Phase Diagrams

1
Phase Diagrams

• Chapter 11

2
Chemical Systems

• Undergo three main processes that change their
  energy
• Chemical reactions
• Heating/cooling
• Phase transitions

3
Phase Diagrams

• Phase diagrams display the state of a substance
  at various pressures and temperatures and the
  places where equilibria exist between phases.

4
Phase Diagrams

• The AB line is the liquid-vapor interface.
• It starts at the triple point (A), the point at
  which all three states are in equilibrium.

5
Phase Diagrams

• It ends at the critical point (B) above this
  critical temperature and critical pressure the
  liquid and vapor are indistinguishable from each
  other.

6
Phase Diagrams

• Each point along this line is the boiling point
  of the substance at that pressure.
• Pressure Force / area
• Increasing pressure causes the molecules to be
  condensed making the molecular arrangement closer

7
Phase Diagrams

• The AD line is the interface between liquid and
  solid.
• The melting point at each pressure can be found
  along this line.

8
Phase Diagrams

• Below A the substance cannot exist in the liquid
  state.
• Along the AC line the solid and gas phases are in
  equilibrium the sublimation point at each
  pressure is along this line.

9
Phase Diagrams

• As temperature increases, the average kinetic
  energy of the substance increases causing the
  spacing between the molecules to increase and
  eventually change phase

10
Phase Diagram of Water

• Note the high critical temperature and critical
  pressure
• These are due to the strong van der Waals forces
  between water molecules.
• London dispersion force
• Dipole-dipole forces
• Hydrogen bonding
• Increase the melting and boiling point of water

11
Phase Diagram of Water

• The slope of the solidliquid line is negative.
• This means that as the pressure is increased at a
  temperature just below the melting point, water
  goes from a solid to a liquid.

12
Phase Diagram of Carbon Dioxide

• Carbon dioxide cannot exist in the liquid state
  at pressures below 5.11 atm CO2 sublimes at
  normal pressures.

13
Phase Diagram of Carbon Dioxide

• The low critical temperature and critical
  pressure for CO2 make supercritical CO2 a good
  solvent for extracting nonpolar substances (such
  as caffeine).

14
Intermolecular Forces and impact on ideal behavior

• Presence of intermolecular forces among gas
  particles (including noble gases) leads to
  deviations from ideal behavior
• Therefore, at sufficiently low temperatures or
  sufficiently high pressures, gases can condense

15
Heating/Cooling

• Between phase changes, all heat (energy) absorbed
  or released changes the average amount of KE
  (therefore, the T) of the substance
• Areas on graph with a slope

16
Heating/Cooling

• There is no slope in areas where a phase change
  is occuring
• All energy is being used to change the phase of
  matter (rather than change the temperature)

17
Heating/Cooling

• Melting and Boiling
• Endothermic
• Freezing and Condensation
• Exothermic
• The amount of energy needed to vaporize amount
  of energy needed to condense
• Reverse phase changes have equal but opposite
  magnitudes of energy gained or released
• Same applies for melting and freezing