Section 5.4

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Section 5.4Polarity of Molecules
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Two atoms sharing equally Draw N2
Each nitrogen atom has an electronegativity of
3.0 They pull evenly on the shared electrons The
electrons are not closer to one or the other of
the atoms This is a non-polar covalent bond.
All compounds that contain Non-polar bonds are
NON-POLAR molecules.
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Atoms sharing unequally Draw H2S
Electronegativities H 2.1 sulfur
2.5 The sulfur pulls on the electrons slightly
more, pulling them slightly towards the
sulfur. This is a polar covalent bond
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Sharing unevenly Draw CH2O
Electronegativities H 2.1 C 2.5
O 3.5 The carbon-hydrogen difference isnt
great enough to create partial charges Its
actually a NON POLAR bond Exception to the
rule But the oxygen atoms pulls significantly
harder on the electrons than the carbon does.
This does create a polar covalent bond This is a
polar covalent bond
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Showing Partial Charges

• There are two ways to show the partial separation
  of charges
• Use of ? for partial
• Use of an arrow pointing towards the partial
  negative atom with a plus tail at the partial
  positive atom. These are referred to as DIPOLES
  which are separation of opposite charge!

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Lets Practice
C H OCl FF CCl
Example If the bond is polar, draw the polarity
arrow
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Lets Practice
2.5 2.1 0.4 non-polarexception 3.5 3.0
0.5 polar 4.0 4.0 0.0 non-polar 2.5
3.0 - 0.5 polar
C H OCl FF CCl
Example If the bond is polar, draw the polarity
arrow
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How do Dipoles Cancel?

• Dipoles must move in equal but opposite
  directions in order for the forces to cancel The
  molecule is classified as NONPOLAR.

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Polar Bonds versus Polar Molecules

• Not every molecule with a polar bond is polar
  itself
• If the polar bonds form Dipoles that cancel out
  then the molecule is overall non-polar.

The dipoles cancel out. No net dipole
The dipoles do not cancel out. Net
dipole
This one is hard to tell!
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The Importance of VSEPR in Predicting Polarity.

• Shape is important. All molecules must be drawn
  in the correct shape to see the proper canceling
  of dipoles to determine if its polar or nonpolar.

Water drawn this way shows all the dipoles
canceling out.
But water drawn in the correct VSEPR structure,
bent, shows the dipoles dont cancel out! Net
dipole
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Draw the molecule NH3
Example Is NH3 a polar molecule?
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Example Is NH3 a polar molecule?
Electronegativities N 3.0 H 2.1 Difference
0.9 Polar bonds VSEPR shape Trigonal
pyramidal
Yes, NH3 is polar
Net dipole
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Draw the molecule for dihydrogen monosulfide, H2S.

• Is it polar or non-polar? What shape?

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• Net dipole
• Yes, H2S is polar
• Is water polar or non-polar? What shape?

Electronegativities S 2.5 H 2.1 Difference
.4 Polar bonds VSEPR shape bent
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Draw the molecule CO2
Example Is CO2 a polar molecule?
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Draw the molecule of carbon dioxide, CO2
Example Is CO2 a polar molecule?
Electronegativities C 2.5 O 3.5 Difference
1.0 Polar bonds VSEPR shape linear
No CO2 is nonpolar
Dipole cancels
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Draw the molecule for carbon tetrachloride, CCl4.
Electronegativities C 2.5 H 2.1 Difference
.4 NonPolar bonds VSEPR shape tetrahedral
H
C
H
H
H
No Net dipole Yes, CH4 is nonpolar
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Lets make this Simple

• Nonpolar bonds Nonpolar molecule.
• Polar bonds with a lone pair on the central atom
  most likely a polar molecule
• Polar bonds no lone pair on central atom all
  terminal atoms are the same nonpolar molecule.
• If terminal atoms are different, its polar.

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Section 5.5Intermolecular Forces
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Intramolecular Forces- versus Inter-molecular
Forces

• So far this chapter has been discussing
  Intramolecular Forces
• Intramolecular forces forces within the
  molecule
• AKAchemical bonds

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Breaking Intramolecular forces

• Breaking of intramolecular forces (within the
  molecule) is a chemical change
• Example 2 H2 O2 ? 2 H2O
• Bonds are broken within the molecules and new
  bonds are formed to form new molecules
• Requires a larger amount of energy to break than
  an intermolecular force

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Inter-molecular Forces

• Intermolecular forces forces between separate
  molecules

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Breaking Intermolecular forces

• Breaking of intermolecular forces (between
  separate molecules) is a physical change
• Breaking glass Boiling water are examples
• Example H2O(l) ? 2 H2O(g)
• Does not require as much energy to break compared
  to an intramolecular force

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London Dispersion Forces (LDF) are the primary
force between nonpolar molecules but are found in
all molecules!
Electrons move around the nuclei. They could
momentarily all gang up on one side
All molecules have electrons.
This lop-sidedness of electrons creates a partial
negative charge in one area and a partial
positive charge in another.
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London Dispersion Forces (LDF)

• Once the electrons have ganged up and created a
  temporary dipole, the molecule is now temporarily
  polar.
• The positive area of one temporarily polar
  molecule can be attracted to the negative area of
  another molecule.

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London Dispersion Forces (LDF)
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Strength of London Dispersion Forces (LDF)
Electrons can gang-up and cause a non-polar
molecule to be temporarily polar
The electrons will move again, returning the
molecule back to non-polar
The polarity was temporary, therefore the
molecule cannot always form LDF.
London Dispersion Forcesthe weakest of the
intermolecular forces because molecules cant
form it all the time, only temporarily
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Strength of London Dispersion Forces (LDF)
All molecules have electronsall molecules can
have London Dispersion Forces
The more electrons that gang-up, the larger the
partial negative charge.
The larger the molecule, the stronger the London
Dispersion Forces
Larger molecules have more electrons
Larger molecules have stronger London Dispersion
Forces than smaller molecules.
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London Forces explain why Chlorine is a gas,
Bromine is a liquid and Iodine is a solid!

• Chlorine Gas 34 e-

• Bromine Liquid 70 e-

GREATER ELECTRONS, STRONGER FORCES
Iodine Solid 106 e-
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Dipole- Dipole Forces primary force between
polar molecules

• Polar molecules have permanent permanent dipoles.
• The positive area of one polar molecule can be
  attracted to the negative area of another
  molecule.
• The partial positive negative poles are shown
  as ? and ?-

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Strength of Dipole Forces
In general, Dipole forces are stronger than
London Dispersion Forces
Polar molecules always have a partial separation
of charge.
Polar molecules always have the ability to form
attractions with opposite charges
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Dipole-Dipole Forces
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Hydrogen Bonding

• A special dipole force between a hydrogen atom of
  1 molecule and a F, O, or N of another molecule.
  
• (ET fon home)
• A very strong dipole forms since F, O, and N are
  all very small, highly electronegative atoms.

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Hydrogen Bond
Hydrogen bond
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Strength of Hydrogen Bond
Its an extreme example of polar bonding with the
hydrogen having a large positive charge.
Hydrogen has no inner electrons to counter-act
the protons charge
This very positively-charged hydrogen is highly
attracted to a lone pair of electrons on another
atom.
This is the strongest of all the intermolecular
forces.
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Hydrogen Bonds

• The ladder rungs in a DNA molecule are hydrogen
  bonds between the base pairs, (AT and GC).

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Rank the forces of attraction in order of weakest
to strongest
Rank the Intramolecular Forces Ionic, Covalent,
and Metallic
Covalentlt Metallic lt Ionic
Rank the Intermolecular Forces Dipole, London
Dispersion, Hydrogen bonding
London Dispersion forceslt Dipole- Dipole forceslt
Hydrogen bonding
Rank ALL the Forces
London Dispersion forceslt Dipole- Dipole forceslt
Hydrogen bonding ltCovalentlt Metalliclt Ionic
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Bond Energy of Bonding Types
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Carbon Allotropes Diamond vs Graphite
Diamond Hard Tetrahedral-Special Network
Covalent Bonds
Graphite soft Strong Sheets of carbon rings but
weak forces holding the sheets together
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NETWORK COVALENT BONDS

• special covalent compounds
• compounds that contain only carbon (diamond,
  graphite) or silicon compounds (silicon
  dioxide- quartz)
• super strong bonding
• super high melting points

• http//www.youtube.com/watch?vfuinLNKkknI

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Tutorial must be in Mozilla

• http//www.wisc-online.com/Objects/ViewObject.aspx
  ?IDGCH6804
• Wisconsin online intermolecular forces

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Section 5.6Intermolecular Forces Properties
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IMFs and Properties

• IMFs are Intermolecular Forces
• London Dispersion Forces
• Dipole interactions
• Hydrogen bonding
• The number and strength of the intermolecular
  forces affect the properties of the substance.
• Energy is needed to break IMFs
• Energy is released when new IMFs are formed

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IMFs and Changes in State
IMFs are broken to go from solid ? liquid.
and from liquid ? gas.
The stronger the IMFs, the more energy is
required to melt, evaporate or boil.
Breaking IMFs requires energy.
The stronger the IMFs are, the higher the
melting and boiling point
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Water

• Water is a very small molecule
• In general small molecules have low melting and
  boiling points
• Based on its size, water should be a gas under
  normal conditions
• However, because water is polar and can form
  dipole interactions and hydrogen bonding, its
  boiling point is much higher
• This is very important because we need liquid
  water to exist!

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Boiling Point of Polar Molecules
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IMFs and Viscosity

• Viscosity is the resistance to flow
• Molasses is much more viscous than
  water
• Larger molecules and molecules with
  high IMFs become inter-twined and stick
  together more
• The more the molecules stick together, the
  higher the viscosity
• An increase in temperature will help break the
  IMFs and make a substance less viscous

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What is More Viscous? Molasses or Water?
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Solubility

• Solute the
• substance that
• is dissolved
• Solvent the
• substance that
• is doing the
• dissolving

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Solubility
Water particles break some intermolecular forces
with other water molecules (to allow them to
spread out) and begin to form new ones with the
sugar molecules.
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Solubility
As new IMFs are formed, the solvent carries
off the solutethis is dissolving
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Solubility

• If the energy needed to break old IMFs is much
  greater than the energy released when the new
  ones are formed, the process wont occur
• An exception to this is if more energy is added
  somehow (such as heating)

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Like Dissolves Like

• Polar solvents dissolve polar solutes
• Nonpolar solvents dissolve nonpolar solutes
• Polar solvents can also dissolve ionic compounds
  because of the charged ends of both

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Oil Water
Water is polar and can hydrogen bond, Oil is
non-polar.
Water has London Dispersion, Dipole forces and
hydrogen bonding. That takes a lot of energy to
break
Water can only form London Dispersion with the
oil. That doesnt release much energy
Much more energy is required to break apart the
water than is released when water and oil combine.
Therefore, oil and water dont mix!
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Surface Tension of Water
metal paper clip on water
water forms beads
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Surface Tension

• Surface tension is the resistance
  of a liquid to spread out.
• This is seen with water on a freshly waxed car
• Due to higher IMFs in the liquid, the more the
  molecules stick together, the less they want to
  spread out.
• The higher the IMFs, the higher the surface
  tension.

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Soap Water

• Soap has a polar head with a non-polar tail
• The polar portion can interact with water (polar)
  and the non-polar portion can interact with the
  dirt and grease (non-polar).

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Soap Water

• The soap surrounds the dirt and the outside of
  the this Micelle can interact with the water.
• The water now doesnt see the non-polar dirt.

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Soap Surface Tension

• The soap disturbs the water molecules ability to
  stick together to form IMFs
• Soap lowers the surface tension of water
• This allows the water to spread over the dirty
  dishes.

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Tutorial must be in Mozilla

• http//www.wisc-online.com/Objects/ViewObject.aspx
  ?IDGCH6804
• Wisconsin online intermolecular forces