Post on 27-Dec-2015
Atkins’ Physical ChemistryEighth Edition
Chapter 4 – Lecture 1Physical Transformations
of Pure Substances
Copyright © 2006 by Peter Atkins and Julio de Paula
Peter Atkins • Julio de Paula
Homework Set #4Homework Set #4
Atkins & de Paula, 8eAtkins & de Paula, 8e
Chap 4 Chap 4
DiscussionDiscussion questionsquestions: 3, 4: 3, 4
ExercisesExercises: all part (b) unless noted: : all part (b) unless noted: 1,5,6,7,81,5,6,7,8
NumericalNumerical ProblemsProblems: 2, 8 (plot this), 16: 2, 8 (plot this), 16
Objectives
• Applications of thermo to phase transitionsof a single, pure substance
• Phase diagrams (P vs T)
• Phase boundaries
• Melting point as function of pressure
• Vapor pressure as function of T
Fig 4.1 A typical phase diagram: P vs T
Fig 4.2 Vapor pressure of a liquid or a solid
≡ the pressure of a vapor measured when a dynamic equilibrium exists between evaporation and condensation
Fig 4.3 Heating of a liquid in a sealed container
For H2O,
Tc = 374 °C
Pc = 218 atm
Fig 4.4 Phase diagram for carbon dioxide
For CO2,
Tc = 304.2 °C
Pc = 72.9 atm
Supercritical COSupercritical CO22
The low critical temperature and critical pressure for CO2 make supercritical CO2 a good solvent for extracting nonpolar substances (like caffeine)
Diagram of a supercritical fluid extraction process
Fig 4.5 Phase diagram for water
Tf 1/P∝ applied
Unique for water!
Fig 4.6 Fragment of structure of ice (ice-I)
Fig 4.7 Phase diagram for Helium-4
Phase Stability and Phase TransitionsPhase Stability and Phase Transitions
• Apply thermodynamics to account for features
in phase diagrams
• All considerations based on molar Gibbs energy, Gm
• For a one-component system,
chemical potential (μ): μ ≡ Gm
Fig 4.8 Two or more phases of a pure substance in equilibrium
According to 2nd law:
At equilibrium, the chemical
potential of a substance is the
same throughout the sample. μ1
μ2
dn
-μ1dn
+μ2dn
For any system in equilibrium: dG = 0
Net: dG = (μ2 - μ2)dn = 0 means μ1 = μ2
Fig 4.9
Schematic of the
temperature dependence
of the chemical potential
mPP
m STT
G
μ
dTSd mμ
Fig 4.10 (a)
Pressure dependence
of the chemical potential
mTT
m VPP
G
dPVd m
Substances for which
Vm(s) < Vm(l)
Fig 4.10 (b)
Pressure dependence
of the chemical potential
Substances for which
Vm(s) > Vm(l)
e.g., water, which expands upon freezing