Transcript Chem 1201 - LSU Department of Chemistry

Watkins
Chapter 11
Chapter 11
Intermolecular Forces
Intermolecular Forces
Gases, Liquids and Solids
Liquid Properties
Phase Changes
Vapor Pressure of a Liquid
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Chapter 11
Gases
Gas particles are in constant, random motion and collide
with their neighbors. True
The average distance between particles is greater than the
particle size. True
All collisions are elastic. False (for a real gas)
Real gas particles (atoms or molecules) are sticky;
intermolecular forces are attractive.
The collisions are not completely elastic. When these
sticky particles collide, there is an instantaneous
interparticle bond formed.
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Chapter 11
Gases
When the collision between gas
particles is at high speed (high
kinetic energy = high T), the
particles bounce apart (nearly
elastic collision).
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Chapter 11
Gases
At the point of collision, an instantaneous weak
bond is formed between the particles, but the
force of a fast collision is able to break the bond,
so they fly apart.
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Chapter 11
Gases
When the collision between gas
particles is at slow speed (low
kinetic energy = low T), the
particles sometimes stick
together (inelastic collision)
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Chapter 11
Gases
The particles clump together to
form droplets which eventually
condense to become liquid.
An ideal gas cannot condense
because collisions at all
temperatures are elastic.
When the collision between gas
particles is at slow speed (low
kinetic energy = low T), the
particles sometimes stick
together (inelastic collision)
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Chapter 11
Compare
Gas ↔ Liquid ↔ Solid
Phase Changes
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Chapter 11
Gases
Gas takes shape of container (fluid) and completely
fills it.
“Fluid motion” is the result of constant and random
motion of the particles
Much empty space between particles (compressible).
Collisions are semi-elastic at high temperatures, but not
at low temperatures.
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Chapter 11
Liquids
At low T, gas particles stick together (condensation).
Liquid takes shape of container (fluid) but only
partially fills it.
“Fluid motion” is the constant and random motion of the
particles
Very little space between particles (incompressible)
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Chapter 11
Solids
Solid does not take shape of container and does not fill it.
Particles are fixed in position but vibrate constantly and in
random directions
Very little space between particles (incompressible)
Liquid-to-Solid phase change is called freezing
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Chapter 11
S, L, G
Gases and Liquids are fluid because the particles move at
random from place to place in the container.
Solids are not fluid because the particles are fixed in place.
Liquids and Solids are condensed and incompressible
because there is little space between particles.
All neutral atoms and molecules are “sticky” - but the
attractive intermolecular forces are much weaker than
covalent or ionic bonds.
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Chapter 11
Relative Bond Energies
2 Intramolecular Bond Energies (kJ/mol)
500
1000
covalent
ionic
intermolecular
4 Intermolecular Bond Energies (kJ/mol)
10
20
30
40
50
ion-dipole
H-bond
dipole-dipole
London dispersion
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Chapter 11
Measuring Intermolecular Bonds
A(s) → A(g) DHsubl > 0
A(l) → A(g) DHvap > 0
All of the intermolecular bonds must be broken!
DHsubl and DHvap are direct measures of the strength
of intermolecular bonds.
Tb is directly proportional to DHvap and is easy to
measure for any liquid.
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Chapter 11
Measuring Intermolecular Bonds
A(l) → A(g) DHvap > 0
Here are three liquids and their Tbs:
H2O (100 oC)
o
N2 (-195.8 C)
o
CH3Br (3.5 C)
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strongest
weakest
stronger
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Arrange these
liquids in
order of
increasing
intermolecular
force.
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Chapter 11
Intermolecular Force
Many molecules have more than one intermolecular
force.
The total intermolecular force between molecules is
the sum of all intermolecular forces.
Intermolecular Forces (Bonds) (kJ/mol)
10
20
30
40
50
ion-dipole
H-bond
dipole-dipole
London dispersion
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Chapter 11
Intermolecular Forces
Polar Molecule
A molecule with one or more
polar covalent bonds
arranged in space such that
one end of the molecule has
a small + charge and the
other end of the molecule
has a small - charge.
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Chapter 11
f = k×z1×z2/d2
Intermolecular Forces
Dipole-Dipole Bond
An attractive force between polar molecules
Force increases with polarity of molecules
431 kJ/mol
+
-
+
-
16 kJ/mol
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Chapter 11
Intermolecular Forces
Ion-Dipole Bond
(aqueous solutions of salts)
Strength depends on two things:
Dipole moment of polar
molecule (solvent)
z1 = + or Ionic charge on ions (solute)
z2
2
f = k×z1×z2/d
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Chapter 11
Intermolecular Forces
Dispersion force
(aka "London", "van der Waals", or "induced
dipole")
Weakest of all intermolecular forces
This force increases with Molar Mass
All atoms, molecules and ions exert this force
Tb oC
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He
-269
Ne
-246
Ar
-186
19
Kr
-153
Xe
-108
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Chapter 11
Intermolecular Forces
H-Bonding
X
H
Y
- +
X
H
Y
X = donor atom
H-bond
XH is a polar covalent bond
X must be very electronegative
X = N, O, F
Y = acceptor atom
Y must be very electronegative
Y must have at least one lone pair
Y = N, O, F
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Chapter 11
H-Bonding - Ice
Lone pairs on O bond to H’s of other water molecules
H
H
H
O
H
H
– ice floats in water
O
H
H
O
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H
• Ice is less dense than
liquid water, so
O
H
H
– solids sink in liquids
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O
O
H
O
H
H
H
O
O
H
H
H
H
• Ice is very hard!
• Almost all solids are
more dense than their
liquid, so
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Chapter 11
H-Bonding - Proteins
-amino acids
(peptide)
-carbon
H
H
O
N
C
C
H
R1
Amino group NH2
(base)
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Although there is an unlimited
number of possible -amino
acids, nature used only about
21 to make us and all living
OH things on this planet.
Carboxyl group COOH
(acid)
Any other atom or
group of atoms
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Chapter 11
H-Bonding - Proteins
Two -amino acids form a peptide bond
H
H
O
N
C
C
H
R1
OH + H
H
O
N
C
C
H
R2
OH
Condensation Reaction
H2O + H
Still an amino
acid!
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H
O
N
C
C
H
R1
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H
O
N
C
C
H
R2
dipeptide
OH
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Chapter 11
H-Bonding - Proteins
Futher condensation leads to
polypeptides (< 50 peptides) and
proteins (> 50 peptides)
H
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H
O
N
C
C
H
R1
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H
O
N
C
C
H
R2
dipeptide
OH
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Chapter 11
H-Bonding - Proteins
Poly--amino acid chains hydrogen bond
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H
O
N
C
C
H
R1
H
O
N
C
C
H
R3
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H
O
N
C
C
H
R2
N-H...N
H
O
N
C
C
H
R4
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Chapter 11
H-Bonding - Proteins
Poly--amino acid chains hydrogen bond
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H
O
N
C
C
H
R1
H
O
N
C
C
H
R3
H
O
N
C
C
H
R2
H
O
N
C
C
H
R2
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N-H...O
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Chapter 11
H-Bonding - Nucleic Acids
• Nucleic acids are large molecules
condensed from nucleotides which have
both acid and base functions.
• Polynucleotides H-bond to produce
– DNA
– Double Helix
– RNA
– t-RNA
– Protein manufacturing blueprints
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Chapter 11
Intermolecular Bonds
Summary: the force of attraction between
any two atoms, molecules or ions is the
sum of all possible forces.
All chemical species have dispersion.
In addition, polar molecules have
dipole-dipole.
In addition, molecules with N-H and O-H
bonds have H-bonding.
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Chapter 11
Measuring Intermolecular Bonds
A(l) → A(g) DHvap > 0
Here are three liquids and their Tbs:
H2O (100 oC)
o
N2 (-195.8 C)
o
CH3Br (3.5 C)
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strongest
weakest
stronger
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Identify the
intermolecular
forces.
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Chapter 11
Measuring Intermolecular Bonds
A(l) → A(g) DHvap > 0
Here are three liquids and their Tbs:
oC)
H2O (100
o
N2 (-195.8 C)
o
CH3Br (3.5 C)
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strongest
H
weakest
N
stronger
H
H C Br
H London & dipole
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O
H
London & dipole & H-bond
N
London
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Chapter 11
Phase Changes
There are six phase changes between the
three common phases
Bond-breaking
(melting, vaporization,
sublimation) is always
endothermic (DH > 0)
Bond-making
(condensation, freezing,
deposition) is always
exothermic (DH < 0)
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endothermic
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exothermic
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Chapter 11
Liquid Properties
All due to intermolecular bonds
Viscosity
surface (gas-liquid interface)
How easily interior
molecules move past
one another.
Surface Tension
Vapor Pressure
How tightly surface
molecules are bound.
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interior
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Chapter 11
Vapor Pressure
As the temperature of the ethanol
is raised from 0 K, some “hot”
molecules escape from the surface
and enter the gas phase. Only
surface molecules with sufficient
kinetic energy can break the
intermolecular bonds and enter the
gas phase. Some gas molecules collide with the surface
and stick, returning to the condensed phase.
heat
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Chapter 11
Vapor Pressure
At each temperature, the number
of molecules leaving the surface is
equal to the number of molecules
returning to the surface. The gas
pressure is called the equilibrium
vapor pressure, Peq.
The number of hot molecules increases with T, so
Peq is proportional to T.
heat
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Peq
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Chapter 11
Vapor Pressure
Peq
At each temperature, the number
of molecules leaving the surface is
equal to the number of molecules
returning to the surface. The gas
pressure is called the equilibrium
vapor pressure, Peq.
The number of hot molecules increases with T, so
Peq is proportional to T.
heat
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T
Peq
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Chapter 11
Vapor Pressure
The vapor pressure curve
has an equation (named
after Rudolf Clausius and
Benoit Clapeyron, 1834).
– DHvap/RT
=
Ke
Peq
T
ln(Peq)
A plot of ln(Peq) vs 1/T has
a slope of –DHvap/R
(R = 0.008314 kJ/mol.K)
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T-1
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Chapter 11
Vapor Pressure
If the container is closed, the hot
molecules return to the surface,
but if the container is open,
most of the hot molecules
escape permanently from the
surface.
They carry energy away from the
condensed phase, so it cools.
The temperature in an open coffee
cup decreases rapidly!
To keep the coffee hot for a
longer period, put a lid on it!
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Chapter 11
Vapor Pressure
Peq
The normal boiling point
Tbo is the temperature at
which Peq = 760 torr
760
T
Tbo
Example: Peq = 760 torr for
water at Tbo = 100.00 oC
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T
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Chapter 11
Vapor Pressure
The boiling point is the
temperature at which
Pext
Peq = Pext
The external pressure is the
pressure on the surface of the
liquid, and can be adjusted to
any value. Thus ...
Peq
T
Tb
Any liquid can boil at any
temperature.
At high altitudes, water boils below 100 oC
Deep in the ocean, water boils above 100 oC
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T
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Chapter 11
Vapor Pressure
Peq
The boiling point is the
temperature at which
Peq = Pext
23.76
T
Example: for water at 25 oC,
Peq = 23.76 torr. In a partial
vacuum (e.g., at very high
altitude) with Pext < 23 torr,
water will boil below 25 oC
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298
T
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Chapter 11
Boiling Point & Vapor Pressure
Volatile: a liquid or solid with
a significant vapor pressure
at room temperature.
Non-volatile: a liquid or solid
with an insignificant Peq at
room temperature.
25 oC
Diethyl ether is the most
volatile of these liquids.
Ethylene glycol is non-volatile; at room temperature,
Peq ≈ 0
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Boiling Point & Vapor Pressure
Volatile: a liquid or solid with
a significant pressure at
room temperature.
Non-volatile: a liquid or solid
with an insignificant vapor
pressure at room
temperature.
25 oC
Given the Normal boiling
points, arrange the liquids in
order of increasing intermolecular force, and name the
forces in each molecule.
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Chapter 11
Boiling Point & Vapor Pressure
H
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C2H5
O
C2H5
C2H5
O
H
H
O
H
O
H
H
C
C
H
H
O
H
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Chapter 11
Boiling Point & Vapor Pressure
H
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C2H5
O
C2H5
London & dipole-dipole
C2H5
O
H
London, dipole-dipole & H-bonding
H
O
H
London, dipole-dipole & H-bonding
O
H
H
C
C
H
H
O
H
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London, dipole-dipole
& H-bonding
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Chapter 11
Phase Diagram
Supercritical Fluid
Pressure
Pc
solid
liquid
gas
Pt
solid
gas
Tt
Tc
The T-P conditions in which
the phases of any pure
substance can exist.
Single phase areas
Two-phase equilibrium lines
Triple point: three phases in
equilibrium at Pt and Tt
Critical Point
Tc and Pc at which the
l g line disappears;
dliquid = dgas
Temperature
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Chapter 11
Phase Diagram
If 1 atm line is above Pt, the substance is normal.
Pressure
Supercritical Fluid
solid
liquid
gas
1 atm
solid
gas
Tfo
Tbo
Normal melting point:
the temperature at which
solid and liquid are in
equilibrium at 1 atm.
Normal boiling point:
the temperature at which
liquid and gas are in
equilibrium at 1 atm.
Most substances are normal
with Tfo > Tt
Some are “abnormal”.
Temperature
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Chapter 11
Phase Diagram
If 1 atm line is below Pt, the substance has no Tfo or Tbo.
Pressure
Supercritical Fluid
solid
liquid
gas
1 atm
solid
gas
-78.5 oC
An example is CO2 ("dry"
ice). Liquid cannot form
in an open container; solid
CO2 can only sublime at
-78.5 oC.
Under pressure, CO2 does
form a liquid - e.g. in a
CO2 fire extinguisher at
room temperature.
25 oC
Temperature
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Chapter 11
Phase Diagram
Water and a very few other substances have an unusual phase diagram,
because the solid is less dense than the liquid: Tfo < Tt
Supercritical Fluid
Pressure
Pressure on ice will melt the
ice even below the normal
melting point - e.g., ice
skating.
solid
liquid
gas
1 atm
solid
gas
Temperature
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0
oC
100
oC
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Chapter 11
Water Phase Diagram
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Chapter 11
Water Phase Diagram
101,325 Pa
More than 15 different kinds of ice are known. The ice in your freezer
and in snowflakes is Ice Ih. Other kinds of ice are formed at high
pressures; for example, Ice II and Ice III are found on the bottom of
glaciers.
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Chapter 11
Helium Phase Diagram
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