Phase Transformations - Vocabulary

• Phase • Allotropes • Phase Transitions • Phase Diagram • Normal vs Standard Transition Temperatures • Critical Temperature • Triple Point

Phase and Allotropes

Phase: Uniform in physical state and chemical composition Mathematically, a phase has 1 equation of state.

Allotrope: One of at least two different crystalline or molecular structures of a substance Examples: Ice, Phosphorous (White and Black), Carbon (Graphite and Diamond)

Phase Transformations - Vocabulary

 Phase  Allotropes • Phase Transitions • Normal vs Standard Transition Temperatures • Phase Diagram • Critical Temperature • Triple Point

Phase Transitions

Spontaneous conversions from one physical state (phase) to another. The phases are in equilibrium during a phase transition.

What does that mean about  S?

What about  G?

Transition Temperature

The temperature, AT A GIVEN PRESSURE, where a phase transition occurs.

Phase Diagram

Normal Melting Point Standard Melting Point

Phase Diagrams

Phase diagrams give us a general idea about phase stability….

But they don’t tell the whole story.

Example: Allotropes of carbon C(diamond) C(graphite) Gibbs Free Energy Diamond 2.9 kJ/Mol Graphite

Phase Diagram

Normal Melting Point Standard Melting Point Super Critical Fluid Critical Point

Critical Point

CO 2 Br 2 H 2 O O 2 CH 4 T c (K) 304.2

584 647.4

154.8

190.6

P c (atm) 72.85

102 218.3

50.14

45.6

CO

2

H

2

O

Ice-I and liquid water boundary has a negative slope There are 6 different types of Ice (7 triple points) There is no such thing as Ice IV Ice VII isn’t shown on this diagram

Helium

Gas and solid never coexist Two allotropes (body centered cubic and hexagonal closed packing) Two liquid forms • A superfluid flows without any viscosity Quantum effects are relevant 3 He and 4 He have different phase diagrams.

Chemical Potential (



)

Is a quantitative measure of a substance’s ability to bring about change.

For a pure substance,



is the potential to bring about physical change (a phase change).

Do we have a quantity that sort of describes that kind of potential?

Entropy is the driving force of the universe.

Molar Gibbs free energy is the definition of  for a pure substance.

 Is going to help us quantify how the system changes under the given conditions.

Chemical Potential

Any system in equilibrium must have the same chemical potential throughout the entire system The chemical potential of a solid phase changes with T differently than that of the liquid phase.  for a liquid changes at a different rate than that of a solid or liquid.