New Van Der Waals Forces

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

HHN

HH

H

CCO O

AlCl

Cl

Cl

Cl

Cl

Cl

ClS

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-dipole2)    dipole-induced dipole3) dispersion

Dipole-DipoleTwo 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 vomitingInduced laborInduced 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 WATERSo…draw CO2 (g) in H2O (l)

CO O CO OOH

H

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 – Dipolebetween two polar molecules

Dipole – Induced Dipoleb/w a polar & a non-polar

moleculeDispersion

between two non-polar molecules

Hydrogen BondingSTRONGEST Intermolecular Force!!A special type of dipole-dipole attractionBonds form due to the polarity of waterDraw 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 cupPennyCapillary tubeneedle

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

# pa

rtic

les

low KE ave KE high KE

25oC 75oC5oC

Which molecules will evaporate?

Only high energy molecules can vaporize

# pa

rtic

les

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 effecthigh 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 escapeEvaporation occurs at all temperatures

# pa

rtic

les

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 evaporateWeak attractive forcesLow boiling pointHigh vapor pressure

Non-volatile substances

Do not easily evaporateStrong attractive forcesHigh boiling pointLow 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 torchProfessor of Industrial

Chemistry and MetallurgyInstrumental 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 + energyAdd 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

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

Produce Heat

Increase Pr.

Evaporation

Condensation

Condensation

produ

ced

usedprodu

ced

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 ChangesTe

mpe

rat u

r e (o C

)

- 5

0

100

105

KEPE

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