1. Section 5.5—Intermolecular Forces
Objective:
Distinguish between intermolecular and intramolecular forces
Distinguish between dipole-dipole forces and hydrogen bonding
Identify forces present between molecules based on Lewis dot structures and polarity of bonds

2. Intra- versus Inter-molecular Forces
So far this chapter has been discussing intramolecular forces
Intramolecular forces = forces within the molecule (example: chemical bonds)
Now let’s talk about intermolecular forces
Intermolecular forces = forces between separate molecules of the same substance

3. Breaking Intramolecular forces
Breaking of intramolecular forces (bonds within the molecule) is a chemical change
2 H2 + O2  2 H2O
Bonds are broken within the hydrogen and oxygen molecules and new bonds are formed to form the water molecules

4. Breaking Intermolecular forces
Breaking of intermolecular forces (forces of attraction between separate molecules) is a physical change
Boiling water is breaking the intermolecular forces between liquid water molecules to allow the molecules to move away from each other and become individual gas molecules.
Dissolving salt means the forces between salt particles need to be broken as well as the forces between water molecules; only after this occurs can the 2 mix and “dissolve”.

5. Special Intermolecular Forces
There are 2 kinds of intramolecular forces that exist between molecules of the same substance that are important for us to discuss.
Dipole-dipole forces occur between molecules of a polar covalent substance
Hydrogen bonding occurs between molecules that contain a hydrogen atom and either a nitrogen, oxygen, or fluorine atom.

6. Dipole-dipole Forces
Polar molecules are molecules that have positive and negative charges on their surface.
One “end” of such a molecule will develop a slight negative charge and the other “end” will develop a slight positive charge
This molecule is because it has
called a dipole ends.
The formation of a dipole occurs due to the presence of polar bonds in the molecule and molecular shapes that are not symmetrical. + -

7. Dipole-dipole Forces These are then called dipole-dipole forces.
When polar molecules are near each other, like they would be in any sample of a polar substance, the positive area of one polar molecule (dipole) can be attracted to the negative area of another molecule (dipole).
These are then called
dipole-dipole forces.

8. Dipole-dipole Forces
Polar molecules always have the ability to form attractions with opposite charges
Forces between the molecules (dipole-dipole forces) are relatively strong
Polar molecules always have charges on their surface

9. Hydrogen Bonding
Hydrogen bonding is an extreme case of dipole-dipole forces.
For molecules that contain a hydrogen atom and either a nitrogen, oxygen, or fluorine atom, the hydrogen end of the molecule develops a “more than slight” positive charge and the nitrogen, oxygen, or fluorine end develops a “more than
slight” negative charge.
The H end of one
molecule will then be
very attracted to the
N, O, or F end of
another molecule.

10. Hydrogen Bonding
Hydrogen bonds are not true bonds – they are very strong attractions between molecules.
Hydrogen bonds give
water many of its
very unique properties.

11. Hydrogen Bonds
It’s an extreme example of dipole-dipole forces: H of 1 molecule attracts a N, O, or F of another
This is the strongest of all the intermolecular forces.
Within a molecule, hydrogen develops a positive charge; N, O, or F develops a negative charge

12. Practice Problems
Determine if dipole-dipole forces or hydrogen bonding occurs in any of the following compounds. H2
H2S
HCl HF