Phase Diagrams

1
Phase Diagrams

• Chapter 8

2
What is a Phase?
3
Phases in Equilibrium

• Two phases in equilibrium have same T P
• The Clapeyron eqn expresses dP/dT for a biphasic
  system in equilibrium
• General Concepts
• same T and P
• Chemical potential same in both phases
• Surface tension issues when the phase boundary is
  curved
• Pressure on concave side of boundary is higher

4
One-Component Systems

• Projections are on the P-T plane.
• Vapor pressure unction given by the line from
  triple point to critical point.
• The critical point more complex than shown
  phenomena such as turbidity occur in this area

5
Phases in a One-Component System

• Chemical potential, m G (molar Gibbs free
  energy)
• Plot of m vs T at const. P
• The stable phase has the lowest m
• Phases with same m have are in equilib.
• Tmelt and Tboil

6
Slopes of the Lines

• S is positive, so slopes are negative and,
  SggtSlgtSS

• molar V is always positive
• VggtgtVlVS

7
First Order Transitions

• So far, we have spoken only of first order
  transitions
• On two sides of a transition temp, the entropies
  are different
• First order transitions also have different molar
  volumes on different sides of a boundary

Discontinuity over phase boundaries
Discontinuity over phase boundaries
Interesting Note CP is infinite when two phases
are in equilibrium