Post on 22-Jan-2020
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Chapter 11: Liquids, Solids and Materials
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The Liquid State
A liquid forms when a gas condenses. • This occurs at low T (and/or high P). • Average intermolecular attraction > average Ek.
Molecules are close together and in constant motion They have a distribution of speeds (and Ek) A few have enough Ek to slide past each other.
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The Liquid State
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High viscosity is caused by: • Large intermolecular attractions. • Large entanglement of molecules.
Higher T = lower viscosity • Ek > Ebarrier required to move past another molecule. • Increase T = increase average Ek.
More molecules have Ek > Ebarrier
The Liquid State
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Liquid molecules attract each other.
Surface molecules experience unbalanced forces. They are not as well stabilized.
Smaller surface area ≡ higher stability.
The Liquid State
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Raindrops are spherical (surface/V ratio minimized).
Substance Formula Surface Tension J/m2 at 20°C
Octane C8H18 2.16 x 10-2
Ethanol C2H5OH 2.23 x 10-2
Chloroform CHCl3 2.68 x 10-2
Benzene C6H6 2.85 x 10-2
Water H2O 7.29 x 10-2 Mercury Hg 46. x 10-2
The Liquid State
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Why don’t all liquids form spheres in all cases?
It’s a balancing act … Gravity pulls down (giving a puddle). Surface tension contracts (into a sphere).
The “winner” depends on the amount of liquid and any surface/liquid attractions. A drop of water:
puddles on a dirty surface “beads” on a waxy surface
The Liquid State
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The Liquid State
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The Liquid State
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low T
velocity or energy
num
ber o
f mol
ecul
es
Threshold for
escape
high T
Volatility increases with increased T.
Increase T = increase Ek = more can escape.
Vapor Pressure
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Open – the liquid can evaporate completely. Closed – the liquid evaporates but cannot disperse.
Vaporized molecules can condense. • more vapor = faster condensation
At some time: evaporation rate = condensation rate.
P increases but reaches a maximum.
Vapor Pressure
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Vapor Pressure
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Boiling Point Boiling points (bp) vary with pressure.
Nor
mal
bp
= 78
.5°C
ethanol Water boils at 100°C
at sea level
1 atm
Nor
mal
bp
-40 -20 0 20 40 60 80 100 120 Temperature, °C
Vapo
r pre
ssur
e, m
mH
g
1000
500
0
H 2O
diethyl ether
bp =
34.
6°C
Salt Lake City (4400 ft) P=650 mmHg
Water boils at 95°C in Salt Lake City
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Clausius-Clapeyron Equation The relationship between vapor pressure and T:
ln = – – P2
P1 ΔHvap
R 1
T2 1
T1
Ethanol
Plot ln P versus 1/T
slope = – ΔHvap R
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Clausius-Clapeyron Equation The normal bp of water = 100°C and ΔHvap = 40.71 kJ/mol. At what T will water boil if P = 500 Torr?
ln = – P2 P1
1 T2
1 T1
-ΔHvap R
ln = – 500 760
1 373
1 T2
- 40710 J g-1 8.315 J K-1mol-1
- 0.4187 = - 4896.3 - 2.681 x 10-3 1 T2
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Phase Changes: Solids, Liquids & Gases
low T
velocity or energy
num
ber o
f mol
ecul
es
Threshold for
escape
high T
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Phase Changes
For H2O(l) → H2O(g) ΔH° = ΔH°vap = +40.7 kJ/mol
For H2O(g) → H2O(l) ΔH° = ΔH°cond = -40.7 kJ/mol
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Melting and Freezing
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Phase Changes
For H2O(s) → H2O(l) ΔH° = ΔH°fus = +6.02 kJ/mol
For H2O(l) → H2O(s) ΔH° = ΔH°cryst = -6.02 kJ/mol
Heat required to melt 1 mol (P = 1 bar).
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Phase Changes
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Phase Changes
endothermic process
(heat added)
exothermic process
(heat released)
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Phase Changes
Substance mp(°C) ΔHfus(kJ/mol) bp(°C) ΔHvap(kJ/mol) O2 (16 e-) –248 0.445 –183.0 6.8 F2 (18 e-) –220 1.020 –188.1 6.54 Cl2 (34 e-) –103 6.406 –34.6 20.39 Br2 (70 e-) –7.2 10.794 59.6 29.54
Nonpolar molecules
London forces increase as the number of e- increase. The data support this trend…
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Phase Changes
Substance mp(°C) ΔHfus(kJ/mol)* bp(°C) ΔHvap(kJ/mol)* SO2 (32 e-) –76 7.4 –10.0 24.9 HCl (18 e-) –115 2.0 –85.1 16.2 HBr (36 e-) –87 2.4 –66.8 17.6 H2O (10 e-) 0 6.0 +100.0 40.7 HF (10 e-) –83 4.6 +19.5 7.5 NH3 (10 e-) –78 5.6 –33.5 23.4
Polar molecules
More difficult to explore trends. Values depend on London, dipole forces and H-bonding.
*At the normal phase-transition temperature (Air Liquide Gas Encyclopedia)
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Phase Changes
Substance mp(°C) ΔHfus(kJ/mol) NaCl 800 30.21 NaBr 747 25.69 NaI 662 21.95
Ionic solids
All Na+ ions / halide ion (-1 charge) compounds.
They only differ in halide size: I- > Br- > Cl- Larger ion → further apart → weaker attraction
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Heating Curve Convert 100 g of ice at -20°C into vapor at 120°C.
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TOTAL heat required = 308.9 kJ
Heat the ice to 0°C. ΔH = mcΔT = (100g)(2.06 Jg-1°C-1)(0-[-20]°C) = 4.1 kJ
Convert the ice to water ΔH = nΔHfus = (100g/18.02g mol-1)(6.020 kJ/mol) = 33.4 kJ
Heat the water from 0°C to 100°C. ΔH = mcΔT = (100g)(4.184 Jg-1°C-1)(100 - 0°C) = 41.8 kJ
Convert water to steam (at its normal bp). ΔH = nΔHvap = (100g/18.02 g mol-1)(40.7 kJ/mol) = 225.9 kJ
Heat steam from 100 to 120°C. ΔH = mcΔT = (100g)(1.84 Jg-1°C-1)(120 - 100°C) = 3.7 kJ
Heating Curve
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Pre
ssur
e (a
tm)
Temperature (°C)
Phase Diagrams
Solid Liquid
Gas
critical point
triple point
Melting point curve
Vapor-pressure curve
Supercritical fluid
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Phase Diagrams
Water T = 0.01°C, P = 4.58 mmHg CO2 T = -57°C, P = 5.2 atm
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Supercritical CO2 is an important solvent. It is used:
It has:
• to extract caffeine from coffee beans • as a dry-cleaning fluid
• a density characteristic of a liquid. • flow properties of a gas.
Critical Temperature and Pressure
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Water Phase Diagram
For most materials, the solid/liquid line has a positive slope. Water is unusual. Ice can be melted by increased P !
4.58
Solid Liquid Gas
Pre
ssur
e (m
m H
g)
Temperature (°C) 0 0.01 100
760
triple point
normal fp normal bp
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Physical Comparison with Importance in Property Other Substances and Biological Environment Specific heat capacity Highest of all liquids and Moderates T in the environment and in (4.18 J g-1 °C-1) solids except NH3 organisms; climate affected by
movement of water (e.g., Gulf Stream) Heat of fusion Highest of all molecular Freezing water releases large quantity (333 J/g) solids except NH3 of thermal E; used to save crops from
freezing by spraying them with liq. water Heat of vaporization Highest of all molecular Condensation of water vapor in clouds (2250 J/g) substances releases large quantities of thermal E
fueling storms Surface tension Highest of all molecular Contributes to capillary action in plants (7.3 x 10-2 J/m2) liquids causes formation of spherical droplets;
supports insects on water surfaces Thermal conductivity Highest of all molecular Aids heat transfer in organisms; (0.6 J s-1 m-1 °C-1) liquids rapidly cools organisms immersed in
cold water, causing hypothermia
Table 11.4 Unusual Properties of Water
Water: A Liquid with Unusual Properties
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Most liquids: Lower T = higher density. Most materials: dsolid > dliquid
• Ice floats on water, insulating the water below it.
These facts allow aquatic life to survive at low T.
Water: A Liquid with Unusual Properties
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Water: A Liquid with Unusual Properties
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Types of Solids
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Types of Solids Solids can be divided into:
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Crystalline Solids
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Crystalline Solids
• The unit cell is a square. • Each corner contributes ¼ of a
circle to the unit cell. • Net of 1 circle/unit cell. • The entire lattice can be formed
by adding unit cells to each face
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Cubic Unit Cells
The simplest are the cubic cells with 3 subtypes: – simple (or primitive) cubic (sc). – body-centered cubic (bcc). – face-centered cubic (fcc).
Faces meet at 90° with equal-length sides.
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Cubic Unit Cells
1
3
2 4 5 6 at rear
Each atom has 6 equivalent neighbors:
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Cubic Unit Cells
68 % of the space is occupied.
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Cubic Unit Cells
74% of the space is filled by atoms
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Closest Packing of Spheres Metal crystals have equal-sized atoms (spheres).
Single layer
A large percentage of the space is occupied.
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Closest Packing of Spheres
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Closest Packing of Spheres Start with “ab” (“b” layer in “a” holes)
Add another “a” layer (Green directly over green).
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Closest Packing of Spheres
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Cubic Close Packed Start with “ab”. (“b” layer in “a” holes)
Add “c” (gold) above holes in the original green “a” layer.
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Ionic Crystal Structures Ionic crystal structures are more complex. The ions making up the crystal:
• are not identical to each other. • may be of very different sizes • may not have the same charge magnitude. • may not be “spheres” (polyatomic ions...)
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Ionic Crystal Structures Many ionic compounds have: • sc or fcc negative-ion lattices. • positive ions occupy “holes”.
CsCl unit cell
Cs+
Cl-
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Ionic Crystal Structures NaCl has an fcc Cl- lattice; Na+ in octahedral holes.
• Each Na+ is surrounded by 6 Cl-. • Each Cl- is surrounded by 6 Na+.
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It also contains 4 Na+: 12 edges x ¼ Na+/edge = 3 Na+ 1 center x 1 Na+/center = 1 Na+
The NaCl unit cell contains 4 Cl- ions:
Ionic Crystal Structures
8 corners x 1/8 Cl- / corner = 1 Cl- 6 faces x ½ Cl- / face = 3 Cl-
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Network Solids Some non-metals link together in extended networks.
Diamond Graphite
335 pm
141 pm
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Materials Science Study of the relationships between the structure and the chemical and physical properties of materials.