1. 14.1 Molecular Substances Intermolecular Forces
Forces between molecules determine the melting and boiling point of substances.
Ionic compounds have high melting and boiling points due to their electrostatic attraction.
Molecular substances are held together by one of three forces.
Dispersion
Dipole
Hydrogen Bond

2. Dispersion Forces
Dispersion forces, also called London Forces, are a result of the temporary location arrangement of the electrons.
Electrons have a probability of being located on one side of the atom.
This side of the atom becomes temporarily charged negative while the other side becomes positive.
This is a temporary situation
This is the weakest of the forces

3. Dipole Forces
Polar molecules have a permanent asymmetrical distribution of electrons.
Although the total molecule is neutral, it has a positive and negative side.
The opposite attraction between +/- sides of the molecule is permanent and thus will cause polar substances to have a higher melting and boiling point than non-polar substances.
Dipole forces are the next strongest.

4. Hydrogen Bonding
The strongest intermolecular force is a subset of dipole forces.
If hydrogen is bonded to oxygen, nitrogen, or fluorine the molecule has the strongest of polarities.
This is called a hydrogen bond.
The hydrogen is highly positive and the negative side is similarly negative.
These substances have the highest melting and boiling points.

5. Evaporation & Vapor Pressure
Evaporation is a natural process in which a liquid converts to a gas in an endothermic process.
H2O(l) → H2O(g) ∆H = +
Evaporation is a cooling process
The energy is supplied from the environment.
Fun Fact! Lake Tahoe evaporates 335 million of water daily

6. If we can contain evaporation by enclosing our liquid source (think of placing a lid on a pot) a pressure would begin to build within the air space.
Eventually the pressure will cause condensation at the same rate as evaporation.
The pressure this happens at is called the vapor pressure of water.
At vapor pressure, Vp, the system is at dynamic equilibrium.
This pressure is above atmospheric pressure
Total Pressure = Vp + 1 atm.