Van der Waals Forces

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Van der Waals Forces

• Johannes Diderik van der Waals

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• E. The motion of particles in these phases

http//itl.chem.ufl.edu/2045_s00/lectures/FG11_001
.GIF
3
Polarity

• Separation of charge
• An asymmetrical difference in electronegativity
  along a bond or in a molecule

4
Circle the polar molecules. Label d and d-
Cl Cl Ca
d-
d-
d
d-
d
d
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Non-Polar

• C. __________ molecules are symmetrical
•  D. What is the bond angle in H2O? _______
•  E. The motion of particles in these
  phases Solid Liquid Gas

104.5o
http//itl.chem.ufl.edu/2045_s00/lectures/FG11_001
.GIF
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Van der Waals Forces

• Small, weak interactions between molecules
•  

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Van der Waals Forces

• Intermolecular between molecules (not a bond)
•  
• Intramolecular bonds within molecules (stronger)

Covalent Bonds
IMF
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What is being attracted?

• d attracted to d-
• ? electrostatic attraction
• e- s of one atom to another atoms nucleus

e-


e-
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Evidence of VDW Forces?

• Non-polar molecules can form gases, liquids and
  solids.
• Ex CO2

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3 Types of Van der Waals Forces

• 1)    dipole-dipole
• 2)    dipole-induced dipole
• 3) dispersion

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Dipole-Dipole
Two polar molecules align so that d and d- are
matched (electrostatic attraction) Ex ethane
(C2H6) vs. fluromethane (CH3F)
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Fluoromethane (CH3F) boiling point 194.7 K
polar or non-polar?
H H H C F H C F H
H
Ethane (C2H6) boiling point 184.5 K
polar or non-polar?
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Try This

• Draw two KBr molecules and draw their
  dipole-dipole interactions with a dashed line.

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What does to induce mean?

• To cause or bring about
• Ex
• Induced vomiting
• Induced labor
• Induced coma

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Dipole-Induced Dipole

• A dipole can induce (cause)
• a temporary dipole to form in a
• non-polar molecule
• The molecules then line up
• to match d and d- charges

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Example
e-
e-
e-
e-
Ar
e-
e-
d-
d
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
INDUCED DIPOLE
non-polar
A DIPOLE (its polar)
Dipole Induced Dipole (weak and short-lived)
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Draw CO2 (aq)

• What does (aq) mean?
• dissolved in WATER
• Sodraw CO2 (g) in H2O (l)

d-
d
d
d-
d-
d
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Where is CO2 (aq) seen?

• Carbonated water
• CO2 is not very soluble
• 1 CO2 in 1000 H2O molecules

http//www.packaging-technology.com/contractor_ima
ges/venus/4_rinser.jpg
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Dispersion Forces

• A temporary dipole forms in a
• non-polar molecule
• which leads to
• a temporary dipole to form in ANOTHER non-polar
  molecule
• Dispersion is the ONLY intermolecular attraction
  that occurs between non-polar molecules

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Dispersion Forces
e-
e-
e-
e-
Cl-Cl
e-
e-
e-
e-
Cl-Cl
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
INDUCED DIPOLE
TEMPORARY DIPOLE
non-polar
non-polar
Dispersion (weakest and very short-lived)
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Tokay GeckoDispersion Forces!
http//www.youtube.com/watch?vTq8Yw19bn7Qfeature
related
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Review

• Dipole Dipole
• between two polar molecules
• Dipole Induced Dipole
• b/w a polar a non-polar molecule
• Dispersion
• between two non-polar molecules

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Hydrogen Bonding

• STRONGEST Intermolecular Force!!
• A special type of dipole-dipole attraction
• Bonds form due to the polarity of water
• Draw 4 H2O molecules in your notes

Covalent Bonds
Ice
Liquid
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Hydrogen Bonding cont

• Hydrogen bonds keep water in the liquid phase
  over a wider range of temperatures than is found
  for any other molecule of its size

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Hydrogen bonds account for the high boiling point
of water
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Expansion of Ice

• Ice expands when water freezes compared to most
  substances that contract when freezing
• Ice bomb video

http//www.youtube.com/watch?verlZb8QiPkg
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Density vs Temperature of H2O
3.98 oCmax density of water liquid!
Solid Ice
Liquid water
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Hexagonal Ice
http//hyperphysics.phy-astr.gsu.edu/hbase/chemica
l/imgche/waterhex.gif
http//www.gala-instrumente.de/images/4420hexagon
al20ice.jpg
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• Halos, Sundogs, Pillars are caused by hexagonal
  ice crystals

http//images.usatoday.com/tech/_photos/2006/09/12
/cloud.jpg
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Ponds Freezing

• Solid water (ice) has a lower density than liquid
  water

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Why is this good?
Ponds freeze from the top down, insulating the
water below and keeping it from freezing solid.
Without this, ponds would freeze solid and thaw
more slowly
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Surface Tension

• Enhancement of the intermolecular attractive
  forces at the surface

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Evidence

• Lab
• Dixie cup
• Penny
• Capillary tube
• needle

http//www.youtube.com/watch?vvajL48mwsCA
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What causes surface tension?

• The cohesive forces between molecules are shared
  with all neighboring atoms.
• Since the surface has no neighboring atoms above,
  they exhibit stronger attractive forces for their
  neighbors next to and below them

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How many drops can you get on a penny?

• Water?
• TTE?
• Why is there a difference???
• Water has strong Hydrogen Bonds and TTE has
  weaker intermolecular forces

http//www.msnucleus.org/membership/html/k-6/wc/wa
ter/1/images/penny.jpg
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How is surface tension affected by soap?

• Breaks the surface tension!

http//www.chemistryland.com/CHM107/Water/SoapDisr
uptsWater.jpg
http//www.chemistry.nus.edu.sg/2500/micelle.jpg
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Capillary Rise
glass
gravity
H2O Hg

• Water rises up the capillary tube because there
  are unbalanced forces between the water and glass
  and the water and gravity

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Which is larger? Adhesion or Cohesion?
Adhesion attraction between H2O (Hg)
glass Cohesion attraction of H2O (Hg) molec. to
each other
Adhesion gt Cohesion
Cohesion gt Adhesion
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Do other liquids exhibit capillary rise?

• As long as they are attracted to glass and have
  enough cohesion

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IM forces and interactions between liquids and
surfaces
Cohesion gt Adhesion Liquid Beads on Surface
Cohesion lt Adhesion Liquid Wets the Surface
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Evaporation

• Diagram the distribution of kinetic energy at a
  temperature

25oC
75oC
5oC
particles
low KE
ave KE
high KE
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Which molecules will evaporate?
This lowers the total kinetic energy
(temperature) of the entire system
Only high energy molecules can vaporize
particles
low KE
ave KE
high KE
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Boiling
Pvap Patm
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Boiling
Pvap
Pvap Patm
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Boiling

• Boiling occurs when
• Vapor Pressure Barometric Pressure
• When Vapor Pressure 760 mmHg, Boiling Point
  100oC

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Evaporation Questions

• Why do we sweat?
• breaking waters bonds has a cooling effect
• high energy molecules are lost

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2. Why does water stay cool in clay containers?

• Since clay is porous, high energy molecules
  escape leaving lower temperature water

When the water added to the sand evaporates in
the Pot-in-Pot Cooler, it pulls heat from the
smaller pot, keeping vegetables cool.
Refrigeration for the other 90
http//www.juneauempire.com/images/050406/13484_50
0.jpg
http//www.npr.org/templates/story/story.php?story
Id11032381scemaf
http//www.juneauempire.com/images/050406/13484_50
0.jpg
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2. Why does water stay cool in clay containers?

• Since clay is porous, high energy molecules
  escape leaving lower temperature water

Refrigeration for the other 90
http//www.npr.org/templates/story/story.php?story
Id11032381scemaf
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3. Why can liquid water change to vapor at room
temperature?

• High energy molecules escape
• Evaporation occurs at all temperatures

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• 4. Define vapor pressure
• Pressure of a vapor above its liquid at
  equilibrium
• Pressure of molecules in their bubbles
• Can solids have a vapor pressure?
• Yes! Solid? Gas
• Ex ice, dry ice, plastics

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• 5. What is the difference between evaporation and
  boiling?
• Evaporation occurs at any temperature high
  energy molecules escape
• Boiling occurs when atmospheric pressure vapor
  pressure

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Volatile Substances

• Easily evaporate
• Weak attractive forces
• Low boiling point
• High vapor pressure

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Non-volatile substances

• Do not easily evaporate
• Strong attractive forces
• High boiling point
• Low vapor pressure

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Chemical Equilibrium

• A B C D

Forward Reaction
Reverse Reaction
Rate of forward reaction Rate of reverse
reaction
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Dynamic Equilibrium

• Acetone (l) Acetone (g)

Reaction looks like it has stopped, but is
dynamic at the molecular level
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What conditions are necessary for equilibrium?

• Closed System
• Rate of fwd rxn rate of rev rxn
• Constant temp, pressure, color
• Both reactants and products are present (but not
  necessarily in equal amounts)

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Henri Louis Le Chatlier(1850-1936)

• Inventor of acetylene torch
• Professor of Industrial Chemistry and Metallurgy
• Instrumental in the development of cement and
  Plaster of Paris

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LeChatliers Principle

• When a stress is applied
• to a system at equilibrium,
• the system will respond
• to partially undo the stress

Add Something, Remove Something, Change
Temperature, Change Pressure,
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produced
used
produced
used

• H2O (l) energy H2O (g)

Use Heat
Evaporation
Produce Heat
Condensation
Evaporation
Increase Pr.
Condensation
Decrease Pr.
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Lab Practice Problem

• NaCl Na Cl-

a) Which direction would the reaction shift if
MgCl2 (Mg2 and Cl-) were added to the system
above? Explain.
b) What would happen to the amount of NaCl if Cl-
were removed from the system? Explain.
Cl-
Na
Cl-
Cl-
Cl-
Na
Cl-
Cl-
Cl-
Na
Na
Cl-
Na
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
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Predicting adjustments (can skip this one)
produced

• Haber process
• N2 3 H2 2 NH3 energy

used
produced
used
Use energy
Produce NH3
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produced
used
produced
used

• 2 H 2 CrO42- Cr2O72- H2O

ORANGE
Use H
(Add H)
(Use H)
Produce H
YELLOW
X CrO4-2 O Cr2O7-2
Add H
x x xx x x x x xox xx x x x xx
xxxx o x x x x x x x x
ox o oo o o o ooo o o o x o oo oo oo o
ooo oo
H
H
H
Add OH-
Na
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2 H 2 CrO42- Cr2O72- H2O
Add H
X CrO4-2 O Cr2O7-2
x x xx x x x x xox xx x x x xx
xxxx o x x x x x x x x
ox o oo o o o ooo o o o x o oo oo oo o
ooo oo
Add OH-
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produced
used
produced
used

• 2 NO2 N2O4 energy

DARKER
Use Heat
LIGHTER
Produce Heat
LIGHTER
Decrease Pr.
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How Do Pressure Cookers Work?
Pressure cookers increase the pressure above the
water so that water boils at a ________
temperature and cooks food ________
HIGHER
http//www.goalfinder.com/images/SPGPRO2/pressur-d
esign-of-pressure-cooker.jpg
QUICKER
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Phase Changes
105
DKE
DPE
100
DKE
Temperature (oC)
DPE
0
DKE
Where is there a DKE?
Where is there a DPE?
- 5
Time
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Terms

• Melting Point
• Temp when substances changes from l ? s
• Boiling point
• Temp when substance changes from l ? g
• DKE
• where there is a change in temperature
• DPE
• where theres a phase change
• (constant temp)

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Calculations

• Calculate the amount of heat needed to raise the
  temperature of 100 ml of water from 15oC to 65oC.

DQ mcDT
DQ (100g)(1 cal/goC)(50oC)
DQ 5000 cal
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• 3. Calculate the amount of heat needed to melt
  100 g of ice.
• REMEMBER Heat of Fusion 80 cal/g

80 cal
x cal
x 8000 cal

1 g
100 g
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• 2. Calculate the amount of heat needed to boil
  100 ml of water.
• HEAT OF VAPORIZATION 540 cal/g

540 cal
x cal
x 54,000 cal

1 g
100 g