Crystalline solids
• Same lattice can be used to
describe many different
designs
• For designs based on the fcc
unit cell: lattice points,
empty spaces, edge lengths
• The only requirement is that
the same design element
must be associated with
each lattice point
Chloride ions in NaCl are associated with the lattice
points of a fcc unit cell. The sodium ions are placed
between the chloride ions.
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
12.6
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
Graphite is good conductor of electricity
Diamond.
diamond
graphite
12.6
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
Graphite is good conductor of electricity
Graphite.
diamond
graphite
12.6
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
Sulfur.
S8
12.6
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
nucleus &
inner shell e-
Copper.
mobile “sea”
of e-
12.6
Types of Crystals
12.6
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. The word glass is
often used as a general term to refer to any amorphous solid.
Melted material that is cooled very slowly has a better chance at forming a
crystalline solid.
Crystalline
quartz (SiO2)
Non-crystalline
SiO2 glass
12.7
Phase Changes
Energy Changes Associated with Changes of State
•
Heat of Fusion: Energy required to change a solid at its melting
point to a liquid.
•
Heat of Vaporization: Energy required to change a liquid at its
boiling point to a gas.
Energy Changes Associated with Changes of State
•
•
Heat of Fusion: Energy required to change a solid at its melting point to a liquid.
Heat of Vaporization: Energy required to change a liquid at its boiling point to a gas.
Why are the heat of vaporization (ΔHvap) values generally larger?
In the transition from the liquid to the vapor state, all intermolecular
forces must be broken. In melting, many of these interactions remain.
Energy Changes Associated with Changes of State
•
•
The heat added to the system at the melting and boiling points goes into pulling the molecules
farther apart from each other.
The temperature of the substance does not rise during the phase change.
1. Put energy in to get to transition temperature
2. Put more energy in to make the phase transition
Vapor Pressure
•
•
At any temperature, some molecules in a liquid have enough energy to escape.
As the temperature rises, the fraction of molecules that have enough energy to
escape increases.
•
As more molecules escape the liquid, the pressure they exert increases.
The liquid and vapor reach a state of dynamic equilibrium: liquid molecules
evaporate and vapor molecules condense at the same rate.
•
•
•
Vapor pressure increases with temperature.
The boiling point of a liquid is the
temperature at which its vapor pressure
equals atmospheric pressure.
The normal boiling point is the temperature
at which its vapor pressure is 760 torr.
At the top of Mt. Everest, where the atmospheric pressure is roughly
1/3 that at sea level, the boiling point of water is roughly 70oC and the
normal boiling point of water is 100oC.
At standard atmospheric pressure, ethylene glycol boils at a much higher
temperature than diethyl ether. Why?
Molar heat of vaporization (DHvap) is the energy required to vaporize 1
mole of a liquid; a measure of how strongly molecules are held in a liquid.
ΔHvap is determined by measuring the vapor pressure of a liquid at different
temperatures.
Clausius-Clapeyron Equation
ln P = -
DHvap
RT
+C
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
12.8
Molar heat of vaporization (DHvap) is the energy required to vaporize 1
mole of a liquid; a measure of how strongly molecules are held in a liquid.
ΔHvap is determined by measuring the vapor pressure of a liquid at different
temperatures.
Clausius-Clapeyron Equation
ln P = -
ln
P1
P2
DHvap
RT
=
+C
DHvap
R
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
(1/T2 – 1/T1)
12.8
ln
P1
=
P2
DHvap
R
(1/T2 – 1/T1)
Diethyl ether is a volatile, highly flammable organic liquid
that is used mainly as a solvent. The vapor pressure of
diethyl ether is 401 mmHg at 18oC. Calculate its vapor
pressure at 32oC.
• Convert temperatures from Celsius to Kelvin because the
gas constant has units of J mol-1 K-1.
• Plug and chug.
ln
401 mmHg
P2
401 mmHg
26,000 J mol-1
=
(1/305 K – 1/291 K)
-1
-1
8.314 J mol K
=
e
-0.493
P2
P2 = 657 mmHg
12.8
Increasing the pressure of a gas can cause it to condense (liquify);
decreasing the temperature can also cause condensation.
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.
12.8
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
H2O (s)
H2O (l)
12.8
Molar heat of fusion (DHfus) is the energy required to melt
1 mole of a solid substance.
12.8
Molar heat of sublimation (DHsub) is the energy required to sublime 1
mole of a solid.
H2O (s)
DHsub = DHfus + DHvap
H2O (g)
(Hess’s Law)
12.8
Phase Diagrams
Phase diagrams display the state of a substance at various pressures
and temperatures and the places where equilibria exist between phases.
Phase Diagrams
• The AB line is the liquid-vapor interface.
• It starts at the triple point (A), the point at which all three states are in
equilibrium.
Phase Diagrams
Each point along this line is the boiling point of the substance at that
pressure.
Phase Diagrams
• The AD line is the interface between liquid and solid.
• The melting point at each pressure can be found along this line.
Phase Diagrams
• Below A the substance cannot exist in the liquid state.
• Along the AC line the solid and gas phases are in equilibrium;
the sublimation point at each pressure is along this line.