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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