Liquids
1
Molecules at interfaces behave differently than those in the interior.
Molecules at surface experience a net INWARD
force of attraction.
This leads to SURFACE TENSION — the energy
req’d to break through the surface.
2
Surface Tension
SURFACE TENSION also leads to spherical liquid
droplets (shape of minimum surface).
Liquids
3
Intermolec. forces also lead to CAPILLARY
ACTION and to the existence of a concave
meniscus for a water column in a glass tube.
concave
meniscus
H2 O in
glass
tube
ADHESIVE FORCES
between water and glass
(with polar Si-O bonds)
COHESIVE FORCES
between water
molecules
Capillary Action
Cohesive forces against the force of gravity
Movement of water up a piece of paper
depends on H-bonds between H2O and
the OH groups of the cellulose in the
paper.
Problem : Search for applications of capillary action
in nature (plants) and in the lab (chromatography)
4
Liquids
5
High surface tension due to cohesive forces
stronger than adhesive forces with the glass
leads to the existence of a convex meniscus
for a column of mercury in a glass tube.
convex
meniscus
Hg in a glass
ADHESIVE FORCES
between Hg and glass
(with polar Si-O bonds)
COHESIVE FORCES
Non-polar mercury
6
Viscosity
VISCOSITY is the tendency or resistance of
liquids to flow.
Do you expect the viscosity of glycerol to be larger or
smaller than the viscosity of ethanol ?
Ethanol
Glycerol
The resistance to flow results from several factors, including
intermolecular interactions, molecular shape and size.
Metallic and Ionic Solids
7
Sections 13.6-8
Solid-state chemistry is one of the booming areas of science, leading
to the development of interesting new materials.
Types of Solids
8
Table 13.6
TYPE
Composition
Ionic NaCl, CaF2, ZnS
Metallic
Na, Fe
Molecular
Ice, I2
Network
BINDING FORCES
Ion-ion
Metallic
Dipole
Ind. dipole
Diamond
Graphite
Amorphous Glass, polyethylene
Extended
covalent
Covalently bonded
Networks with no
Long-range
Regularity.
Network Solids
Diamond
Graphite
9
Network Solids
A comparison of diamond (pure carbon)
with silicon.
10
Properties of Solids
1. Molecules, atoms or ions
locked into a CRYSTAL
LATTICE
2. Particles are CLOSE
together
3. STRONG IM forces
4. Highly ordered, rigid,
incompressible
5. No translations (only
vibrations, or rotations on
lattice sites)
ZnS, zinc sulfide
11
Crystal Lattices
• Regular 3-D arrangements of equivalent
LATTICE POINTS in space.
• Lattice points define UNIT CELLS
– smallest repeating internal unit that has the symmetry
characteristic of the solid.
12
Cubic Unit Cells
There are 7 basic crystal systems, but we are
only concerned with CUBIC.
All sides
equal length
All angles
are 90 degrees
13
Cubic Unit Cells of Metals
Figure 13.24
Simple
cubic (SC)
1 atom/unit cell
Bodycentered
cubic (BCC)
2 atoms/unit cell
Facecentered
cubic (FCC)
4 atoms/unit cell
14
Units Cells for Metals
Figure 13.25
15
Atom Packing in Unit Cells
Assume atoms are hard spheres and that crystals are built
by PACKING of these spheres as efficiently as possible.
16
Number of Atoms per Unit Cell
Unit Cell Type
SC
BCC
FCC
Net Number Atoms
1
2
4
17
18
Atom Sharing
at Cube Faces and Corners
Atom shared in corner
--> 1/8 inside each unit cell
Atom shared in face
--> 1/2 inside each unit cell
Simple Ionic Compounds
CsCl has a SC lattice of
Cs+ ions with Cl- in the
center.
1 unit cell has 1 Cl- ion
plus
(8 corners)(1/8 Cs+ per
corner)
= 1 net Cs+ ion.
19
Simple Ionic Compounds
Salts with formula
MX can have SC
structure — but
not salts with
formula MX2 or
M2X
20
21
Two Views of CsCl Unit Cell
Either arrangement leads to formula of 1 Cs+ and 1 Cl- per unit cell
NaCl Construction
FCC lattice of Cl- with
Na+ in holes
Na+ in
octahedral
holes
22
The Sodium Chloride Lattice
Many common salts have FCC arrangements
of anions with cations in OCTAHEDRAL
HOLES — e.g., salts such as CA = NaCl
• FCC lattice of anions ----> 4 A-/unit cell
• C+ in octahedral holes ---> 1 C+ at center
+ [12 edges • 1/4 C+ per edge]
= 4 C+ per unit cell
23
24
Comparing NaCl and CsCl
• Even though their formulas have one
cation and one anion, the lattices of
CsCl and NaCl are different.
• The different lattices arise from the fact
that a Cs+ ion is much larger than a Na+
ion.
Phase Diagrams
25
Lines connect all conditions of T and P where EQUILIBRIUM
exists between the phases on either side of the line.
Phase Equilibria — Water
Solid-liquid
26
Gas-Liquid
Gas-Solid
27
Phases
Diagrams—
Important Points for
Water
Normal boil point
T(˚C)
P(mmHg)
100
760
Normal freeze point 0
760
Triple point
4.58
0.0098
Solid-Liquid Equilibria
In any system, if you increase P the DENSITY
will go up.
Therefore — as P goes up, equilibrium favors
phase with the larger density (or SMALLER
volume/gram).
Liquid H2O
Solid H2O
Density
1 g/cm3
0.917 g/cm3
cm3/gram
1
1.09
28
Solid-Liquid Equilibria
29
Raising the pressure at
constant T causes
water to melt.
The NEGATIVE SLOPE
of the S/L line is
unique to H2O.
Almost everything
else has positive
slope.
30
Solid-Vapor Equilibria
At P < 4.58 mmHg and T < 0.0098 ˚C
solid H2O can go directly to vapor. This
process is called SUBLIMATION
This is how a frost-free refrigerator works.