1. Intermolecular Attractions and the Properties of Liquids and Solids
Chapter 12
Intermolecular Attractions and the Properties of Liquids and Solids

2. Gases, Liquids and Solids

3. Intermolecular Attractions
Intermolecular forces depend on distance
Gases have very small attractive forces
Solids/liquids have larger attractive forces since molecules are closer to each other
Intermolecular forces - attractions between two molecules
Intramolecular forces - chemical bonds that hold molecule together
Intermolecular forces weaker than Intramolecular forces

4. van der Waals’ Forces HCl molecules
H and Cl atoms held tightly by covalent bond
Strength of chemical bond keeps molecule intact
Attractions between HCl molecules are weaker (4% as strong)
Attraction between molecules determine physical properties
Notice disorientation!

5. Dipole-dipole attractions
HCl(g) - polar molecule with partial charges
Polar molecules tend to line up so partial negative and near partial positive
Still net attraction!! (Dipole-dipole!)
Why weak?
Charges associated are only partial charges
Ordinary temperatures (Thermal energy) causes the dipoles to be somewhat misaligned reducing effectiveness of attractions

6. Hydrogen Bonds
Important Dipole-dipole attraction when hydrogen bonds to very small, highly electronegative atom
Think FON (HF, OH, and NH)
Why Hydrogen bonding?
Ends of bond contain substantial positive and negative charges
Charges highly concentrated due to small size
Positive ends can get very close to negative of another molecule due to small size

7. Hydrogen Bonds in Water
In Liquid water - molecules experience hydrogen bonds that continually break and re-form
As water freezes, molecules become locked and participate in 4 hydrogen bond
Resulting structure has larger volume than liquid water
Ice cubes float in more dense liquid

8. London Dispersion forces
Nonpolar molecules still have attraction (although weak) to hold substance together
Fritz London, German Scientist
Nonpolar substances can still have attraction
Atoms constantly moving
Motion in one particle affects neighboring particles
Electrons repel and push away
At any given moment, the electron density of molecule can be unsymmetrical
At particular instant, instantaneous dipole!