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Understanding Phase Equilibria: Pure Substances and Phase Diagrams

This guide covers the fundamentals of phase equilibria and the critical concepts of pure substances. It introduces the definitions of phases, phase transitions, and key terms such as transition temperature, boiling points, and critical points. Detailed explanations of phase diagrams for CO2 and H2O illustrate stable regions, phase boundaries, and important properties such as vapor pressure and supercritical fluids. Prepare for Exam #3 by reviewing the key concepts outlined here, and ensure your grasp of phase equilibrium principles is strong.

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Understanding Phase Equilibria: Pure Substances and Phase Diagrams

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  1. CHEM 433 – 11/1/11 V. Phase Equilibria (Pure Substances) • Intro & definitions. • Phase diagrams - CO2 , H2O, C • Phase Stability vs. T READ: CHAPTER 4 HW #6: via e-mail – DUE Th. EXAM #3 on Tuesday – review(s) Monday Web up to date with review sheet today.

  2. Phase: A form of mater, uniform in chemical composition and physical state. Phase Transition: The spontaneous conversion of one phase to another (e.g. g —> l) Transition Temperature (Ttrs): Chemical potentials of 2 phases are equal and the 2 phases are in equilibrium Phase Diagram: Shows regions of P and T in which phases are stable Phase boundaries: p and T where 2 phases co-exist (in eq. , ’s are =). Vapor pressure: pressure of vapor in eq. with liquid (the l-v boundary). Sublimation vapor pressure: pressure of vapor in eq. with solid (s-v phase boundary)

  3. Boiling: A condition of free vaporization throughout the liquid. Boiling temperature(Tb): T at which vapor pressure equals external pressure Normal boiling: boiling T at 1 atm external pressure Standard boiling point: boiling T at 1 bar external pressure. Freezing/Melting Temperature (Tf): T at which liquid and solid are in EQ at a given p. Normal/Standard Tf: as above, for 1 atm or 1 bar, respectively.

  4. Triple point: Temperature and pressure at which 3 phases coexist (usually s, l, g). Critical temperature: T when surface between phases disappears (closed system). Density of (l) and (g) become equal! Critical pressure: vapor pressure at critical temperature. Supercritical Fluid: The name of the phase above the critical point.

  5. Phase Diagram :CO2 • How much pressure is needed to liquefy CO2? • What is its normal boiling point? • What is T of “dry ice bath”? • What phase does one have at T = 310K? • Is slope (dP/dT) of solid-liquid boundary “+” or “–”?

  6. Phase Diagram :H2O • Notice slope of “Ice I” – liquid boundary – positive or negative? • How many triple points • What is Tc (presumably)?

  7. Phase diagram for Carbon (note 1 GPa = 109 Pa = 104 bar) http://www.youtube.com/watch?v=O42K4EwVssQ

  8. Phase diagram of H2O and the greenhouse effect

  9. This picture is a “slice” through the phase diagram at p > T.P. • You re-draw this for: • p < that of triple point • p = to that of triple point

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