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
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
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
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 CO2 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/Papplied 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 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 +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
Fig 4.9 Schematic of the temperature dependence of the chemical potential
Fig 4.10 (a) Pressure dependence of the chemical potential 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