1. Intermolecular Forces

2. Intra- vs. Inter- Intra-: inward Inter-: between or among
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

3. Intramolecular vs. Intermolecular Forces - Similarities
Attractive forces
Force due to electron sharing (charge)
Affect spatial arrangements of atoms and molecules, respectively

4. 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

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

6. Lava Lamp Experiment What materials will mix together?
What happened to food coloring in oil? How would you describe the color, shape, size, and movement of the food coloring?
What happened to food coloring in water? How would you describe the color, shape, size, and movement of the food coloring?
What do you think the phrase “like dissolves like” means?
How would you apply “like dissolves like” to the materials used in the beaker?

7. Intermolecular Forces
London dispersion forces
Dipole-dipole forces
Hydrogen bonding
Ion-dipole forces

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

9. Why don’t oil and water mix?

10. 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.

11. London dispersion forces

12. Why don’t oil and water mix?

13. Lava Lamp Experiment Which materials exhibit London dispersion forces?
What observations can be explained by London dispersion forces?

14. Polar or nonpolar molecules?
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)
Oil and water

15. 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”.

16. Dipole Example: HCl Example: H2O H Cl O H H δ - δ - δ - δ+ δ+ δ+
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17. Dipole-Dipole Forces
Attractive force between neutral, polar molecules (molecules that possess a dipole).
The larger the dipole, the greater the force.
Animation
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18. Lava Lamp Experiment
What materials exhibit dipole-dipole interactions?
What observations can be explained by dipole-dipole interactions?

19. Polar or nonpolar molecules?
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)
Oil and water
Dipole-dipole forces 3
Polar
Alcohol in water

20. Why does salt dissolve?

21. 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.
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22. Why does salt dissolve? How would heating affect solubility? Stirring?
NaCl in Water
How would heating affect solubility? Stirring?

23. Lava Lamp Experiment
Were there any ion-dipole forces in the lava lamp experiment?
Can any observations be explained by ion-dipole forces?

24. Polar or nonpolar molecules?
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)
Oil and water
Dipole-dipole forces 3
Polar
Alcohol in water
Ion-dipole forces 1
Yes
Salt in water

25. 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

26. Hydrogen Bonding of Water

27. Snowflakes

28. Hydrogen Bonding & Boiling Point
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29. Lava Lamp Experiment
Were there any hydrogen bonds in the lava lamp experiment?
Can any observations be explained by hydrogen bonding?

30. Polar or nonpolar molecules?
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)
Oil and water
Dipole-dipole forces 3
Polar
Alcohol in water
Ion-dipole forces 1
Yes
Salt in water
Hydrogen bonding 2
Water

31. “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

32. Summary
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