Transcript Thermochemistry Set 2

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THERMODYNAMICS: PHASE
DIAGRAMS
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WHAT IS A PHASE DIAGRAM?
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A phase diagram is common way to represent the various
phases of a substance and the conditions under which each
phase exists.
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
At the triple-point temperature, T3, and triple-point pressure,
P3, three phases can coexist at equilibrium. The point at
T3, P3 is called the triple point.
Critical Point
At a temperature above the critical temperature, Tc, and a
pressure above the critical pressure, Pc, it is no longer
possible to distinguish between the gas and liquid
phases. At T > Tc and P > Pc the substance is referred to as a
super-critical fluid.
http://www.chm.davidson.edu/vce/phasechanges/PhaseDiagram.html
WHAT IS THIS EQUILIBRIUM OF WHICH
YOU SPEAK?
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:
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Rate of freezing = rate of melting
 Rate of boiling = rate of condensing
 Rate of sublimation = rate of deposition
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At the triple point, all of the rates listed above
are happening at the same time!
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EXAMPLE: CO2
http://www.teamonslaught.fsnet.co.uk/co2%20phase%20diagram.GIF
WHAT IS “NORMAL”?
Sometimes you’ll see freezing point and boiling
point referred to as the normal freezing point or
normal boiling point.
 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
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These are actually the normal f.p. and b.p. because
they’re happening at standard atmospheric pressure
so pressure isn’t specified
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EXAMPLE: WATER
http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg
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EXAMPLE: UF6
http://web.ead.anl.gov/uranium/guide/prodhand/images/DUF6PH22.gif
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EXAMPLE: CARBON
http://dao.mit.edu/8.231/carbon_phase_diagram.jpg
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EXAMPLE: SILICA
http://www.quartzpage.de/cr/silica_phase_diagram.png
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DID 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: a different bonding structure of the
same compound or element
 Example: carbon as graphite v. carbon as
diamond
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Both are solid forms of carbon!
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ONE OTHER THING…
Solid lines represent two phases at equilibrium
 So any time you cross a line, a phase change has
occurred
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And that, my friends, is that.