1. Lecture 69 Intermolecular Forces Ozgur Unal
NIS – CHEMISTRY
Lecture 69
Intermolecular Forces
Ozgur Unal

2. What does the word internet mean?
inter- vs intra-
What does the
word internet mean?
How about intranet?

3. Intramolecular Forces
Remember the forces between atoms in a compound.
We have ionic bonds, covalent bonds and metallic bonds
Check out Table 12.2!!
These forces are intramolecular forces because they are within a compound.

4. Intermolecular Forces
Remember from previous chapters that there are also attractive forces between molecules  intermolecular forces
There are three types of intermolecular forces:
Dispersion forces
Dipole-dipole forces
Hydrogen bond
All of these intermolecular forces are weaker then the intramolecular forces.

5. Dispersion Forces Remember nonpolar molecules..
Example: O2, CH4 etc.... Can you give one more example?
Consider a nonpolar molecule, such as O2 gas at 300 K.
It can be made to change into liquid or solid  attraction between O2 molecules
The force of attraction between O2 molecules is called dispersion force.
Dispersion force (London force) is due to the temporary shifts in the density of electron clouds.
Dispersion forces increase as the size of molecule increases.

6. Dipole-dipole Forces Remember polar molecules??
Polar molecules contain permanent dipoles.
The attraction between oppositely charged regions of polar molecules are called dipole-dipole forces.

7. Hydrogen Bonds
A hydrogen bond is a special type of dipole-dipole force.
A hydrogen bond occurs between molecules containing H to F, O or N.
Example: H2O, or NH3
Hydrogen bonds dominate
dispersion and dipole-dipole
forces.

8. Follow Up Questions
Why F2 and Cl2 are gases, while Br2 is a liquid and I2 is a solid at room temperature?
Why is water liquid at room temperature, while molecules similar in molar masses are gases?  Check out Table 12.3!!
What kinds of interparticle forces must be overcome to
Melt ice
Boil water
Melt NaCl
Sublime I2