atkinsphysicalchemisrrychapter4lecture1

1. Atkins’ Physical Chemistry
Eighth Edition
Chapter 4 – Lecture 1
Physical Transformations
of Pure Substances
Copyright © 2006 by Peter Atkins and Julio de Paula
Peter Atkins • Julio de Paula

2. Homework Set #4
Atkins & de Paula, 8e
Chap 4
Discussion questions: 3, 4
Exercises: all part (b) unless noted: 1,5,6,7,8
Numerical Problems: 2, 8 (plot this), 16

3. Objectives
• Applications of thermo to phase transitions
of a single, pure substance
• Phase diagrams (P vs T)
• Phase boundaries
• Melting point as function of pressure
• Vapor pressure as function of T

4. Fig 4.1 A typical phase diagram: P vs T

5. 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

6. Fig 4.3 Heating of a liquid in a sealed container
For H2O,
Tc = 374 °C
Pc = 218 atm

7. Fig 4.4 Phase diagram for carbon dioxide
For CO2,
Tc = 304.2 °C
Pc = 72.9 atm

8. Supercritical CO2
The low critical
temperature and critical
pressure for CO2 make
supercritical CO2 a
good solvent for
extracting nonpolar
substances (like
caffeine)

9. Diagram of a supercritical fluid extraction process

10. Fig 4.5 Phase diagram for water
Tf ∝ 1/Papplied
Unique for water!

11. Fig 4.6 Fragment of structure of ice (ice-I)

12. Fig 4.7 Phase diagram for Helium-4

13. Phase 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

14. 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

15. Fig 4.9
Schematic of the
temperature dependence
of the chemical potential
m
P
P
m
S
T
T
G


















 μ
dT
S
d m


μ

16. Fig 4.10 (a)
Pressure dependence
of the chemical potential
m
T
T
m
V
P
P
G



















dP
V
d m


Substances for which
Vm(s) < Vm(l)

17. Fig 4.10 (b)
Pressure dependence
of the chemical potential
Substances for which
Vm(s) > Vm(l)
e.g., water,
which expands upon freezing