Thermochemistry ch' 10

1
Changes of State(p. 342 - 348)

• Thermochemistry (ch. 10)

2
A. Phase Changes
Exothermic - gives off heat Endothermic - takes
in heat
3
A. Phase Changes

• Evaporation
• molecules at the surface gain enough energy to
  overcome attractions between particles
• Condensation
• some gas particles strike the surface of the
  liquid and re-enter the liquid phase.

4
A. Phase Changes

• Equilibrium
• trapped molecules reach a balance between
  evaporation condensation
• rate of evap. rate of condensation

5
A. Phase Changes
p.344

• Vapor Pressure
• pressure of vapor above a liquid at equilibrium

v.p.

• depends on temp and strength of attractions
  between particles.

temp
6
A. Phase Changes

• Boiling Point
• temp at which v.p. of liquid equals external
  pressure

• depends on Patm strength of attractions between
  particles
• Normal B.P. - b.p. at 1 atm

7
A. Phase Changes

• Changing pressure changes b.p.
• pressure cooker
• cooking at high altitudes
• boiling water in a vacuum

8
A. Phase Changes

• Melting Point
• equal to freezing point

• TEMPERATURE DOES NOT CHANGE during a phase
  change.

• No change in KE (temp measures avg. KE), but PE
  between particles changes as attractions are
  broken.

9
A. Phase Changes

• Sublimation
• solid ? gas

• EX dry ice, mothballs, solid air fresheners

• Deposition
• gas ? solid

• EX frost

10
B. Phase Diagrams

• Show the phases of a substance at different temps
  and pressures.

Cant be liquid above this P and T
Negative slope solid less dense than liquid
Water
11
B. Phase Diagrams
What is the critical temp of CO2? What is the
critical pressure of CO2? At what conditions will
solid, liquid and gas exist at the same
time? Does CO2 have a normal boiling point? What
happens to CO2 at 1 atm?
Positive slope solid more dense than liquid
carbon dioxide
31.1oC
73 atm
5.11 atm, -56.4oC
No sublimation