Lecture20A - Lecture 20 1 Phase Equilibria(Chapter 23 We...

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Unformatted text preview: Lecture 20 1 Phase Equilibria (Chapter 23) We will begin to look at phase diagrams, initially for pure materials to understand the relationships between various phases. Fig 23.1 From this diagram, we can analyze the degrees of freedom of the system. Within a region of single phase, pressure and temperature can freely vary. Along a phase boundary line, P=P(T), i.e. the two variables are coupled. When two phase boundary lines intersect at a point there are no independent variables. Triple point (3 phases) has a fixed T and P. ( 29 degrees of freedom 3 =# of phases f p p = =- Phase diagram for benzene Lecture 20 2 Fig. 23.2 Fig. 23.3 Typical behavior for most compounds: slow increase in T fus with P and rapid increase in T vap with P. Benzene is typical of most compounds we know in that as we increase its temperature solid liquid vapor under normal atmospheric conditions. This is because the triple point occurs at a temperature where P<1bar, so the liquid phase will exist under typical atmospheric conditions. Lecture 20 3 CO 2 (first member of “weird” compounds). Triple Point is 5.11 atm so at 1 atm solid sublimes and goes directly to the vapor phase. H 2 O is 2 nd member of weird compounds T fus decreases with increasing pressure. Fig. 23.7 Benzene Fig. 23.6 H 2 O Fig. 23.4 CO 2 o l v vap At 200 C, a vertical line shows and at equilibrium. Note at the critical point the densities are the same and 0. H ρ ρ ∆ = Lecture 20...
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Lecture20A - Lecture 20 1 Phase Equilibria(Chapter 23 We...

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