van der waals forces johannes diderik van der waals
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Van der Waals Forces
Johannes Diderik van der Waals
Polarity
Separation of charge
An asymmetrical difference in electronegativity along a bond or in a molecule
Circle the polar molecules. Label + and -
O
HH
N
HH
H
CCO O
Al
Cl
Cl
Cl
Cl
Cl
Cl
Cl
S
HH
C. __________ molecules are symmetrical
D. What is the bond angle in H2O? _______
E. The motion of particles in these phases:Solid Liquid Gas
Non-Polar104.5o
http://itl.chem.ufl.edu/2045_s00/lectures/FG11_001.GIF
Van der Waals Forces
Small, weak interactions between molecules
Van der Waals Forces
Intermolecular: between molecules (not a bond)
Intramolecular: bonds within molecules (stronger)
What is being attracted?
+ attracted to -
electrostatic attraction
e- s of one atom to another atom’s nucleus
e-
e-+ +
Evidence of VDW Forces?Non-polar molecules can form gases,
liquids and solids.
Ex: CO2
CO
O
CO O CO
O
CO O
CO
O
CO O CO
O
CO OCO
O
CO O CO
O
CO O
3 Types of Van der Waals Forces
1) dipole-dipole
2) dipole-induced dipole
3) dispersion
Dipole-Dipole
Two polar molecules align so that + and - are matched (electrostatic attraction)
Ex: ethane (C2H6) vs. fluromethane (CH3F)
Fluoromethane (CH3F) – boiling point = 194.7 K
H H
H C F H C F
H H
polar or non-polar?
- -
Ethane (C2H6) – boiling point = 184.5 K
H H H H
H C C H H C C H
H H H H
polar or non-polar?
Dipole-Dipole
NOT Dipole-Dipole
Try This:
Draw two KBr molecules and draw their dipole-dipole interactions with a dashed line.
BrBrK K
What does to “induce” mean?
– To cause or bring about
Ex:
Induced vomiting
Induced labor
Induced coma
Dipole-Induced DipoleA dipole can induce (cause)
a temporary dipole to form in a non-polar molecule
The molecules then line up to match + and - charges
Example
H Cl+ Are-e-
e-e-
e-
e-
e-e- e-
e-
e- e-
e-
e-e-
e-e-
e-
A DIPOLE (it’s polar)
non-polarINDUCEDDIPOLE
Dipole – Induced Dipole (weak and short-lived)
Draw CO2 (aq)What does (aq) mean?
dissolved in WATER
So…draw CO2 (g) in H2O (l)
CO O CO OO
HH
Where is CO2 (aq) seen?
Carbonated water
CO2 is not very soluble…
1 CO2 in 1000 H2O molecules
http://www.packaging-technology.com/contractor_images/venus/4_rinser.jpg
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
Dispersion Forces
Cl-Cle-e-
e-
e-
e-
e-
e- e-e-e-
e-e-
e-
e-e-
e-e-
e-
non-polarINDUCEDDIPOLE
TEMPORARY DIPOLEnon-polar
Cl-Cle-e- e-
e-
e-
e-e- e-e-
e-
e-e-e-e-
e-
e- e-
Dispersion (weakest and very short-lived)
Tokay Gecko:Dispersion
Forces!
Review
Dipole – Dipole
between two polar molecules
Dipole – Induced Dipole
b/w a polar & a non-polar molecule
Dispersion
between two non-polar molecules
Hydrogen BondingSTRONGEST Intermolecular Force!!A special type of dipole-dipole attractionBonds form due to the polarity of water
Draw 3 H2O molecules in your notes
Ice Liquid
Hydrogen Bonding con’t
Hydrogen bonds keep water in the liquid phase over a wider range of temperatures than is found for any other molecule of its size
Hydrogen bonds account for the high boiling point of water
Expansion of Ice
Ice expands when water freezes compared to most substances that contract when freezing
Ice bomb video
Denisty vs Temperature of H2O4 oC—max density of water – liquid!
SolidIce
Liquidwater
Hexagonal Ice
http://www.gala-instrumente.de/images/44%20hexagonal%20ice.jpg
http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/imgche/waterhex.gif
Halos, Sundogs, & Pillars are
caused by hexagonal ice crystals
http://www.lummox.net/celestial/pics/ak1999-sundog.jpg
http://images.usatoday.com/tech/_photos/2006/09/12/cloud.jpg
Ponds Freezing
Solid water (ice) has a lower density than liquid water
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
Surface Tension
Enhancement of the intermolecular attractive forces at the surface
Evidence
Lab:
Dixie cup
Penny
Capillary tube
needle
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
Surface tension is a result of cohesive intermolecular forces
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/water/1/images/penny.jpg
How is surface tension affected by soap?
Breaks the surface tension!
http://www.chemistryland.com/CHM107/Water/SoapDisruptsWater.jpg http://www.chemistry.nus.edu.sg/2500/micelle.jpg
Capillary Rise
Water rises up the capillary tube because there are unbalanced forces between the water and glass and the water and gravity
H2O Hggravity
glass
Which is larger? Adhesion or Cohesion?
Adhesion: attraction between H2O (Hg) & glassCohesion: attraction of H2O (Hg) molec. to each other
Adhesion > Cohesion Cohesion > Adhesion
Do other liquids exhibit capillary rise?
As long as they are attracted to glass and have enough cohesion
Cohesion > AdhesionLiquid “Beads”
on Surface
Cohesion < AdhesionLiquid “Wets”
the Surface
IM forces and interactions between liquids and surfaces
EvaporationDiagram the distribution of kinetic energy
at a temperature
# p
arti
cles
low KE ave KE high KE
25oC 75oC5oC
Which molecules will evaporate?
Only high energy molecules can vaporize
# p
arti
cles
low KE ave KE high KE
This lowers the total kinetic energy (temperature) of the entire system
Boiling
t = 1 min
P atm
Pvap
P atm
t = 5 minBOILING!
Pvap
t = 0 min
P atm
Pvap
Pvap = Patm
Boiling
Boiling occurs when
Vapor Pressure = Barometric Pressure
When Vapor Pressure = 760 mmHg, Boiling Point = 100oC
Evaporation Questions1. Why do we sweat?
breaking water’s bonds has a cooling effect
high energy molecules are lost
2. Why does water stay cool in clay containers?
Since clay is porous, high energy molecules escape leaving lower temperature water
http://www.juneauempire.com/images/050406/13484_500.jpg
When the water added to the sand evaporates in the Pot-in-Pot Cooler, it pulls heat from the smaller pot, keeping vegetables cool.
http://www.npr.org/templates/story/story.php?storyId=11032381&sc=emaf
Refrigeration for the other 90%
3. Why can liquid water change to vapor at room temperature?
High energy molecules escape
Evaporation occurs at all temperatures
# p
arti
cles
low KE ave KE high KE
4. Define vapor pressure
Force of particles leaving a liquidPressure of molecules in their bubbles
Can solids have a vapor pressure?Yes! Solid Gas Ex: ice, dry ice, plastics
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
Volatile Substances
Easily evaporate
Weak attractive forces
Low boiling point
High vapor pressure
Non-volatile substances
Do not easily evaporate
Strong attractive forces
High boiling point
Low vapor pressure
Equilibrium
A + B C + D
Forward Reaction
Reverse Reaction
Rate of forward reaction = Rate of reverse reaction
Dynamic Equilibrium
Acetone (l) Acetone (g)
Reaction looks like it has stopped, but is dynamic at the molecular level
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)
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
LeChatlier’s 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,…
Predicting adjustments
Haber process
N2 + 3 H2 2 NH3 + energy
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
prod
uced usedprod
uced
used
(Use H+)
2 H+ + 2 CrO42- Cr2O7
2- + H2O Add HCl System wants? Shift? Color?
Add NaOH System wants? Shift? Color?
Use H+
Produce H+
ORANGE
YELLOW
Na+H+
H+H+
prod
uced
used
used
prod
uced
(Add H+)
2 H+ + 2 CrO42- Cr2O7
2- + H2O
x x xx x x x x xox xx x x x xx xxxx
o x x x xx x x x
ox o oo o o o ooo o
o o x o oo oo oo o ooo
oo
X = CrO4-2
O = Cr2O7-2
Add H+
Add OH-
2 NO2 N2O4 + energy
Add Heat System wants?
Shift? Color?
Remove Heat
System wants?
Shift? Color?
Increase Pressure
System wants?
Shift? Color?
Use Heat
Produce Heat
Decrease Pr.
DARKER
LIGHTER
LIGHTER
producedused
produce
d
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
Produce Heat
Increase Pr.
Evaporation
Condensation
Condensation
prod
uced
usedprod
uced
used
Decrease Pr.
Evaporation
How Do Pressure Cookers Work?
http://www.goalfinder.com/images/SPGPRO2/pressur-design-of-pressure-cooker.jpg
Pressure cookers increase the pressure above the water so that water boils at a ________ temperature and cooks food ________
HIGHER
QUICKER
Lab Practice Problem
NaCl Na+ + Cl-
Cl-
Na+Na+
Na+
Cl-Cl-
NaClNaClNaCl
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.
Na+Cl-Cl-
Cl-Cl-
NaClNaClNaCl
Na+
Cl-
Phase ChangesT
empe
ratu
re (
o C)
- 5
0
100
105
KE
PE
KE
PE
KE
Where is there a KE?
Where is there a PE?
Time
TermsMelting Point
Temp when substances changes from l s
Boiling pointTemp when substance changes from l g
KE—where there is a change in temperature
PE—where there’s a phase change
(constant temp)
Calculations
1. Calculate the amount of heat needed to raise the temperature of 100 ml of water from 15oC to 65oC.
Q = mcT
Q = (100g)(1 cal/goC)(50oC)
Q = 5000 cal
3. Calculate the amount of heat needed to melt 100 g of ice.
REMEMBER: Heat of Fusion = 80 cal/g
80 cal
1 g=
x cal
100 gx = 8000 cal
2. Calculate the amount of heat needed to boil 100 ml of water.
HEAT OF VAPORIZATION = 540 cal/g
540 cal
1 g=
x cal
100 gx = 54,000 cal