1. Liquids & Vapor Pressure

2. Vapor
Gas phase of substance that is normally liquid at room temperature
Some evaporation occurs at all temperatures
The easier a substance evaporates, the weaker the IMF

3. Fig a: open system
Evaporation continues until all liquid is gone
Fig b: closed system
Rate of evaporation = rate of condensation
System appears static due to equilibrium

4. How does evaporation occur?

5. molecules in liquid phase have range of KE;
some have enough KE to “escape” from liquid phase

6. Vapor Pressure Pressure exerted by vapor over its liquid
VP is affected by temperature:
higher the T, higher the VP

7. all liquids:
VP ↑as T ↑

8. Vapor Pressure VP does NOT depend on how much liquid is present
VP depends ONLY on temperature

9. IMF in Liquids WEAK FORCES High VP High rate evaporation Low bp
Small Hv
STRONG FORCES
Low VP
Low rate evaporation
High bp
Large Hv

10. Phase Changes Melting Boiling Vaporization
occurs throughout liquid; T constant)
Evaporation (occurs only at surface; T ↓)
Sublimation
Freezing
Deposition
Condensation

11. Phase Changes are accompanied by energy changes
Endothermic Process: Energy is absorbed
Exothermic Process: Energy is released

12. melting, boiling (vaporization), sublimation
Which phase changes absorb energy (endothermic)?
Which phase changes release energy (exothermic)?
melting, boiling (vaporization), sublimation
freezing , condensation, deposition

13. Boiling Point Temperature at which: VP liquid = external atmospheric P
Normal Boiling Point:
temp at which VP liquid = 1 atm
Substances with high bp’s have strong IMF’s

15. Melting Point
= temp at which liquid & solid phases of substance coexist at equilibrium
higher the mp, the stronger the IMF

16. Freezing Point They’re the same: for H2O = 0˚C, 273K
Temp at which liquid is converted to crystalline solid
How does fp compare to mp?
They’re the same: for H2O = 0˚C, 273K

17. Strong or Weak attractive forces?
High vapor pressure
Large Hv
High boiling point
Low boiling point
Low vapor pressure
Small Hv
Evaporates rapidly
Evaporates slowly
Weak
Strong

18. > < Boiling and Pressure
If increase external pressure (ex: camping in Death Valley), boiling point is ____ than 100oC
If decrease external pressure (ex: eating Raman noodles at top of Mt. Whitney), the boiling point is ____ than 100oC
>
<

19. Boiling vs. Evaporation
occurs at surface of liquid (or solid)
occurs at all T
liquid cools off as evaporates
spontaneous all the time
BOILING
occurs throughout liquid
occurs at definite T
occurs at constant T
only spontaneous when VP = external pressure