Chapter 12
Intermolecular Forces: Liquids, Solids,
and Phase Changes
Properties of liquid
Crystal structure
Macroscopic properties of liquid :
Surface tension – is the energy required to increase the surface area by
a unit amount.
Capillarity – is the phenomenon of rising a liquid through the narrow
space against the pull of gravity.
Capillarity results from a competition between
intermolecular forces within the liquid and those between the
molecule and the wall of the tube.
Viscosity – is the resistance to flow. Viscosity results from the
intermolecular attraction resisting the movement
ATTRACTIVE FORCES electrostatic in nature
Intramolecular forces
bonding forces
These forces exist within each molecule.
They influence the chemical properties of the substance.
Intermolecular forces
nonbonding forces
These forces exist between molecules.
They influence the physical properties of the substance.
1
Intermolecular Forces- Van der Waals forces
• Attractive forces between molecules and ions.
• Determine bulk properties of matter.
• Much weaker than intramolecular forces.
• Several types of forces:
– Dipole–dipole
– Dipole induced dipole
– Instantaneous induced dipole (dispersion forces)
– Ion–dipole
– Hydrogen “bonds.”
– Longdon dispersion
• Intermolecular forces are electrical in origin
• These forces result from the mutual attractions of unlike charges.
»
repulsion of like charges.
The major types of intermolecular forces
2
Intermolecular Forces
• Dipole–Dipole: Between polar molecules.
•
Neutral but polar molecules experience dipole-dipole forces.
•
Electrical interactions are among dipoles on neighboring molecules.
•
The forces can be negative or positive based on the orientation.
•
The forces are weak and significant only when the molecules are in close contact.
• Ion–Dipole: Between polar molecules and ions.
•
Ion-dipole forces are electrical interactions.
•
These forces are between ions and the partial charges in polar molecules.
•
The favored orientation of a positive end in a dipole molecules is near the anion.
•
The energy magnitudes depend on the ion charge ion size, dipole moment. E= zµ/r2
3
Intermolecular Forces
• London Dispersion Forces: Attraction is due to instantaneous,
temporary dipoles formed due to electron motions.
•
London dispersion forces arise among non-polar molecules.
•
All molecules and atoms experience London dispersion due to the electron
motion.
•
In symmetrical molecules, there can be more electrons at one end that another,
giving a molecule short-lived dipole.
•
The short-lived dipole will affect the electron distribution and induce temporary
dipole.
Intermolecular Forces
• Hydrogen Bond: Molecules containing N–H, O–H, or F–H
groups, and an electronegative O, N, or F.
•
Hydrogen bond is an attractive interaction between H atom and O,N, F.
•
Hydrogen bond arises because O-H, N-H, H-F bonding are highly polar.
•
H atom has no core electron to shield its nuclei and H has a small size.
4
THE HYDROGEN BOND
a dipole-dipole intermolecular force
A hydrogen bond may occur when an H atom in a molecule, bound to small
highly electronegative atom with lone pairs of electrons, is attracted to the lone
pairs in another molecule.
The elements which are so electronegative are N, O, and F.
..
O
..
O
..
..
H
..
..
F
..
hydrogen bond
acceptor
hydrogen bond
donor
..
N
hydrogen bond
donor
hydrogen bond
acceptor
H
..
F
..
hydrogen bond donor
..
..
N
hydrogen bond
acceptor
H
hydrogen bond
donor
hydrogen bond acceptor
5
Hydrogen bonding and boiling point.
6
SAMPLE PROBLEM
Drawing Hydrogen Bonds Between Molecules of a Substance
PROBLEM:
Which of the following substances exhibits H bonding? For those
that do, draw two molecules of the substance with the H bonds
between them.
O
(a)
PLAN:
(b) CH3OH
(c)
CH3C NH2
Find molecules in which H is bonded to N, O or F. Draw H bonds in
the format -B: H-A-.
SOLUTION:
(a) C2H6 has no H bonding sites.
(c)
H
H C O H
(b)
C2H6
H
H
H
H
H O C H
O
CH3C N H
H
O
H N CH3C
H N
CH3C
CH3C
O
H
N H
O
H
SAMPLE PROBLEM
Predicting the Type and Relative Strength of Intermolecular Forces
PROBLEM:
For each pair of substances, identify the dominant intermolecular
forces in each substance, and select the substance with the higher
boiling point.
(a) MgCl2 or PCl3
(b) CH3NH2 or CH3F
(c) CH3OH or CH3CH2OH
PLAN:
CH3
(d) Hexane (CH3CH2CH2CH2CH2CH3)
CH3CCH2CH3
or 2,2-dimethylbutane
CH3
•
Bonding forces are stronger than nonbonding (intermolecular) forces.
•
Hydrogen bonding is a strong type of dipole-dipole force.
•
Dispersion forces are decisive when the difference is molar mass or molecular
shape.
7
SAMPLE PROBLEM
continued
Predicting the Type and Relative Strength of
Intermolecular Forces
SOLUTION:
(a) Mg2+ and Cl- are held together by ionic bonds while PCl3 is covalently bonded and
the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger
than dipole interactions and so MgCl2 has the higher boiling point.
(b) CH3NH2 and CH3F are both covalent compounds and have bonds which are polar.
The dipole in CH3NH2 can H bond while that in CH3F cannot. Therefore CH3NH2 has
the stronger interactions and the higher boiling point.
(c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH for more
dispersion force interaction. Therefore CH3CH2OH has the higher boiling point.
(d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to
hold the molecules together. Hexane has the larger surface area,
area thereby the greater
dispersion forces and the higher boiling point.
The molecular basis of surface tension.
hydrogen bonding
occurs across the surface
and below the surface
the net vector
for attractive
forces is downward
hydrogen bonding
occurs in three
dimensions
Molecules in the interior of a liquid experience intermolecular attractions in all direction.
Molecules in the surface of a liquid experience intermolecular attractions down.
The molecules tend to decrease the number of molecules in the surface, which result in
the surface tension.
8
Shape of water or mercury meniscus in glass (made of silicon dioxide).
concave meniscus.
convex meniscus
capillarity
stronger cohesive
forces
adhesive forces
Hg
H2O
Water and the glass wall has adhesive forces.
Water molecules can form hydrogen bond.
Adhesive forces are stronger than adhesive forces
between water molecules.
The cohesive force between Hg and inner tube
wall is weaker than the cohesive forces within
Hg molecules.
A convex meniscus occurs when the molecule of
the liquid repels the molecule of the container.
The thin film of water creeps up the wall.
Viscosity of Water at Several Temperatures
viscosity - resistance to flow
Intermolecular forces resist the movement of liquid or gas.
Intermolecular forces operates in the short distance.
Temperature and molecular shape play the important roles.
Temperature( °C)
Viscosity (N*s/m2)*
20
1.00 x 10-3
40
0.65 x 10-3
60
0.47 x 10-3
80
0.35 x 10-3
*The units of viscosity are newton-seconds per square meter.
Viscosity decreases if the temperature increases since molecules move faster
at higher temperature and can overcome the intermolecular forces readily.
Longer molecules have larger viscosity since long molecules make more
contacting surface with each other than the spherical ones.
9
Phase diagrams for CO2 and H2O.
CO2
H2O
The crystal lattice and the unit cell.
A crystal system is described by three basis vectors.
There are seven crystal system - Triclinic, monoclinic, orthorhombic, hexagonal,
rhombohedral, tetragonal, cubic
Triclinic
hexagonal
orthorhombic
monoclinic
rhombohedral
tetragonal
cubic
10
The crystal lattice and the unit cell.
The crystal systems are a grouping of crystal structures according to the axial system used to
describe their lattice.
Each crystal system consists of a set of three axes in a particular geometrical arrangement.
The unit cell is a spatial arrangement of atoms which is tiled in three-dimensional space to
describe the crystal.
The unit cell is given by its lattice parameters, the length of the cell edges and the angles
between them,
lattice point
unit cell
unit cell
portion of a 3-D lattice
portion of a 2-D lattice
The three cubic unit cells.
Simple Cubic
1/8 atom at
8 corners
Atoms/unit cell = 1/8 * 8 = 1
coordination number = 6
11
The three cubic unit cells.
Body-centered
Cubic
1/8 atom at
8 corners
1 atom at
center
Atoms/unit cell = (1/8*8) + 1 = 2
coordination number = 8
The three cubic unit cells.
Face-centered
Cubic
1/8 atom at
8 corners
1/2 atom at
6 faces
coordination number = 12
Atoms/unit cell = (1/8*8)+(1/2*6) = 4
12
The levitating power of a superconducting oxide.
rare earth magnet
superconducting
ceramic disk
liquid nitrogen
Schematic of a liquid
crystal display (LCD).
The hexagonal structure of ice.
SAMPLE PROBLEM - optional
Determining Atomic Radius from Crystal Structure
PROBLEM:
PLAN:
Barium is the largest nonradioactive alkaline earth metal. It has a
body-centered cubic unit cell and a density of 3.62 g/cm3. What is the
atomic radius of barium?
(Volume of a sphere: V = 4/3πr3)
We can use the density and molar mass to find the volume of 1 mol of Ba.
Since 68% (for a body-centered cubic) of the unit cell contains atomic
material, dividing by Avogadro’s number will give us the volume of one
atom of Ba. Using the volume of a sphere, the radius can be calculated.
density of Ba (g/cm3)
radius of a Ba atom
V = 4/3πr3
reciprocal divided by M
volume of 1 mol Ba metal
volume of 1 Ba atom
multiply by packing efficiency
divide by Avogadro’s number
volume of 1 mol Ba atoms
13
SAMPLE PROBLEM - optional
Determining Atomic Radius from Crystal Structure
continued
SOLUTION:
Volume of Ba metal =
37.9 cm3/mol Ba
26 cm3
mol Ba atoms
x
r3 = 3V/4π
1 cm3
3.62 g
x 0.68
x
= 37.9 cm3/mol Ba
mol Ba
= 26 cm3/mol Ba atoms
mol Ba atoms
6.022x1023 atoms
r =3
137.3 g Ba
= 4.3x10-23 cm3/atom
3V 3 3(4.3x10-23 cm3 ) = 2.2 x 10-8cm
=
4π
4 x 3.14
Take-home message
Summary diagram for analyzing the intermolecular forces in a sample.
INTERACTING
INTERACTING PARTICLES
PARTICLES
(atoms,
(atoms, molecules,
molecules, ions)
ions)
ions present
ions
ions only
only
IONIC
IONIC BONDING
BONDING
ions not present
polar
polar molecules
molecules only
only
DIPOLE-DIPOLE
DIPOLE-DIPOLE
FORCES
FORCES
ion
ion ++ polar
polar molecule
molecule
ION-DIPOLE
ION-DIPOLE FORCES
FORCES
nonpolar
nonpolar
molecules
molecules only
only
DISPERSION
DISPERSION
FORCES
FORCES only
only
H bonded to
N, O, or F
HYDROGEN
HYDROGEN
BONDING
BONDING
polar
polar ++ nonpolar
nonpolar
molecules
molecules
DIPOLEDIPOLEINDUCED
INDUCED DIPOLE
DIPOLE
FORCES
FORCES
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