1. Evaporation and Vapor Pressure
14.2

2. Phase Changes

3. Evaporation
Evaporation occurs when molecules at the liquids surface are moving fast enough to escape into the gas phase
Also called vaporization
Evaporation requires energy to overcome intermolecular forces between the molecules of the liquid

4. Evaporation – Endothermic or Exothermic?
Do the demo with acetone, ethanol, and water on filter paper on the end of a thermometer – as the liquid evaps the temp decreases

5. Is evaporation endothermic or exothermic?
Fastest molecules leave from the liquid to the gas phase
Now there is a decrease in Ave. KE
To equilibrate - heat will flow into the system (thus endothermic)

6. Evaporation in a closed container…
When we put an amount of liquid into a closed container, it first decreases slightly as liquid changes to vapor
Some of the vapor will begin to change back to liquid

7. Dynamic Equilibrium
Eventually, the same number of molecules are leaving the liquid as a returning to it
The rate of condensation = the rate of evaporation
Dynamic because still moving
Equilibrium because no net change
It occurs ONLY in a closed container
Like a bridge in
and out of a city

8. Vapor Pressure
The pressure of the vapor present at equilibrium in a closed container is called vapor pressure of the liquid
Explain real quick how pressure works
24 molecules = high pressure
12 molecules
= low pressure

9. Vapor Pressure
The vapor pressure of a liquid depends on intermolecular forces
Liquids with high vapor pressures are called volatile (evaporate readily)
Why do Liquids with high intermolecular forces have a low vapor pressure?
Liquids with high intermolecular forces must have particles moving fast enough to break these attractions in order to enter the gas state. Few particles can move into the gas phase; therefore, there is a low vapor pressure

10. Why do Liquids with high intermolecular forces have a low vapor pressure?
High intermolecular force means particles are held tightly in the liquid state
The tighter held molecule means it must move FAST to overcome the attraction
Less likely molecules are moving fast enough so few reach the gas phase
Fewer as a gas means less pressure provided.

11. Boiling Point is different from Vaporization…..
The temperature at which the vapor pressure of a liquid equals the pressure exerted on the liquid is called the boiling point.

12. The formation of the bubble is opposed by atmospheric pressure.

13. Boiling Point The normal boiling point is the boiling point at 1 atm.
Why would the boiling point of a liquid vary throughout the world?
The higher altitude  the lower the atmospheric pressure, therefore, a lower boiling point will occur since a lower vapor pressure is required