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P1
Temperature (oC)
Pre
ssu
re (
atm
)
Solid Liquid Vapor
P2
Melting point decreases with Pressure
Melting + Vaporization
Sublimation
4.6torr
0.01
Phase Diagram of HPhase Diagram of H22OO
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Phase Diagram of “Normal” SubstancesPhase Diagram of “Normal” Substances
Temperature (oC)
Pre
ssu
re (
atm
)
P1
Solid Liquid Vapor
P2
Melting point increases with Pressure
Melting + Vaporization
Sublimation
CO25.1
-56
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Pressure dependence of TPressure dependence of Tmpmp
in “normal” substancesin “normal” substances
Temperature (oC)
Pre
ssu
re (
atm
)
1
100 Solid Liquid
d(sol) > d(liq)
V(sol) < V(liq)
Increased pressure shiftsequilibrium in directionof lower volume.
Solid Liquid
Tmpo Tmp
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Phase TransitionsPhase Transitions
Solid
Gas
Liquid
Melting Freezing
Vaporization
En
thal
py
Sublimation Deposition
Condensation
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Energy Changes of Phase TransitionsEnergy Changes of Phase Transitions
Solid
Liquid
Melting Freezing
En
thal
py
Melting (Fusion)
Hfus = Hliq - Hsol
= 6.01 kJ/mol (for H2O)
ENDOTHERMIC
Freezing (Crystallization)
Hcrys = Hsol - Hliq
= -6.01 kJ/mol (for H2O)
EXOTHERMIC
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Gas
Liquid
Vaporization
En
thal
py
Condensation
Vaporization
Hvap = Hgas - Hliq
= 40.7 kJ/mol (for H2O)
ENDOTHERMIC
Condensation
Hcond = Hliq - Hgas
= -40.7 kJ/mol (for H2O)
EXOTHERMIC
Energy Changes of Phase TransitionsEnergy Changes of Phase Transitions
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Heating CurvesHeating Curves
Heat Added (Joules)
Tem
per
atu
re (
oC
)
Tmp
Tbp
1. Heating solid
1
2. Melting solid to liquid2 3. Heating liquid
3
4. Vaporizing liquid to gas
4
5. Heating gas
5
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Quantitative considerations of heating curves
Hfus= 6.0 kJ/mol
Hvap= 40.7 kJ/mol
Tmp= 0 oC
Tbp = 100 oC
Heat capacities:
Cs(sol)= 2.09 J/g-oC
Cs(liq)= 4.18 J/g-oC
Cs(gas)= 1.84 J/g-oC
For water:
To heat 18 g of H2O from -40o to 140oC:
18 x 2.09 x 40 = 1.5 kJ (heating the ice)
1.0 x 6.01 = 6.0 kJ (melting the ice)
18 x 4.18 x 100 = 7.5 kJ (heating the water)
1.0 x 40.7 = 40.7 kJ (boiling the water)
18 x 1.84 x 40 = 1.3 kJ (heating the steam)
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Properties of LiquidsProperties of LiquidsViscosityViscosity
Viscosity is the resistance of a liquid to flowing.
High viscosity liquids (e.g. molasses, motor oil) flow slowly.
Low viscosity liquids (e.g. water, gasoline) flow quickly.
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Surface tension is a measure of the strength of intermolecular attractions which pull on molecules at the surface of a liquid.
It is because of its high surface tension thatwater tends to “bead” up on a waxy surface.
That’s because a sphere gives the minimum ratioof surface area to volume.
The high surface tension of water also causes molecules onthe surface to pack very closely together. This is why someinsects can “walk on water”.
Properties of LiquidsProperties of LiquidsSurface TensionSurface Tension
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Structure of SolidsStructure of Solids
Crystalline Solid
Ordered arrangement of atoms(or molecules) in 3-dimensionalstructure, called a lattice
e.g. Quartz (SiO2)
Amorphous Solid
Irregular (disordered)arrangement of atoms(or molecules)
e.g. Silica Glass (SiO2)
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The Crystalline LatticeThe Crystalline Lattice
Lattice: Three Dimensional array of points representing the centers of each atom in the crystal.
Lattice Point: Point corresponding to atom center
Unit Cell: Parallelopiped corresponding to minimum unit which can be used to replicate crystal
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Bonding in SolidsBonding in Solids
There are four classifications of solids, dependingon the type of bonds that are present.
• Covalent-Network Solids
• Ionic Solids
• Metallic Solids
• Molecular Solids
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Covalent-Network SolidsCovalent-Network SolidsForm of particles: Atoms connected in network of covalent bonds
Forces between particles: Covalent bonds
Properties: Very Hard Very high melting point Usually poor thermal and electrical conductivity
Examples: Diamond (C), Quartz (SiO2)
Diamond
Each carbon is connected to 4 others by a covalent bond
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Ionic SolidsIonic Solids
Form of particles: Positive and negative ions
Forces between particles: Electrostatic attractions
Properties: Hard and Brittle High melting point Poor thermal and electrical conductivity
Examples: All typical salts. e.g. NaCl, Ca(NO3)3, MgBr2
++
-- -
-+
++
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Metallic SolidsMetallic SolidsForm of particles: Atoms
Forces between particles: Metallic Bonds (due to delocalized valence electrons)
Properties: Soft to very hard Low to very high melting point Excellent thermal and electrical conductivity Malleable and Ductile
Examples: All metals. e.g. Cu, Fe, Sn, Au, Ag
Bonding due to delocalized valenceelectrons (shown in blue)
Strength of bonding varies betweendifferent metals, resulting in widerange of physical properties
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Molecular SolidsMolecular SolidsForm of particles: Atoms or molecules
Forces between particles: Dispersion Dipole-Dipole (if molecules are polar) Hydrogen Bonds (if O-H, N-H, F-H)Properties: Fairly soft Moderately low melting point (usually <200 oC) Poor thermal and electrical conductivity
Examples: Argon , CH4, CO2, C6H12O6 (sucrose), H2O