Van der Waals Forces

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Van der Waals Forces

• Johannes Diderik van der Waals

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Polarity

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

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Circle the polar molecules. Label d and d-
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)

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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!
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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 3 H2O molecules in your notes

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

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Denisty vs Temperature of H2O
4 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

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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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Surface tension is a result of cohesive
intermolecular forces
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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

• 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
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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

particles
low KE
ave KE
high KE
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• 4. Define vapor pressure
• Force of particles leaving a liquid
• 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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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?

1. Closed System
2. Rate of fwd rxn rate of rev rxn
3. Constant temp, pressure, color
4. Both reactants and products are present (but not
   necessarily equal)

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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 Reactant, Add Product, Remove Reactant,
Remove Product, Add Heat, Increase Pressure,
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Predicting adjustments
produced

• Haber process
• N2 3 H2 2 NH3 energy

used
produced
used
Add energy System wants? Shift? Amount of N2 and H2? Amount of NH3?
Remove NH3 System wants? Shift? Amount of N2 and H2? Amount of NH3?
Use energy
Produce NH3
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produced
used
produced
used

• 2 H 2 CrO42- Cr2O72- H2O

Add HCl System wants? Shift? Color?
Add NaOH System wants? Shift? Color?
ORANGE
Use H
(Add H)
(Use H)
Produce H
YELLOW
H
H
H
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

Add Heat System wants? Shift? Color?
Remove Heat System wants? Shift? Color?
Increase Pressure System wants? Shift? Color?
DARKER
Use Heat
LIGHTER
Produce Heat
LIGHTER
Decrease Pr.
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produced
used
produced
used

• H2O (l) energy H2O (g)

Add Heat System wants? Shift? Observation?
Remove Heat System wants? Shift? Observation?
Decrease Pressure System wants? Shift? Observation?
Increase Pressure System wants? Shift? Observation?
Use Heat
Evaporation
Produce Heat
Condensation
Evaporation
Increase Pr.
Condensation
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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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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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

1. 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