Intermolecular Forces

1
Intermolecular Forces
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Intra- vs. Inter-

• Intra- inward
• Ex. Intradermal, Intravenous
• Inter- between or among
• Ex. Interstate, International
• Intramolecular forces act within a molecule.
• Ionic, covalent, and metallic bonding
• Intermolecular forces act between molecules.
• London dispersion, dipole-dipole interactions,
  ion-dipole interactions, and Hydrogen bonding

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Intramolecular vs. Intermolecular Forces -
Similarities

• Attractive forces
• Force due to electron sharing (charge)
• Affect spatial arrangements of atoms and
  molecules, respectively

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Intramolecular vs. Intermolecular Forces -
Differences
Intramolecular Forces Intermolecular Forces
Strong Weak
Act within molecules Act between molecules
Persist for life of molecule More brief in life of molecule
Not strongly effected by physical changes Strongly effected by physical changes
Stabilize individual molecules Responsible for bulk properties of matter
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Lava Lamp Experiment

1. Each pair of students should write their names on
   the top of 1 sheet of paper.
2. One student should obtain a 250 mL beaker and
   fill halfway with water.
3. A second student should add about 1 cm of oil to
   top (between ¼ and ½ inch or about the width of
   pinky finger).
4. On your piece of paper, draw and describe the
   beaker and label it as Drawing 1.
5. When Drawing 1 is complete, raise your hand for
   addition of 1 drop of food coloring.
6. Draw the beaker and its contents immediately
   after the addition of food coloring. Label this
   Drawing 2.
7. Wait 1-2 minutes and draw the beaker again. This
   is Drawing 3.
8. When Drawings 2 and 3 are complete, raise your
   hand for addition of sugar.
9. Draw and describe what happens right after the
   sugar is added as Drawing 4.
10. Draw and describe what happens about one minute
    after the sugar is added as Drawing 5.

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Lava Lamp Experiment

1. What materials will mix together?
2. What happened to food coloring in oil? How would
   you describe the color, shape, size, and movement
   of the food coloring?
3. What happened to food coloring in water? How
   would you describe the color, shape, size, and
   movement of the food coloring?
4. What do you think the phrase like dissolves
   like means?
5. How would you apply like dissolves like to the
   materials used in the beaker?

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Intermolecular Forces

• London dispersion forces
• Dipole-dipole forces
• Hydrogen bonding
• Ion-dipole forces

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Name of force Rank of strength Ion involved? Polar or nonpolar molecules? Is H involved? Example




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Why dont oil and water mix?
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London dispersion forces

• Weakest intermolecular force
• Only attractive force between non-polar
  molecules
• Created from temporary fluctuations in electron
  density around atoms
• The larger the molecule, the greater the
  dispersion force.

https//www.chem.unsw.edu.au/coursenotes/CHEM1/non
unipass/hainesIMF/dispersion.html
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London dispersion forces
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Why dont oil and water mix?
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Lava Lamp Experiment

• Which materials exhibit London dispersion
  forces?
• What observations can be explained by London
  dispersion forces?

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Name of force Rank of strength Ion involved? Polar or nonpolar molecules? Is H involved? Example
London dispersion forces 4 No Both (strongest for nonpolar) No Oil and water



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Dipole

• In polar molecules electrons are not equally
  shared between atoms.
• In areas of the electron cloud where electrons
  are more likely to be found, a dipole is
  formed. This end of the molecular has a partial
  negative charge. The opposing side of the
  molecule will have a partial positive charge.
• These molecules are polar.

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Dipole
Example HCl
Example H2O
d -
O
d -
d -
H
H
d
d
d
Images modified from http//employees.csbsju.edu/
hjakubowski/classes/ch123/ch123ch2mcmfay5th.htm
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Dipole-Dipole Forces

• Attractive force between neutral, polar
  molecules (molecules that possess a dipole).
• The larger the dipole, the greater the force.
• Animation

Image modified from http//www.chem.ufl.edu/itl/
2041_u01/lectures/lec_g.html
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Lava Lamp Experiment

• What materials exhibit dipole-dipole
  interactions?
• What observations can be explained by
  dipole-dipole interactions?

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Name of force Rank of strength Ion involved? Polar or nonpolar molecules? Is H involved? Example
London dispersion forces 4 No Both (strongest for nonpolar) No Oil and water
Dipole-dipole forces 3 No Polar No Alcohol in water


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Why does salt dissolve?
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Ion-Dipole Forces

• Interaction between charged molecule (ion) and
  polar molecule (dipole).
• Strength depends on charge and size of ion and
  magnitude and size of dipole
• Cations interact more strongly with dipoles than
  anions.

Image from http//www.chem.purdue.edu/gchelp/liqu
ids/iondip.html
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Why does salt dissolve?
NaCl in Water
How would heating affect solubility? Stirring?
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Lava Lamp Experiment

• Were there any ion-dipole forces in the lava
  lamp experiment?
• Can any observations be explained by ion-dipole
  forces?

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Name of force Rank of strength Ion involved? Polar or nonpolar molecules? Is H involved? Example
London dispersion forces 4 No Both (strongest for nonpolar) No Oil and water
Dipole-dipole forces 3 No Polar No Alcohol in water
Ion-dipole forces 1 Yes Polar No Salt in water

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Hydrogen Bonding

• Permanent dipole-dipole interaction
• Only occurs in molecules containing H-F, H-N, or
  H-O bonds

A H B
Where A and B are F, N, or O
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Hydrogen Bonding of Water
Hydrogen Bonding of Water
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Snowflakes
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Hydrogen Bonding Boiling Point
Image from http//faculty.ycp.edu/peterman/chm13
6/chm136s07ex1a.htm
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Lava Lamp Experiment

• Were there any hydrogen bonds in the lava lamp
  experiment?
• Can any observations be explained by hydrogen
  bonding?

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Name of force Rank of strength Ion involved? Polar or nonpolar molecules? Is H involved? Example
London dispersion forces 4 No Both (strongest for nonpolar) No Oil and water
Dipole-dipole forces 3 No Polar No Alcohol in water
Ion-dipole forces 1 Yes Polar No Salt in water
Hydrogen bonding 2 No Polar Yes Water
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Like dissolves like

• To be soluble a compound must interact with the
  solute by
• Dipole-dipole forces
• Ion-dipole forces
• London dispersion forces
• Polar solutes dissolve in polar solutions
• Non-polar solutes dissolve in non-polar solutions

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Summary
Image from http//www.chem.ufl.edu/itl/2041_u01/
lectures/lec_g.html