Download Lecture 11 Notes

Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
February 1, 2010 – Lecture 11
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11.3 Intermolecular Forces: The Forces That Hold
Condensed Phases Together
– Dispersion Force
– Dipole-Dipole Force
– Hydrogen Bonding
– Ion Dipole Force
Dr. L. Dawe
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11.2 Solids, Liquids, and Gases: A Molecular
Comparison
– Changes between phases
Chem 1011
Lecture 11
Winter 2010
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Solids, Liquids, and Gases: A Molecular Comparison
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St mo s
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ow
A
Fl
it le?
ill
b
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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
• fixed = keeps shape when placed in a container
• indefinite = takes the shape of the container
Chem 1011
Dr. L. Dawe
Winter 2010
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Solids, Liquids, and Gases: A Molecular Comparison
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Changes between phases
Lecture 11
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Phase Change
State Change
Example
Melting (fusion)
solid liquid
Ice cube melting
Freezing
(solidification)
liquid solid
Water freezing
Vaporization
liquid gas
Water boiling
Condensation
gas liquid
Dew forming on grass
Sublimation
solid gas
Dry ice: CO2(s) → CO2(g)
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Sublimation
gas solid
I2(g) → I2(s)
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Chem 1011
Dr. L. Dawe
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Winter 2010
Changes between phases
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Chem 1011
Dr. L. Dawe
Winter 2010
Intermolecular Forces: The Forces That Hold
Condensed Phases Together
Intramolecular Forces: attraction within a molecule. A covalent bond is
an example of an intramolecular force.
Intermolecular Forces: an attraction between molecules.
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Responsible for condenses phases (solids and liquids).
Intermolecular forces explain many properties of liquids.
(a) Molecules at the
surface interact with
other surface
molecules and with
molecules directly
below the surface.
(b) Molecules in the
interior experience
intermolecular
interactions with
neighboring
molecules in all
directions.
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Intermolecular Forces: The Forces That Hold
Condensed Phases Together
Lecture 11
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There are four primary types of intermolecular forces:
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1.
(London) Dispersion Forces
2.
Dipole-Dipole Forces (van der Waal's Forces)
3.
Hydrogen Bonding
4.
Ion-Dipole Force
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Recall: polarizability: the ease with which a particle’s
electron cloud can be distorted.
Chem 1011
Dr. L. Dawe
Winter 2010
Dispersion Force
Dispersion forces: Intermolecular forces associated with
instantaneous and induced dipoles.
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Electron location is expressed in terms of probabilities only
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For non-polar molecules it is most probable that the electrons
will be evenly distributed in the atomic or molecular orbitals.
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It is possible for the electrons in one molecule to flicker into an
arrangement that results in partial positive (δ+) and partial
negative (δ-) charges.
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When this occurs the molecules acquire an instantaneous (it
lasts less than 10-16 seconds!) dipole.
–
A molecule that has acquired an instantaneous dipole can then
induce a dipole in another molecule.
Chem 1011
Dr. L. Dawe
Winter 2010
Dispersion Force
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a) Normal Condition: A non-polar molecule has a symmetrical charge distribution
b) Instantaneous Condition: A displacement of the electronic charge produces an
instantaneous dipole with a charge separation represented as δ+ and δ-.
c) Induced Dipole: The instantaneous dipole on the left induces a charge
separation in the molecule on the right. The result is a dipole-dipole
interaction.
The two dipoles, in the two molecules, will attract each other, and the
result is that the potential energy of the two is lowered.
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Dispersion Force
•
•
Dispersion forces are present in both polar and non-polar
molecules, however, it is the main type of intermolecular
force between non-polar molecules.
There are two factors that effect the magnitude of dispersion
forces:
Lecture 11
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1. Number of electrons: Larger molecules with more
electrons more easily undergo vibrations that lead to uneven
distribution of charge.
Molecules with more electrons will have stronger
dispersion forces.
Chem 1011
Dr. L. Dawe
Winter 2010
Dispersion Force
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Chem 1011
Dr. L. Dawe
Winter 2010
Dispersion Force
•
There are two factors that effect the magnitude of dispersion
forces:
2. Surface area: Molecules with a larger surface area offer a
greater opportunity for a molecule to induce a dipole in a
nearby molecule.
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Spherical molecules with the same number of
electrons as more branched molecules will have weaker
dispersion forces.
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Dispersion Force
Lecture 11
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Chem 1011
Dr. L. Dawe
Winter 2010
Dispersion Force
•
Example: F2 has 18 electrons and Cl2 has 34 electrons. The
dispersion forces for Cl2 are stronger than those of F2.
•
Example: Cl2 and C4H10 each have 34 electrons. C4H10 has
a larger, more complex shape therefore it has stronger
dispersion forces than Cl2.
•
When a substance melts or boils, the intermolecular forces
are overcome.
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•
Molecules with greater dispersion forces will have higher
boiling points because more energy is required to overcome
the attraction between molecules.
Chem 1011
Dr. L. Dawe
Winter 2010
Dipole-Dipole Force
Dipole-Dipole Forces: intermolecular attractions associated with
molecules with permanent dipoles.
•
Recall that the bond dipoles for molecules do not always
cancel. When the bond dipoles do not cancel the resulting
molecule is polar (it has a permanent dipole).
•
This leads to polar molecules trying to line up with the
positive of one dipole directed toward the negative end of
neighbouring dipoles.
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Dipole-Dipole Force
Dipole-dipole forces add to
dispersion forces (which are
present for all molecules).
Dipole-dipole forces also
affect physical properties
such as melting and boiling
point.
Dr. L. Dawe
Winter 2010
Dipole-Dipole Force
NO is a polar molecule, hence it has both dispersion
forces and dipole-dipole forces present. It has a higher
boiling point than O2(l) or N2(l) because extra energy is
required to overcome the dipole-dipole interactions.
Dr. L. Dawe
Winter 2010
Intermolecular Forces
Problem: Which would you expect to have the higher
boiling point?
(a) C4H10 or (CH3)2CO
(b) C3H8, CO2, CH3CN
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Example: The boiling point of N2(l) is -195.81 oC. The
boiling point of O2(l) is -182.96 oC. If only dispersion
forces are considered, one would predict the boiling
point of NO(l) to be in between that of O2(l) and N2(l) .
The actual boiling point of NO(l) is -151.76 oC, much
higher than either that of O2(l) or N2(l) .
Chem 1011
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Dipole-dipole forces involve
the displacement of
electrons in bonds, rather
than the displacement of all
the electrons in a molecule,
as in dispersion forces.
Chem 1011
Lecture 11
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Chem 1011
Dr. L. Dawe
Lecture 11
Problem: Which would you expect to have the higher boiling point?
(a) C4H10 or (CH3)2CO
(b) C3H8, CO2, CH3CN
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
Hydrogen Bonding: an intermolecular force of attraction in
which an H-atom covalently bonded to one highly
electronegative atom is simultaneously attracted to
another highly electronegative atom of the same or a
nearby molecule.
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The only atoms that are electronegative enough to participate
in hydrogen bonding are fluorine, nitrogen and oxygen.
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Hydrogen bonding, like dispersion forces and dipole-dipole
forces, also affects physical properties, like melting and boiling
points.
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Notice the anomalies in the following graph of boiling points for
hydrides of Groups 4A, 5A, 6A and 7A.
Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
Lecture 11
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The values for NH3, H2O and HF are unusually high compared to those of other
member of their groups!
Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
•
Consider the molecule HF as our example.
•
Fluorine is highly electronegative, and pulls the bonding pair
of electrons closer to itself, leaving the hydrogen nucleus
unshielded with a partial positive charge.
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•
The hydrogen nucleus is then attracted to the lone pair of
electrons on another highly electronegative atom.
•
Recall that the other highly electronegative atom will have a
partial negative charge, acquired by attracting electrons
involved in its molecular bonds.
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
Lecture 11
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Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
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The result for gaseous HF is that it often forms cyclic (HF)6.
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Covalent Bond
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Hydrogen Bond
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Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
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N-H Bond Example:
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Urea is CO(NH2)2 and it undergoes extensive hydrogen
bonding due to its four hydrogen atoms bonded directly
to nitrogen, and also H-bonded to both nitrogen and
oxygen. Several of these interaction are depicted
below.
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
Lecture 11
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O-H Bond Example:
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In liquid H2O each hydrogen atom is bonded to at least
four other H2O molecules.
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Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
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O-H Bond Example:
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In liquid H2O each hydrogen atom is bonded to at least
four other H2O molecules.
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A few of these bonds are illustrated below.
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The molecules can still move around
because they have enough kinetic
energy to break the hydrogen bonds,
which then reform with another H2O
molecule.
Chem 1011
Dr. L. Dawe
Winter 2010
Hydrogen Bonding
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Hydrogen bonding in H2O also explains why ice floats!
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To float a solid must be less dense than the liquid.
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In the liquid there is enough kinetic energy to overcome
some of the hydrogen bonds, but in the solid the kinetic
energy is no longer sufficient to overcome the hydrogen
bonds.
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This results in the molecules being
organized into a crystalline
arrangement, which is less dense
than the arrangement of the H2O
molecules in the liquid.
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Chem 1011
Dr. L. Dawe
Chem 1011
Dr. L. Dawe
Winter 2010
Comparing Melting and Boiling Points
Melting/Boiling Point
Compound
Highest
1. Ionic
3. Molecular
(Dipole-Dipole + Dispersion
Forces)
Chem 1011
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2. Molecular
(H-Bonding + Dipole-Dipole
+ Dispersion Forces)
Lowest
Lecture 11
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4. Molecular
(Dispersion Forces Only)
Dr. L. Dawe
Winter 2010
Ion Dipole Force
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• In a mixture, ions from an ionic compound are
attracted to the dipole of polar molecules.
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• The strength of the ion–dipole attraction is one of the
main factors that determines the solubility of ionic
compounds in water.
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Chem 1011
Dr. L. Dawe
Winter 2010
Ion Dipole Force
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An ionic crystal dissolving in water. Clustering of water dipoles around the surface
of the ionic crystal and the formation of hydrated ions in solution are the key
factors in the dissolving process.
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Chem 1011
Chem 1011
Dr. L. Dawe
Dr. L. Dawe
Winter 2010
Looking Ahead February 3, 2010 – Lecture 12
11.5 Vaporization and Vapor Pressure
– The Process of Vaporization
– The Energetics of Vaporization
– Vapor Pressure and Dynamic Equilibrium
– The Critical Point: The Transition to an Unusual
Phase of Matter
11.6 Sublimation and Fusion
– Sublimation
– Fusion
– Energetics of Melting and Freezing
Lecture 11
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