1. T HERMODYNAMICS : P HASE D IAGRAMS 1

2. W HAT IS A PHASE DIAGRAM ? phase diagram A phase diagram is common way to represent the various phases of a substance and the conditions under which each phase exists. Lines on the diagram equilibrium A phase diagram is a plot of pressure ( P ) vs temperature ( T ). Lines on the diagram represent conditions ( T, P ) under which a phase change is at equilibrium. That is, at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. In other regions of the plot, only one phase exists at equilibrium. Triple Point three phases can coexist at equilibrium Triple Point At the triple-point temperature, T 3, and triple-point pressure, P 3, three phases can coexist at equilibrium. The point at T 3, P 3 is called the triple point. Critical Point no longer possible to distinguish between the gas and liquid phases Critical Point At a temperature above the critical temperature, T c, and a pressure above the critical pressure, P c, it is no longer possible to distinguish between the gas and liquid phases. At T > T c and P > P c the substance is referred to as a super-critical fluid. http://www.chm.davidson.edu/vce/phasechanges/PhaseDiagram.html 2

3. W HAT IS THIS EQUILIBRIUM OF WHICH YOU SPEAK ? Equilibrium reversible no net change Equilibrium is a condition where two reversible processes are occurring at the same rate so that there is no net change. The states of matter can exist at equilibrium when: Rate of freezing = rate of melting Rate of boiling = rate of condensing Rate of sublimation = rate of deposition At the triple point, all of the rates listed above are happening at the same time!

4. E XAMPLE : CO 2 http://www.teamonslaught.fsnet.co.uk/co2%20phase%20diagram.GIF 4

5. W HAT IS “ NORMAL ”? Sometimes you’ll see freezing point and boiling point referred to as the normal freezing point or normal boiling point. normal standard pressure The word “ normal ” means that the phase change is occurring at standard pressure (1 atm). Technically, f.p and b.p. are both a temperature AND pressure (points on a graph have two coordinates) But we usually refer to the freezing point of water as 0°C and the boiling point of water as 100°C These are actually the normal f.p. and b.p. because they’re happening at standard atmospheric pressure so pressure isn’t specified

6. E XAMPLE : W ATER http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg 6

7. E XAMPLE : UF 6 http://web.ead.anl.gov/uranium/guide/prodhand/images/DUF6PH22.gif 7

8. E XAMPLE : C ARBON http://dao.mit.edu/8.231/carbon_phase_diagram.jpg 8

9. E XAMPLE : S ILICA http://www.quartzpage.de/cr/silica_phase_diagram.png 9

10. D ID YOU NOTICE …? Some substances had more than three “states” of matter! The extra “states” are not truly states of matter, they are allotropic forms of the same substance. Allotrope Allotrope : a different bonding structure of the same compound or element graphiteas diamond Example: carbon as graphite v. carbon as diamond solid Both are solid forms of carbon! 10

11. O NE OTHER THING … Solid lines represent two phases at equilibrium So any time you cross a line, a phase change has occurred And that, my friends, is that. 11