1. Intermolecular Forces and
Liquids and Solids
Chapter 11
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Acknowledgement
Thanks to The McGraw-Hill Companies, Inc. for allowing usage in the classroom.

2. A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1

3. Kinetic-Molecular Description
Intermolecular forces are attractive forces between molecules
Intramolecular forces hold atoms together in a molecule. (covalent bond)
Intermolecular vs Intramolecular
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 force
boiling point
melting point
DHvap
DHfus
DHsub
Generally, intermolecular forces are much weaker than intramolecular forces.(only about 15% as strong)
11.2

4. Q+Q- E = k r Types of Intermolecular Forces (IMF)
• Should actually be called Interparticulate Forces (molecules, ions, and/or atoms)
• Ion - ion forces
• Ion-dipole forces
• Dipole-dipole forces
• Dispersion Forces
• Hydrogen Bonds
IMF determines the boiling point (b.p.), melting point(m.p.), vapor pressure, solubility, viscosity, surface tension, etc.
Greater IMF greater b.p., m.p., solubility, surface tension , and viscosity; lower vapor pressure.
Ion - ion forces: (lattice energy-ionic compound)
• Remember Chapter 9
• Force depends on the charge on the ions and
the distance separating the ions
• (STRONG FORCES)
E = k
Q+Q- r

5. Intermolecular Forces
1. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Example: ions in solution
Ion-Dipole Interaction
11.2

6. The strength of the interaction depends on the charge and size of the ion and on the magnitude of the dipole moment and size of the molecule.
Water molecules
Higher charge, smaller size (that is, higher charge density)  strong interaction
11.2

7. Intermolecular Forces
2. Dipole-Dipole Forces (permanent dipole moment)
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2

8. Intermolecular Forces
3. Temporary dipoles (Dispersion forces)
What attractive force occurs in nonpolar substances?
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules; based on molar mass; the weakest intermolecular force.
– Ion induced
– Dipole induced
– Instantaneous dipole
ion-induced dipole interaction
The electron distribution of atom is distorted by the force exerted by the ions or polar molecules.
dipole-induced dipole interaction
11.2

9. Polarizability increases with: greater number of electrons
The likelihood of a dipole moment being induced depends not only on the charge of the ion or the strength of the ddipole but also on the polarizability of the atom or molecules.
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 (more electrons), or size of the atom.
Melting point increases as the number of electrons in the molecule increase.
11.2

10. London Dispersion Force (Dispersion force)
Polarizability allows gases containing atoms or nonpolar molecules (e.g.He,N2) to condense.
Induced dipoles interacting with each other. This type of interaction produces dispersion forces, which arise as a result of temporary dipoles induced in atoms or molecules, is responsible for the condensation of nonpolar gases.
**Dispersion force exists between all species as long as they have mass.
At any instant it is likely that the atom has a dipole moment created by a specific positions of electrons. This dipole moment is called instantaneous dipole.

11. 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 molecules.
CH4
CH4 is nonpolar: dispersion forces. S O
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
11.2

12. Intermolecular Forces
4. Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B or
A & B are N, O, or F
11.2

13. Hydrogen Bonds:
• Strength of H bonds: up to 40 kJ/mol
• Lots of H bonds = strong
• compare with strength of typical covalent bonds:
250 kJ/mole)

14. Why is the hydrogen bond considered a “special” dipole-dipole interaction? Why water has a high boiling point?
Decreasing molar mass
Decreasing boiling point
11.2

16. Strong intermolecular forces
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
The intermolecular attraction tend to pull molecules into liquids and cause the surface to tighten like a plastic film.
11.3

17. Strong intermolecular forces
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong intermolecular forces
High viscosity
CH2-OH
CH-OH
11.3

18. Solid---crystal structure
A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. (e.g. NaCl)
An amorphous solid does not possess a well-defined arrangement and long-range molecular order. (e.g. glass)
Structures of crystalline solids:
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

19. Figure 11.15

20. 11.4

21. Corner atom edge atom face centered atom
Shared by 2 unit cells
Shared by 8 unit cells
Shared by 4 unit cells
11.4

22. 1 atom/unit cell
2 atoms/unit cell
4 atoms/unit cell
(8 x 1/8 = 1)
(8 x 1/8 + 1 = 2)
(8 x 1/8 + 6 x 1/2 = 4)
11.4

23. 11.4

24. 4 atoms/unit cell in a face-centered cubic cell
When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver.
d = m V
V = a3
= (409 pm)3 = 6.83 x cm3
4 atoms/unit cell in a face-centered cubic cell
107.9 g
mole Ag x
1 mole Ag
6.022 x 1023 atoms x
m = 4 Ag atoms
= 7.17 x g
d = m V
7.17 x g
6.83 x cm3 =
= 10.5 g/cm3
11.4

25. 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

26. 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
sp2
sp3
diamond
graphite
allotrope
11.6

27. 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

28. Cross Section of a Metallic Crystal
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

29. amorphous solid
An amorphous solid does not possess a well-defined arrangement and long-range 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)
Non-crystalline
quartz glass
11.7

30. 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

31. On top of a mountain
• Water boils at a lower temp. on top of a mountain than at sea level.
• Why? because the external pressure (atmospheric pressure) is less on top of a mountain.
In an autoclave
• An autoclave is used to sterilize medical instruments
• The pressure is often 2 atmospheres.
• Will water inside the autoclave boil at 100°C?

32. Vapor Pressure Versus Temperature
Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point.
ln P = -
DHvap RT
+ C
Clausius-Clapeyron Equation
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
Vapor Pressure Versus Temperature
11.8

33. Molar heat of fusion (DHfus) is the energy required to melt 1 mole of a solid substance at its freezing point.
11.8

34. Heating Curve
• Heat of fusion = energy needed to convert a solid to a liquid
• Heat of vaporization = energy needed to
convert a liquid to a gas
• Be able to draw a heating curve
Energy is needed to
• heat a solid
• heat a liquid
• heat a gas
• convert a solid to a liquid
• convert a liquid to a gas
• convert a solid to a gas
11.8

35. Phase Diagram of Water phase diagram
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.
Phase Diagram of Water
Solid line:
Two phases are in equilibrium
Triple point
All three phases are in equilibrium
11.9