Chapter 11: Liquids & Solids
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Transcript Chapter 11: Liquids & Solids
Chapter 11: Liquids & Solids
The molecular compounds like water, ammonia, and
carbon dioxide have different physical properties because
of the intermolecular forces.
Comparison of all three phases:
Liquids & Solids
Liquids & Solids
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State Shape Volume
Solid
fixed
fixed
Liquid indefinite fixed
Gas indefinite indefinite
Density
high
high
low
No No
No Yes
Yes Yes
very strong
intermediate
weak
Compressibility
Changes of State
Changes in state can be induced by a change in
temperature or pressure.
Intermolecular Forces
Forces between molecules.
Always LESS in energy than actual bond.
The attractive force between two HCl molecules is
about 16 kJ/mol.
The bond dissociation energy of the HCl bond is
about 431 kJ/mol.
Intermolecular Forces
One method to compare the strength of
intermolecular forces is to examine the substance’s
boiling point.
When the forces are relatively weak, then the boiling
point is small.
Ex) HCl, bp = -85oC.
There are three main types of intermolecular forces
between neutral substances.
Intermolecular Forces
In the LS packet, we
identified molecules
as being polar or
non-polar based on
shape, types of
atoms, etc.
Polar molecules have
a dipole – that is a
positive and negative
end.
Intermolecular Forces
Thus, the first type of force is called Dipole-Dipole
(or DD) forces and occurs for any polar molecule.
The larger the dipole moment, the more DD forces.
Intermolecular Forces
If you cool any non-polar molecule or atom to a low
enough temperature, then it will liquefy.
Yet, these have no reason to be attractive to each
other.
Fritz London first proposed a theory in 1930.
On average, electrons in an atom like He are evenly
distributed.
But, in one INSTANT, the two electrons may both be
on the same side.
Intermolecular Forces
Thus, in that one INSTANT, a He atom would have
an instantaneous dipole.
This is called the London Dispersion (or LD) force.
Intermolecular Forces
Since all molecules have electrons, they all have a LD
force.
The polarizability of an atom or molecules electrons
depends on two factors.
The number of electrons. More electrons = More LD
forces.
2. The shape of molecule. More spread out = more LD
forces.
1.
Intermolecular Forces
Non-polar Alkanes
Intermolecular Forces
The formula C5H12 has three structural isomers.
CH3 – CH2 – CH2 – CH2 – CH3
CH3
CH3 – C – CH3
CH3
CH3
CH3 – CH2 – CH – CH3
Non-polar Branched Alkanes
Name
Molar Mass
Boiling Point
Pentane
72.15 g/mol
36.1oC
Methylbutane
72.15 g/mol
27.7oC
Dimethylpropane
72.15 g/mol
10oc
Butane
58.12 g/mol
-0.5oC
Methylpropane
58.12 g/mol
-11.7oC
Comparison of the group 4A, 5A, 6A, and 7A
hydrides shows an interesting result.
What type of forces do the group 4A have?
Group 6A?
What is the notable exception?
Intermolecular Forces
The third type of force is a
special case of DD force and is
called Hydrogen Bonding (or
HB).
The name “Hydrogen Bonding”
is a misnomer!
HB can only occur when:
H is bonded to either N, O, or
F.
The N, O, or F atom has at least
one lone pair.
Intermolecular Forces
The strength of Hydrogen Bonding varies from 5 kJ
to 40 kJ, which is still much weaker than a covalent
bond (200 – 1000 kJ).
However, it is MUCH stronger than DD or LD forces.
Thus, it can greatly increase the boiling point
temperatures of molecules.
Intermolecular Forces
HB forces are very important in biochemistry.
Proteins are made from the twenty amino acids.
The structure of the amino acid has both an –OH
group and an –NH2 group that can HB.
R O
H–N–C–C –O –H
H H
Intermolecular Forces
Predicting relative boiling points.
1. Determine the molecular weight.
2. Determine the type(s) of intermolecular forces
present.
If weights are similar, then LD < DD < HB
If weights are very dissimilar, then #2 probably
does not matter.
However, HB can really distort the bp’s!
Ex) H2O, bp = 100oC, MW = 18 g/mol versus CCl4,
bp = 76oC, MW = 154 g/mol.
Intermolecular Forces
The strengths of
attractions between
the molecules may
affect a liquids
properties.
Viscosity
Surface Tension
Intermolecular Forces
Viscosity is the resistance of a
liquid to flow.
Liquids with low viscosity, like
water, will produce a “splash”
whereas liquids with high
viscosity, like corn syrup or
ketchup, will not.
Intermolecular Forces
Viscosity tends to increase with more intermolecular
forces and molecular weight.
Many liquids, like water, have a consistent viscosity
over a wide range of temperatures.
Some liquids, like corn syrup, will decrease in
viscosity as the temperature increases.
Multi-weight motor oil actually increases with an
increase in temperature (ie. 5W – 30).
Non-Newtonian liquids (ie. “Slime”) have a variable
viscosity at the same temperature.
Surface Tension
Surface Tension is the
“skin-like” appearance
of the surface.
Results from surface
molecules seeking six
nearest neighbors like
interior molecules.
Surface Tension
Phase Changes
Phase Changes
Energy when changing between solid and liquid
phase is called the Heat of Fusion and denoted as
DHfus.
DHfus for water is 6.01 kJ/mol or 334 J/g.
Energy when changing between liquid and gas is
called the Heat of Vaporization and denoted as
DHvap.
DHvap for water is 40.67 kJ.mol or 2,260 J/g.
Heating Curve
Refrigeration
The basics of
refrigeration.
First law of
thermodynamics
at work again!
Coolant is CF2Cl2
(old) or CF3CH2F
(new).
Vapor Pressure
Above the surface of any liquid, some liquid molecules
will have enough energy to escape and become gas
molecules.
In a closed system, an equilibrium will be achieved
between the gas molecules and the liquid.
This is the vapor pressure.
Vapor Pressure
As the temperature of the liquid increases, its vapor
pressure will increase.
Vapor Pressure
When the vapor pressure
equals 1 atmosphere,
then the liquid
spontaneously becomes a
gas. You would call this
the boiling point.
Does pure water always
boil at 100oC?
Clausius-Clapeyron Equation
The graphs of vapor
pressure versus
temperature are
approximately an
exponential function.
Mathematically, if you
take the natural
logarithm (ln key on
calculator) of the vapor
press versus 1/T, then
you get a linear
relationship.
Clausius-Clapeyron Equation
P2 DH vap 1 1
n
R T1 T2
P1
•R is molar gas constant = 8.314 J/K mol and the T is
the temperature in Kelvin
•Heat of vaporization must be in J/mol.
•Pressures can be in either atm or mmHg (must agree).
Phase Diagrams
Display a single’s substances states of matter over a
wide range of P and T.
Carbon Dioxide
The phase diagram of
CO2 shows that the
liquid phase can only be
found above a pressure
of 5.11 atm.
As the temperature of
solid CO2 increases, it
undergoes sublimation.
Water
The phase diagram of
water has one very
important difference.
What is it?
Solids
Solids can be either
amorphous (random) or
crystalline (repeating pattern).
Unit cell is the smallest
repeating pattern for the
crystalline structure.
Analogy: a hotel with many
floors.
Structure of unit cell can have
various lengths and angles.
Solids
While many types of unit cells are possible, a few are
seen many times in structures of metals, molecular,
and ionic compounds.
Cubic unit cells – two main versions.
Body-centered cubic (BCC) – has atoms at each corner
and an atom in the body-center.
Face-centered cubic (FCC) – has atoms at each corner
and an atom on each face.
Important – just like a hotel room shares walls,
floors, and ceilings with other rooms, so does a unit
cell share atoms with other unit cells.
Solids
Solids
Solids
Can also have atoms on edges in larger unit cells –
namely for ionic compounds.
Thus, the following are the contributions for
locations on or in a unit cell:
Solids
Unit cell calculations will follow the formula:
MW C
Vc
D NA
Where Vc is the volume of the cubic unit cell, MW is
the molar mass, C is the number of atoms per unit
cell, D is the density (m/V), and Na is Avogadro’s
Number.
Solids
Another view – Closest
Packing Model.
Assumes that atoms are
hard spheres.
Maximize the density,
minimize the empty spaces.
Solids
First layer – what is the most efficient method of
arrangement?
Solids
Second layer is placed so that spheres sit in gaps from
previous row.
Third layer can either repeat first layer yielding an
ABABAB… pattern.
OR, the third layer is offset from the first two producing
an ABCABCABC… pattern.
Solids
The ABABAB…
pattern produces a
unit cell called
hexagonal closest
packing or HCP.
This is NOT a
cubic unit cell!
Solids
The ABCABCABC… pattern produces a unit cell
called cubic closest packing or CCP.
However, CCP is the same as FCC!
Solids
H
He
Hcp
Li
Bcc
Be
Hcp
B
C
N
O
F
Ne
Fcc
Na
Bcc
Mg
Hcp
Al
Fcc
Si
P
S
Cl
Ar
Fcc
K
Bcc
Ca
Fcc
Sc
Hcp
Ti
Hcp
V
Bcc
Cr
Bcc
Mn
Bcc
Fe
Bcc
Co
Hcp
Ni
Fcc
Cu
Fcc
Zn
Hcp
Ga
Ge
As
Se
Br
Kr
Fcc
Rb
Bcc
Sr
Fcc
Y
Hcp
Zr
Hcp
Nb
Bcc
Mo
Bcc
Tc
Hcp
Ru
Hcp
Rh
Fcc
Pd
Fcc
Ag
Fcc
Cd
Hcp
In
Sn
Sb
Te
I
Xe
Fcc
Cs
Bcc
Ba
Bcc
Hf
Hcp
Ta
Bcc
W
Bcc
Re
Hcp
Os
Hcp
Ir
Fcc
Pt
Fcc
Au
Fcc
Hg
Tl
Hcp
Pb
Fcc
Bi
Po
At
Rn
Solids
All crystalline solids can be catagerized into one of
four types.
Type 1: Molecular Solids
Consist of atoms or molecules like Ne, CH4, and H2O.
Are held together by relatively weak intermolecular
forces.
Are soft and have low melting points (unless they have
a high MW).
Poor conductors of heat and electricity.
Solids
Type 2: Ionic Solids
Consist of ions held together by their electrostatic
attractions.
Unit cells are always larger since the smallest repeating
pattern must include two ions.
When cation and anion are of similar sizes, get BCC type
arrangement. When anion is much larger, get a CCP
arrangement of anions with cations stuck in the “holes.”
Hard and brittle and have high melting points.
Poor electrical conductors as solids, but excellent when
melted.
Solids
(a) CsCl
(b) ZnS
(c) CaF2
Solids
View of NaCl
Solids
Type 3: Metallic Solids
Atoms are held together by
a “sea of valence
electrons.”
Can be soft (Na, Au) or
very hard (Fe, Co) with low
to very high melting
points.
Excellent conductors of
both heat and electricity.
Malleable and ductile.
Solids
Type 4: Covalent Network Solids
Consist of atoms held together in large networks of
covalent bonds.
There are not many of these – C(diamond), SiO2,
quartz, SiC, and BN.
Very hard with very high melting points.
Poor conductors.
Solids
Two forms of carbon, diamond and graphite.
Solids
Comparing metal points of solids.
First – determine the type of solid.
Molecular is always the lowest of the four types.
Second – if both are the same type of solid, then:
Molecular is like bp’s. LD < DD < HB.
Ex) CH4 (-182 C) < COCl2 (-118 C) < H2O (0 C)
Ionic depends on lattice energy – the larger the lattice
energy, the higher the mp.
Ex) NaCl (801 C) < MgO (2852 C)
Solids
Metallic depends on the number of unpaired electrons.
More unpaired electrons = higher melting point.
K, 1 unpaired electron, mp = 64 C
Ti, 2 unpaired electrons, mp = 1668 C
Cr, 6 unpaired electrons, mp = 1907 C
Cu, 1 unpaired electron, mp = 1065 C
Covalent network are always very high.
Quartz, mp = 1670 to 1710 C
Diamond, mp = 3550 C (highest of any naturally occurring
substance)