Intermolecular Forces and
Liquids and Solids
Chapter 11
Copyright © The McGraw­Hill Companies, Inc. Permission required for 1
A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well­defined boundary.
2 Phases
Solid phase ­ ice
Liquid phase ­ water
11.1
2
Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O­H bonds in 1 mole of water (intra)
“Measure” of intermolecular force
Generally, boiling point
intermolecular melting point
forces are much ∆Hvap
weaker than ∆Hfus
intramolecular ∆Hsub
forces.
11.2
3
Intermolecular Forces
Dipole­Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Nonpolar molecules do not have dipole dipole forces.
11.2
4
Intermolecular Forces
Ion­Dipole Forces
Attractive forces between an ion and a polar molecule
Pretty strong attractive force. This is the attractive between water and ionic solute.
Ion­Dipole Interaction
11.2
5
Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
Present in all molecules, but the only intermolecular force in NONPOLAR MOLECULES, or in Gaseous atoms.
ion­induced dipole interaction
dipole­induced dipole interaction
11.2
He
He
the temporary dipole induces a temporary dipole in a neighboring He atom
both electrons on the same side atom set up a TEMPORARY DIPOLE.
6
Intermolecular Forces
Dispersion Forces Continued
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
• greater number of electrons
• more diffuse electron cloud
Dispersion forces usually increase with molar mass.
11.2
7
What type(s) of intermolecular forces exist between each of the following molecules?
HBr
HBr is a polar molecule: dipole­dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2 O
S
O
SO2 is a polar molecule: dipole­dipole forces. There are also dispersion forces between SO2 molecules.
11.2
8
Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole­dipole interaction between they hydrogen atom in a polar N­H, O­H, or F­H bond and an electronegative O, N, or F atom.
or
A H …B
A H …A
A & B are N, O, or F
11.2
9
Why is the hydrogen bond considered a “special” dipole­dipole interaction?
Decreasing molar mass
Decreasing boiling point
11.2
10
11
Rank the following in order of increasing boiling point:
HBr CH3OH
CH2O
NaCl
CH3CH3
He
CH4
12
Properties of Liquids
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Strong intermolecular forces
High surface tension
11.3
13
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
11.3
14
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong intermolecular forces
High viscosity
11.3
15
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
11.3
16
A crystalline solid possesses rigid and long­range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions.
An amorphous solid does not possess a well­defined arrangement and long­range molecular order.
A unit cell is the basic repeating structural unit of a crystalline solid.
At lattice points:
• Atoms
• Molecules
• Ions
lattice
point
Unit Cell
Unit cells in 3 dimensions
11.4
17
11.4
18
11.4
19
11.4
20
11.4
21
Shared by 8 unit cells
Shared by 2 unit cells
11.4
22
1 atom/unit cell
(8 x 1/8 = 1)
2 atoms/unit cell
(8 x 1/8 + 1 = 2)
4 atoms/unit cell
(8 x 1/8 + 6 x 1/2 = 4)
11.4
23
11.4
24
When silver crystallizes, it forms face­centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver.
m
V = a3 = (409 pm)3 = 6.83 x 10­23 cm3
d = V
4 atoms/unit cell in a face­centered cubic cell
m = 4 Ag atomsx
1 mole Ag
107.9 g
x
= 7.17 x 10­22 g
23
mole Ag 6.022 x 10 atoms
7.17 x 10­22 g
m
=
d = = 10.5 g/cm3
­23
3
V
6.83 x 10 cm
11.4
25
11.5
26
Extra distance = BC + CD =2d sinθ = nλ (Bragg Equation)
11.5
27
X rays of wavelength 0.154 nm are diffracted from a crystal at an angle of 14.170. Assuming that n = 1, what is the distance (in pm) between layers in the crystal?
nλ = 2d sin θ
d =
n = 1
θ = 14.170 λ = 0.154 nm = 154 pm
nλ
1 x 154 pm
= 77.0 pm
2 x sin14.17
2sinθ
=
11.5
28
Types of Crystals
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
CsCl
ZnS
CaF2
11.6
29
Types of Crystals
Covalent Crystals
• Lattice points occupied by atoms
• Held together by covalent bonds
• Hard, high melting point
• Poor conductor of heat and electricity
carbon
atoms
diamond
graphite
11.6
30
Types of Crystals
Molecular Crystals
• Lattice points occupied by molecules
• Held together by intermolecular forces
• Soft, low melting point
• Poor conductor of heat and electricity
11.6
31
Types of Crystals
Metallic Crystals
• 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
11.6
A metallic bond is the attractive force between the nucleus of the metal atoms and the "sea" of mobile electrons.
32
Types of Crystals
11.6
33
An amorphous solid does not possess a well­defined arrangement and long­range molecular order.
A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing
Crystalline
quartz (SiO2)
Non­crystalline
quartz glass
11.7
34
Least
Order
Condensation
Evaporation
T2 > T1
Greatest
Order
11.8
35
The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation
H2O (l) H2O (g)
Dynamic Equilibrium
Rate of
=
condensation
Rate of
evaporation
11.8
36
Before
Evaporation
At Equilibrium
11.8
37
Molar heat of vaporization (∆Hvap) is the energy required to vaporize 1 mole of a liquid.
Clausius­Clapeyron Equation
∆Hvap
ln P = ­
+ C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
11.8
38
The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure.
The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.
11.8
39
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.
11.8
40
H2O (s) H2O (l)
The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium
Freezing
Melting
11.8
41
Molar heat of fusion (∆Hfus) is the energy required to melt 1 mole of a solid substance.
11.8
42
11.8
43
H2O (s) H2O (g)
Molar heat of sublimation (∆Hsub) is the energy required to Sublimation
sublime 1 mole of a solid.
Deposition
∆Hsub = ∆Hfus + ∆Hvap
( Hess’s Law)
11.8
44
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.8
45
11.8
46
47