intermolecular forces, liquid, and solid kinetic-molecular view of liquid and solid intermolecular...
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Intermolecular Forces, Liquid, and Solid
• Kinetic-Molecular View of Liquid and Solid
• Intermolecular Forces
• Liquid Properties
• Crystal Structure
• Phase Changes and Phase Diagram
I. Kinetic–Molecular View
Gas
• Gas– Assume volume and shape of the container– Low density, very compressible, free motion
Liquid
• Liquid– Has definite volume, assume shape of the container– High density, slightly compressible, flow readily
Solid
• Solid– Definite volume, definite shape– High density, incompressible
T
P
T
II. Intermolecular Forces
• Intermolecular force: attractive force between molecules
• Intramolecular force: hold atom together in a molecule
Ex: 41 kJ to vaporize 1 mole of water (inter)
930 kJ to break all O-H bonds in 1 mole of water (intra)
• Measure of intermolecular forces
–Boiling point, melting point, Hvap Hsub, Hfus
intramolecular forces > intermolecular forces
Types of Intermolecular Forces
• Ion-Dipole Forces– Attractive forces between an ion and a polar molecule
• Dipole-Dipole Forces– Attractive forces between polar molecules
• Dispersion Forces– Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
• Hydrogen Bond– Special dipole-dipole forces involving H and F, O, N
• van der Waals Forces– includes dipole-dipole, dipole-induced dipole and dispersion forces
II. Intermolecular Forces
A. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
II. Intermolecular Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
B. Ion-Dipole Forces
II. Intermolecular Forces
(1) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules (exist in all molecules)
(2) Resulted from electron cloud distortion
ion-induced dipole interaction
dipole-induced dipole interaction
C. Dispersion Forces
II. Intermolecular Forces
(3) Related to the polarizability of the molecule
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
C. Dispersion Forces
Polarizability generally increases with:
• greater number of electrons
• more diffuse electron cloud
• larger molar mass
Ex: Arrange boiling point
CH3CH3 CH3CH2CH3
He Ne Kr
<
< <
II. Intermolecular Forces
(4) Depend on the molecular shape
C. Dispersion Forces
Ex: C5H12 two isomers
H H H CH3
H3C - C - C - C - C H3 H3C - C - CH3
H H H CH3
>
II. Intermolecular Forces
D. Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.
A H…B A H…Aor
A & B are N, O, or F
II. Intermolecular Forces
D. Hydrogen Bond
Decreasing molar massDecreasing boiling point
SO
O
(1) What type(s) of intermolecular forces exist between each of the following molecules?
HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr.
CH4CH4 is nonpolar: dispersion forces.
SO2
SO2 is a polar molecule: dipole-dipole forces and dispersion forces between SO2 molecules.
Lets try these:
(2) Which molecule can form H bond with each other? (a) CH3OH(b) CH3OCH3 (c) F2
Only (a) CH3OH CH3 O H …. O CH3
H
Predict the trend in boiling point for the following species:
BaCl2, H2, CO, HF, Ne
One more example:
BaCl2 > HF > CO > Ne > H2
BaCl2 : ionic bonding
HF: HB + dispersion CO: dipole-dipole + dispersion Ne: dispersion H2 : dispersion
Stronger intermolecular forces, higher BP, MP
III. Properties of Liquids
A. Surface Tension
(1) The amount of energy required to stretch or increase the surface of a liquid by a unit area.
Unit: J/m2
Stronger intermolecular forces, higher surface tension
III. Properties of Liquids
A. Surface Tension
(2) Capillary action
Cohesion is the attraction between like molecules
Adhesion is an attraction between unlike molecules
Capilliary rise:adh > coh
coh > adh
III. Properties of Liquids
B. Viscosity - fluid’s resistance to flow
(1) Viscosity increases as interaction increases
(2) Viscosity decreases as T increases
(3) Viscosity depends on molecular shape
larger for long flexible molecule
III. Properties of Liquids
C. Water: a unique substance due to HB
Maximum Density40C
Density of WaterEach H2O has 4 HB, hexagonal arrangement
Unit Cell Unit cells in 3 dimensions
IV. Crystal Structure
A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions.
An amorphous solid does not possess a well-defined arrangement and long-range molecular order (no sharp MP)
A. Unit Cell the basic repeating structural unit of a crystalline solid.
latticepoint
At lattice points:
• Atoms
• Molecules
• Ions
B. Cubic Cells
IV. Crystal Structure
a. Simple cubic lattice (scc)
B. Cubic Cell
1. No. of atom per unit cell
8 x 1/8 =1
2. No. of nearest neighbors (coordination number): 6
3. In a unit cell, 52% space is occupied by atom
4. Size of unit cell to size of atom
r = radius of atom
a = edge length of unit cell
2 r = a
b. body-centered cubic (bcc)
B. Cubic Cell
1. No. of atom per unit cell
8 x 1/8 +1 =2
2. No. of nearest neighbors (coordination number): 8
3. In a unit cell, 68% space is occupied by atom
4. Size of unit cell to size of atom
r = radius of atom
a = edge length of unit cell
b2 = a2 + a2 C2 = a2 +b2 = 3 a2
€
C = 3 a = 4 r
€
a = 4r
3
c. face-centered cubic (fcc)
B. Cubic Cell
1. No. of atoms per unit cell
8 x 1/8 + 6 x 1/2 =4
2. No. of nearest neighbors (coordination number): 12
3. In a unit cell, 74 % space is occupied by atom
4. Size of unit cell to size of atom
r = radius of atom
a = edge length of unit cell
b2 = a2 + a2 b = 4 r
2 a2 = 16 r2
€
a = 8 r
Question: When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver.
d = mV
V = a3 = (409 pm)3 = 6.83 x 10-23 cm3
4 atoms/unit cell in a face-centered cubic cell
m = 4 Ag atoms107.9 gmole Ag
x1 mole Ag
6.022 x 1023 atomsx = 7.17 x 10-22 g
d = mV
7.17 x 10-22 g6.83 x 10-23 cm3
= = 10.5 g/cm3
V. Bonding in Solids
• Amorphous solid
• Crystalline solid Crystallinequartz (SiO2)
Non-crystallinequartz glass
A. Ionic Crystals• Lattice points occupied by cations and anions• Held together by electrostatic attraction• Hard, brittle, high melting point• Poor conductor of heat and electricity in solid form• Conduct electricity when melted
CsCl ZnS CaF2
B. Molecular Crystals
• Held together by intermolecular forces
• Soft, low melting point
• Poor conductor of heat and electricity
• Examples:
CH4, Ar, P4, S8
C. Covalent Crystals• Form 3-D network • Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity• Examples include C, B, Si, SiO2 (quartz)
diamond graphite
D. Metallic Crystal• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricity
Cross Section of a Metallic Crystal
nucleus &inner shell e-
mobile “sea”of e-
VI. Phase ChangesA substance can be converted into different phases by adding Or removing heat
Gas
Liquid
Solid
Tem
pera
ture
End
othe
rmic
Vap
oriz
atio
nM
elti
ng
Con
dens
atio
nF
reez
ing S
ubli
mat
ion
Dep
osit
ion
BeforeEvaporation
At Equilibrium
A. Liquid-Vapor Equilibrium
A. Liquid-Vapor Equilibrium
The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation
Rate ofcondensation
Rate ofevaporation
=
1. Dynamic Equilibrium
2. Equilibrium vapor pressure
a. as T
b. as intermolecular forces
3. Molar heat of vaporization (Hvap) is the energy required to vaporize 1 mole of a liquid.
ln P = -Hvap
RT+ C
Clausius-Clapeyron Equation
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
€
lnP1
P2
=ΔHvapR
1
T2
−1
T1
⎛
⎝ ⎜
⎞
⎠ ⎟
Question: The vapor pressure (VP) of ethanol is 100 mmHg at
34.9oC. What is the VP a 52.6oC? (Hvap = 39.3 kJ/mol)
P1=100 mmHg
T1=34.9oC = 308.1 K T2=52.6oC = 325.8 K
P2=?
€
ln100
P2
=39.3 ×103J /mol
8.314J /K • mol
1
325.8K−
1
308.1K
⎛
⎝ ⎜
⎞
⎠ ⎟= −0.834
€
lnP1
P2
=ΔHvapR
1
T2
−1
T1
⎛
⎝ ⎜
⎞
⎠ ⎟
€
100
P2
= e−0.834 = 0.434P2=230 mmHg
(a) The boiling point is the temperature when the (equilibrium) vapor pressure = the external pressure.
(b) The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.
4. Boiling point
5. Critical point
The critical temperature (Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure.
The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature.
B. Liquid-Solid Equilibrium
Molar heat of fusion (Hfus) is the energy required to melt 1 mole of a solid substance.
C. Solid-Vapor Equilibrium
Molar heat of sublimation (Hsub) is the energy required to sublime 1 mole of a solid.
Solid
Sub
lim
atio
n,
Dep
osit
ion
Gas
Hsub = Hfus + HvapHsub
Hvap
Hfus
Liquid
Heating (cooling curve)
Question: How much heat (in kJ) is needed to convert 80.0 g of ethanol at -130oC to vapor at 96oC? The specific heat of solid, liquid, and vapor ethanol are 0.97, 2.4, and 1.2 J/g.oC, respectively. Hfus= 5.02 kJ/mol,
Hvap= 39.3 kJ/mol. The mp is -114oC and bp is 78oC.
Step 1: heat solid up to mp (-114oC), remember q=smT q1=0.97 J/g.oC x 80.0 g x (-114oC -(-130oC))x 1 kJ/1000J=1.24 kJ
Step 2: melt solid to liquid q2= 80.0g x 1 mol C2H5OH/46.07 g x 5.02 kJ/mol=8.72 KJ
Step 4: convert liquid to vapor q4= 80.0g x 1 mol C2H5OH/46.07 g x 39.3 kJ/mol=68.2 KJ
Step 3: heat liquid from mp (-114oC) to bp (78oC) q3=2.4 J/g.oC x 80.0 g x (78oC -(-114oC))x 1 kJ/1000J=36.9 kJ
Step 5: heat vapor from 78oC to 96oC q5=1.2 J/g.oC x 80.0 g x (96oC -78oC)x 1 kJ/1000J=1.73 kJ
Total heat=q1 +q2 + q3+ q4 + q5 =117 kJ
VII. Phase diagram
Solid
Liquid
Gassublimation
deposition
melting
freezingvaporization
condensation
Critical point
Triple point
Pre
ssur
e, P
Temperature, T
– summarizes the conditions at which a substance exists as a solid, liquid, or gas.
Phase Diagram of Water Phase Diagram of CO2