Chapter 10
Liquids, Solids and
Phase Changes
Chapter 10
1
KMT of Liquids and Solids
• Gas molecules have little or no interactions.
• Molecules in the Liquid or solid state have significant
interactions.
• Liquids and solids have well-defined volume.
• Liquid molecules “flow,” while solids are held “rigid.”
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Properties of Liquids
1. Liquids have a variable shape, but a fixed volume.
2. Liquids usually flow readily.
•
However, not all liquids flow at the same rate.
3. Liquids do not compress or expand significantly
•
The volume of a liquid varies very little as the temperature
and pressure change.
4. Liquids have a high density compared to gases.
•
Liquids are about 1000 times more dense than gases.
5. Liquids that are soluble mix homogeneously.
•
Liquids diffuse more slowly than gases but eventually will
form a homogeneous mixture.
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Properties of Solids
1.
Solids have a fixed shape and volume.
•
2.
Solids are either crystalline or non-crystalline.
•
3.
Assuming no change in physical state, temperature and pressure have
a negligible effect on the volume of a solid.
Solids have a slightly higher density than their corresponding
liquid
•
5.
Crystalline solids contain particles in a regular, repeating pattern.
Solids do not compress or expand to any degree
•
4.
Unlike liquids, solids are rigid.
One important exception is water; ice is less dense than liquid water.
Solids do not mix by diffusion
•
Chapter 10
The particles are not free to diffuse in a solid heterogeneous mixture.
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2
Intermolecular Force Concept
• An intermolecular force is an attraction between molecules
– Intramolecular bonds occur between atoms within a
molecule.
• Intermolecular forces are much weaker than intramolecular
bonds
• These forces are due to dipole moments within the molecules
• There are three main intermolecular forces:
Dipole forces
Hydrogen bonds
Chapter 10
Dispersion forces
5
Dipole Moments
• Polar covalent bonds form between atoms of
different electronegativity.
• This is described as a bond dipole.
• The figure with the electronegativity values in in
CHP 7 (pg. 248)
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3
Rules for Determining
the Polarity of Molecules
Rule 1: If the central atom has an odd number of lone pairs, the
molecule is polar
• One exception is a Linear Molecule (MG) that has a
Trigonal Bipyramidal EPA
Rule 2: If there are no lone pairs and the central atom is bound to
only one type of atom (for example, CH4) then the molecule
is non-polar.
Rule 3: If there are no lone pairs on the central atom, but it is
surrounded by more than one type of atom (for example,
CH3Cl), you must look at the shape of the molecule.
• Linear, Trigonal planar & Tetrahedral = Polar
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• Trigonal Bipyramidal & Octahedral = Look at structure
Dipole Moments
• Dipole Moment (µ): The measure of net
molecular polarity or charge separation.
µ=Q•r
r = distance between charges
δ+ = Q, δ– = –Q
• Dipole moments are expressed in debyes (D)
1 D = 3.336 x 10–30 C·m
• A proton and electron separated by 100 pm have µ = 4.80 D (This is
the dipole moment for a fully IONIC bond!)
Chapter 10
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4
Dipole Moments
Chapter 10
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Dipole Moments
• Which of the following compounds will have a
dipole moment? Show the direction of each.
SO2
Chapter 10
NH3
CF4
TeH4
PF6– XeOF4
AlCl3
BF4– SiCl4
ICl4–
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Dipole Forces
• Two types of Dipole Forces:
Dipole-Dipole
Interactions
Ion-Dipole
Interactions
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Dipole Forces and the Boiling Point
• For polar molecules, the dipole-dipole attractions influence
temperatures at which state changes occurs
– In particular, the boiling point of the liquid
• As the temperature of a substance is increased, what do you
think happens to the molecules?
• Eventually, the molecules will gain enough Kinetic energy
to override the intermolecular forces and escape their state
and move into gaseous state.
• The strength of the IMFs is directly related to the
temperature at which this occurs. HOW?
Chapter 10
IMFs ∝ Boiling Point
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6
London Dispersion Forces
• London Dispersion Forces are dipole attractions that result
from the formation of instantaneous, temporary dipoles in
non-polar molecules due to electron motion.
• In a molecule, electrons are constantly
orbiting the nucleus and a region may
become temporarily electron poor and
slightly positive while another region
becomes slightly negative.
• This creates a temporary dipole and
two molecules with temporary dipoles
are attracted to each other.
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Hydrogen Bonds
• Hydrogen bonds are present when a molecule
has an N-H, O-H, or F-H bond.
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Hydrogen Bonds
• Hydrogen bonds are particularly important to
your DNA and protein structure and in water
Chapter 10
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How to Determine What Type of
Intermolecular Forces are Present
When trying to determine what type of intermolecular forces are
involved in the attraction between two molecules ask yourself the
following questions:
1) Is there any potential for Hydrogen bonding between the two
molecules?
o In other words, look for O-H, N-H, or F-H groups
2) Is one or both of the molecules polar (or an ion)?
o If both, then usually dipole-dipole (or ion-dipole) interactions
o If one is polar and the other is nonpolar, then should be dipole-induced
dipole interaction
3) If both molecules are nonpolar, look and see if one molecule is
charged.
o If yes, then should be an induced dipole interaction
o If no, then London dispersion force interaction
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8
Intermolecular Forces
Chapter 10
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Phase Changes
• Heat is necessary to
raise the temperature
and change the
physical state of a
substance.
• Specific heat is the
amount of heat
required to raise 1.00 g
of a substance 1°C.
• Liquid water is the reference and its specific heat is
4.184 J/(g×°C)
• The specific heats of ice and steam are about half that
of liquid water.
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9
Heating and Cooling Curves
• We can graph the amount of energy required to change
the temperature and physical state of a substance.
Hvap
Hcon
Hfus
Hsol
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Energy from Heating Curves
• We can use the energy curves and Enthalpy values for a
molecule to calculate how much energy is required to
change the temperature and/or state of a sample.
• These problems can be broken into two types of
calculations:
1: The amount of energy required to raise the temperature:
heat = (Specific Heat) × (∆T) × (m)
2: The amount of energy required to change the state:
heat = (Hxxx) × (m)
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10
Energy Calculation Problem
•
Calculate the amount of energy (kJ) needed to heat 346
g of liquid water from 0°C to 182°C. Assume that the
specific heat of water is 4.184 J/g·°C over the entire
liquid range and that the specific heat of steam is 1.99
J/g·°C.
•
The molar heats of fusion and sublimation of molecular
iodine are 15.27 kJ/mol and 62.30 kJ/mol, respectively.
Estimate the molar heat of vaporization.
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Physical Properties of Liquids
• There are four physical properties of liquids
that we can relate to their intermolecular
attractions :
Vapor Pressure
Boiling Point
Viscosity
Surface tension
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Vapor Pressure
• At the surface of a liquid, some molecules gain enough energy
to escape the intermolecular attractions of neighboring
molecules and enter the vapor state. This is evaporation.
• The reverse process is condensation.
• When the rates of evaporation and condensation are equal, the
pressure exerted by the gas molecules above a liquid is called
the vapor pressure.
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Vapor Pressure Trend
• The stronger the intermolecular forces between the molecules in
the liquid, the less molecules escape into the gas phase.
Intermolecular Forces are indirectly
proportional to the Vapor Pressure
• As the attractive force between molecules increases, vapor pressure
decreases.
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Vapor Pressure vs. Temperature
• As the temperature
increases, the vapor
pressure of a liquid
increases.
They are directly proportional
• Again, the stronger the
intermolecular attractions,
the lower the vapor
pressure at a given
temperature.
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Vapor Pressure
• Clausius–Clapeyron Equation: Provides a link between vapor
pressure (P), temperature (T), and molar heat of vaporization
(∆Hvap).
⎛ ∆H vap ⎞⎛ 1 ⎞
ln Pvap = ⎜ −
⎟⎜ ⎟ + C
R
⎝
⎠⎝ T ⎠
y
=
m
x
+ b
• By taking measurements
at two temps, we get:
ln
P1 ∆H vap ⎛ 1 1 ⎞
⎜ − ⎟
=
P2
R ⎜⎝ T1 T2 ⎟⎠
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Vapor Pressure
• Ethyl ether is a volatile, highly flammable
organic liquid that is used mainly as a solvent.
The vapor pressure of ethyl ether is 401 mm Hg
at 18°C. Calculate its vapor pressure at 32°C.
• The vapor pressure of ethanol is 100 mm Hg at
34.9°C. What is its vapor pressure at 63.5°C?
(∆Hvap for ethanol is 39.3 kJ/mol)
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Boiling Point
• The normal boiling point of a substance is the
temperature where the vapor pressure is equal to 1
atm.
• The stronger the intermolecular attractions, the
higher the boiling point of the liquid.
Intermolecular Forces ∝ Boiling Point
Vapor Pressure ∝
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Boiling Point
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Viscosity
• The viscosity of a liquid is a liquid’s resistance to flow.
– Viscosity is the result of an attraction between
molecules.
• The stronger the intermolecular forces, the higher the
viscosity.
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Surface Tension
• The attraction between molecules at the surface of a
liquid is called surface tension.
• The stronger the intermolecular attractions, the
stronger the surface tension of a liquid.
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The Solid State
• Solids are divided into two categories:
Crystalline: Rigid and long-range order
Amorphorous: Lacks well-defined arrangement
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Crystalline Solids
• Crystalline solids can be classified in two ways:
– The arrangement of the particles (Crystal Lattice)
– The types of forces that hold the particles together
• Structure of a crystalline solid is based on the unit cell, a basic
repeating structural unit.
• Cell Types of crystal lattices based on a cube include:
–
–
–
–
–
Simple Cubic (SC)
Body-centered Cubic (BCC)
Face-centered Cubic (FCC)
Hexagonal or Cubic Close Packing
There are many others!
• The number of particles touching each atom (the coordination
number) and the number of atoms in each cell depends on the cell type
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Crystalline Solids
• To determine the number of atoms in a unit cell, remember
the following:
• An atom is considered a corner, edge, face or center.
– Corner = 1/8 in unit cell
– Edge = ¼ in unit cell
– Face = ½ in unit cell
– Center = 1 in unit cell
• The number of particles touching each atom (the
coordination number) and the number of atoms in each
cell depends on the cell type
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Crystalline Solids
Simple Cube and Body-Centered Cube:
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Crystalline Solids
Simple Cube and Body-Centered Cube:
Coordination # # atoms in cell
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SC
6
1
BCC
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Crystalline Solids
Hexagonal Close Packing Arrangements (a-b-a-b):
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Crystalline Solids
Cubic Close Packing Arrangements (a-b-c-a-b-c):
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Crystalline Solids
Face-Centered Cube:
Coordination # # atoms in cell
Chapter 10
FCC
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Crystalline Solids
Chapter 10
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Density of Crystalline Solids
Silver metal crystallizes in a
FCC arrangement with the edge
of the unit cell having a length of
d = 407 pm. What is the radius
(in picometers) of a silver atom?
Nickel has a FCC arrangement with the edge of the unit cell
having a length of d = 352.4 pm. What is the density of the
Nickel in g/cm3?
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Density of Crystalline Solids
Sodium has a density of 0.971 g/cm3 and crystallizes with
a body-centered cubic unit cell. What is the radius of a
sodium atom, and what is the edge length of the cell (in
pm)?
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Ionic Solids
• A crystalline ionic solid is composed of cations and anions
arranged in a regular, repeating pattern.
– Usually a Face-centered cubic arrangement but not always
– The face positions are occupied by one of the ions, with the other ion
filling in the holes as needed.
– The ratio of cations to anions in a unit cell must be consistent with the
ionic formula for the compound
– Ionic solids are very hard and brittle with high melting points
– They are also good conductors of electricity but only in their aqueous or
liquid state
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Ionic Solids
Chapter 10
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Molecular Solids
• A crystalline molecular
solid has molecules
arranged in a particular
conformation.
– A separate water molecule is
found at each lattice point
• The solids are held together by intermolecular forces
• These compounds are fragile (compared to ionic
compounds) with melting points dependent on the
intermolecular forces holding them together.
• They do not conduct electricity
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Covalent Network Solids
• A crystalline covalent
network solid has
molecules arranged in
a particular
conformation.
– These molecules are
held together by
covalent bonds
– They are generally hard
and high melting
– They do not conduct
electricity
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Metallic Crystals
• A crystalline metallic solid is
composed of metal atoms
arranged in a definite pattern.
– BCC, FCC or Hex Arrangement
– A metallic crystal is made up of
positive metal ions surrounded
by a sea of valance electrons.
– Metallic solids are good
conductors of electricity because
electrons are free to move about
the crystal.
• Most metals are malleable and ductile as well.
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Phase Diagrams
• A Phase Diagram is a graphical display of the
temperatures and pressures at which two phases of a
substance are in equilibrium.
– Triple Point: The only
condition under which all
three phases can be in
equilibrium with one another.
– Critical Temperature (Tc):
The temperature above which
the gas phase cannot be made
to liquefy at any pressure.
– Critical Pressure (Pc): The
minimum pressure required
to liquefy a gas at its critical
temp.
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Phase Diagrams
Water
Chapter 10
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Phase Diagrams
•
Approximately, what is the normal boiling
point and what is the normal melting point of
the substance?
•
What is the
physical state
when:
A) T = 150 K, P = 0.5 atm
B) T = 325 K, P = 0.9 atm
C) T = 450 K, P = 165 atm
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