1. Peter Atkins • Julio de Paula Atkins’ Physical Chemistry
Eighth Edition
Chapter 4 – Lecture 1
Physical Transformations
of Pure Substances
Copyright © 2006 by Peter Atkins and 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 = °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
+μ2dn μ2
According to 2nd law:
At equilibrium, the chemical
potential of a substance is the
same throughout the sample. dn μ1
-μ1dn
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

16. Fig 4.10 (a)
Pressure dependence
of the chemical potential
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