chem101/3, wi2010 pe 21‐1 chem101/3, wi2010 pe 21‐2 General
States of Matter
Chem101/3:
SM VII
Apply to pure substances only!
(not to mixtures)
(pre)
Phase: at molecular level, has
uniform chem. composition &
Phase Diagrams
uniform physical structure
(includes crystal arrangement)
normally: G, L, S
Ref
12: 2
Prob
reproduce phase diagrams
often: several phases exist for solids,
having diff. crystal structures
Phase Diagram
Adv Rdg Interchapter Topic 4, p.559; 18:3
chem101/3, wi2010 pe 21‐3 •
P vs T plot
•
showing existence of phases (at equil. !!)
•
substance specific (to each its own)
chem101/3, wi2010 General Phase Diagram
pe 21‐4 Idealized Phase Diagram
P
P
supercritical
fluid
S
L
O
T
not necessarily straight lines
O = triple point (sometimes T)
C = critical point
C
G
T
chem101/3, wi2010 pe 21‐5 Ex. Phase Diagram for Butane (lighter fluid)
chem101/3, wi2010 pe 21‐6 Comments
1.) at O (T), triple point,
3 phases can co-exist in equil.;
P
supercritical
fluid
S
L
defines unique temp. & pressure
(can be used for calibration)
for H2O : T = 0.01 °C, P = 0.006 atm
C
2.) at C, critical point,
(generally at high T & P),
G
O
L & G become indistinguishable
(for more see SB. p.449)
T
chem101/3, wi2010 pe 21‐7 3.) Phase Transitions
chem101/3, wi2010 5.) Phase diagrams can be used
to assess phase transitions,
esp. heating curves
(what happens to a substance if T↑ )
G
Generally
at “low” P, (i.e., at P’s below triple point O )
S
L
only S → G occurs;
e.g., I2 below 92 torr, CO2 at 1 atm
4.) at “separation lines”,
2 phases can co-exist;
e.g., O - C line = VP curve (vapor pressure)
at “high” P, (i.e., at P’s above triple point O )
transitions S → L → G are possible,
e.g., H2O at 1 atm
pe 21‐8 chem101/3, wi2010 pe 21‐9 chem101/3, wi2010 pe 21‐10 HT Fig. 21.1 Phase Diagram of Water (after Pet. Fig. 12.21)
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 pe 21‐11 chem101/3, wi2010 pe 21‐12 specific ...
HT Fig 21.2 Crystal Structure of Ice
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 VP curve)
• see demo for more details
chem101/3, wi2010 pe 21‐13 Pet. Fig. 12.19 Phase Diagram of CO2
chem101/3, wi2010 pe 21‐14 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 HT Fig. 21.3 CO2 Liquefaction Demo
pe 21‐15