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chem101/3, wi2010 p0 21‐1
States of Matter
SM VII (post)
Phase Diagrams
Ref 12: 2 Prob reproduce phase diagrams;
HMWK #11
Adv Rdg Interchapter Topic 4, p.559; 18:3
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General Chem101/3: Apply to pure substances only! (not to mixtures)
Phase: at molecular level, has
uniform chem. composition &
uniform physical structure (includes crystal arrangement)
normally: G, L, S
often: several phases exist for solids, having diff. crystal structures Phase Diagram • P vs T plot • showing existence of phases (at equil. !!) • substance specific (to each its own)
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General Phase Diagram
P see next page
T not necessarily straight lines
O = triple point (sometimes T)
C = critical point
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Idealized Phase Diagram
O
C
P
T
S
G
L
supercriticalfluid
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Ex. Phase Diagram for Butane (lighter fluid) C4H10
O
C
P
T
S
G
L
supercriticalfluid
~ 20oC
P=1 atm
P>1atmclosed
open;imagine a small cloud of C4H10forms;of course will ultimately dissipate due to convection
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Comments 1.) at O (T), triple point,
3 phases can co-exist in equil.;
defines unique temp. & pressure (can be used for calibration)
for H2O : T = 0.01 °C, P = 0.006 atm
2.) at C, critical point,
(generally at high T & P),
L & G become indistinguishable
(for more see Pet. p.486)
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3.) Phase Transitions
G
S L
evaporation/condensation
sublimation/deposition
melting/freezing
4.) at “separation lines”,
2 phases can co-exist;
e.g., O - C line = liquid VP curve (vapor pressure)
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5.) Phase diagrams can be used
to assess phase transitions,
esp. heating curves (what happens to a substance if T↑ )
Generally
at “low” P, (i.e., at P’s below triple point O )
only S → G occurs;
e.g., I2 below 0.12 atm, CO2 at 1 atm
at “high” P, (i.e., at P’s above triple point O )
transitions S → L → G are possible,
e.g., H2O at 1 atm
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Specific Phase Diagrams 1.) H2O
(see HT Fig. 21.1)
remarkable:
has negative slope (tilting right to left) for S / L line
∴ increase in P ( near 0°C) causes transition S → L
molecular explanation:
P↑ causes collapse of the rigid H - Bonding structure in solid ice (which has lots of empty space = voids)
see HT Fig 21.2
∴ less voids in H2O(l); H2O(l) denser than H2O(s)
Other consequences: lakes don’t freeze to bottom; water pipes fail if left unprotected below 0°C
chem101/3, wi2010 p0 21‐10 HT Fig. 21.1 Phase Diagram of Water (after Pet. Fig. 12.21)
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HT Fig 21.2 Crystal Structure of Ice
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2.) Dry Ice, CO2
(see Pet. Fig. 12.19)
• dry ice, CO2(s) will not melt at 1 atm
• “VP” of CO2(s) at -78°C is 1 atm
• “subliming point” is -78°C
• if heated in a closed (sealed) tube,
P will increase; can go through triple point O &
liquefaction (melting) occurs
• further heating will move system
along L / G line (typical liquid VP curve)
• see demo for more details
chem101/3, wi2010 p0 21‐13 Pet. Fig. 12.19 Phase Diagram of CO2
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Demo: Liquefaction of CO2 see HT Fig. 21.3
• initially, the system is at point (1),
“near” equilibrium conditions (but open system)
near the surface of the solid CO2 piece:
PCO2 = 1 atm; T = -78°C
• if tube is sealed,
P can go up as T increases, (like in an a pressure cooker) (following S/G separation line, if at equil.)
• may observe 3 phases but probably not at equil.;
(may go temporarily go through triple point, O)
• ultimately, re-establish equilibrium at point (2):
T ≈ 20°C, P > 5.1 atm.; only L & G present
chem101/3, wi2010 p0 21‐15 HT Fig. 21.3 CO2 Liquefaction Demo