Chem 1011 – Intersession 2011
Class #11
20-May-11
1
Class 11: Comparing Solids, Liquids
and Gases; Introduction to
Intermolecular Forces
•
Sec 11.2 – Solids, Liquids, and Gases: A
Molecular Comparison
▫
•
Changes between phases
Sec 11.3 – Intermolecular Forces: The Forces
That Hold Condensed Phases Together
▫
▫
▫
▫
Dispersion Force
Dipole-Dipole Force
Hydrogen Bonding
Ion Dipole Force
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Comparison of Phases
Density
Compressible?
Strength of
Intermolecular
Attractions
Volume
Fixed
High
No
No
Very Strong
Fixed
High
No
Yes
Intermediate
Low
Yes
Yes
Weak
Gas
Indefinite Indefinite
Will it Flow?
Shape
Fixed
Indefinite
State
Solid
Liquid
• fixed = keeps shape when placed in a container
• indefinite = takes the shape of the container
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Liquid vs. Gas
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
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Chem 1011 – Intersession 2011
Class #11
20-May-11
4
Phase Changes
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)
Sublimation
gas  solid
I2(g) → I2(s)
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Phase Changes
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Forces hold Molecules Together
•
Intramolecular Forces
▫
•
attraction within a molecule. A covalent bond is an example of an
intramolecular force.
Intermolecular Forces
▫
▫
▫
attraction between molecules.
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.
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
2
Chem 1011 – Intersession 2011
Class #11
20-May-11
7
Intermolecular Forces
•
There are four primary types of intermolecular forces:
1.
(London) Dispersion Forces
2.
Dipole-Dipole Forces (van der Waal's Forces)
3.
Hydrogen Bonding
4.
Ion-Dipole Force
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(London) Dispersion Forces
•
Dispersion forces
▫
Intermolecular forces associated with instantaneous and
induced dipoles.
•
For non-polar molecules, it is most likely that the electrons will be
evenly distributed in the orbitals throughout the molecule.
•
However, it is possible for the electrons in one molecule to flicker
into an arrangement that results in partial positive (d+) and
partial negative (d–) charges (think of electrons in the molecule
like water in a water balloon)
•
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.
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Dispersion Forces
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 d+ and d-.
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 then each other.
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
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Chem 1011 – Intersession 2011
Class #11
20-May-11
10
Dispersion Forces
•
Dispersion forces are present in all types of molecules both
polar and non-polar), however, it is the predominant type of
intermolecular force between non-polar molecules.
•
There are two factors that effect the magnitude of dispersion
forces:
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
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Dispersion Forces
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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.
Spherical molecules with the same number of electrons as more
branched molecules will have weaker dispersion forces.
• Both have the
formula C5H12
• Both have a total
of 42 e–
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
4
Chem 1011 – Intersession 2011
Class #11
20-May-11
13
Dispersion Force – Examples
•
▫
•
▫
F2 has 18 electrons and Cl2 has 34 electrons.
The dispersion forces for Cl2 are stronger than those of F2.
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.
•
Molecules with greater dispersion forces will have higher
boiling points because more energy is required to overcome
the attraction between molecules.
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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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Dipole-Dipole Force
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.
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.
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
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Chem 1011 – Intersession 2011
Class #11
20-May-11
16
Dipole-Dipole Force 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).
▫
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.
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Problem
•
Which would you expect to have the higher boiling
point?
(a) C4H10 or (CH3)2CO
(b) C3H8, CO2, CH3CN
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Hydrogen Bonding
•
Hydrogen Bonding
▫
an intermolecular force of attraction in which a H-atom
covalently bonded to a highly electronegative atom is
simultaneously attracted to another highly
electronegative atom of the same or a nearby molecule.
•
The only atoms that are electronegative enough to
participate in hydrogen bonding are fluorine, nitrogen and
oxygen.
•
Hydrogen bonding, like dispersion forces and dipole-dipole
forces, also affects physical properties, like melting and
boiling points.
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
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Chem 1011 – Intersession 2011
Class #11
20-May-11
19
Hydrogen Bonding
The values for NH3, H2O and HF are unusually high compared to those of
other member of their groups!
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Hydrogen Bonding
•
Consider the molecule HF as our example.
•
Fluorine is very electronegative, and there is a large ∆EN between
H and F.
▫
This pulls the bonding pair of electrons much closer to F, leaving
the hydrogen nucleus electron deficient and therefore it will have a
very large partial positive charge.
•
The hydrogen atom, because it now electron deficient (and has
such a large partial positive charge), 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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Hydrogen Bonding
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
7
Chem 1011 – Intersession 2011
Class #11
20-May-11
22
Hydrogen Bonding
•
N-H Hydrogen Bonding Example
▫
Urea is CO(NH2)2 and it undergoes extensive hydrogen
bonding due to its four hydrogen atoms bonded directly
to nitrogen, which can then be Hydrogen bonded to either
nitrogen or oxygen. Several of these interaction are
depicted below.
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Hydrogen Bonding
•
O-H Bond Example
▫
▫
In liquid H2O, each water molecule is hydrogen bonded to
at least four other H2O molecules.
This occurs because each water molecule has 2 H-atoms
looking to participate in hydrogen bonding, as well as an
O-atom with 2 lone pairs available for hydrogen bonding
with other H-atoms
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Hydrogen Bonding
•
O-H Bond Example
▫
▫
In liquid H2O each hydrogen atom is bonded to at least four
other H2O molecules.
A few of these bonds are illustrated below.
The molecules can still move
around because they have enough
kinetic energy to break the
hydrogen bonds, which then
reform with another H2O
molecule.
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
8
Chem 1011 – Intersession 2011
Class #11
20-May-11
25
Hydrogen Bonding
•
Hydrogen bonding in H2O also explains why ice floats!
•
To float a solid must be less dense than the liquid.
•
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.
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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Comparison of Melting and Boiling
Points
• When examining the melting or boiling points of compounds,
consider the intermolecular forces in order of strength:
1.
2.
3.
Hydrogen Bonding
Dipole-Dipole
Dispersion
Strongest
Weakest
• Therefore, molecules containing a hydrogen bonding force
will have the highest boiling points, followed by the others.
• However, none of these interactions can rival a true ionic
attraction
▫ This is why ionic compounds have the highest boiling points
▫ Molecular compounds can also be attracted towards ions as a
type of intermolecular force.
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Problem
• Complete the table below, and arrange the
molecules in order of lowest boiling point to
highest
Molecule
Total # of e–
Dispersion?
Dipole-Dipole?
H-Bonding?
H2S
N2
CH3NH2
C2H6
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
9
Chem 1011 – Intersession 2011
Class #11
20-May-11
28
Ion-Dipole Force
• In a mixture, ions from an ionic compound are attracted
to the dipole of polar molecules.
• 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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Ion-Dipole Force
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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Later On...
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Sec 11.5 – Vaporization and Vapor Pressure
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▫
▫
▫
•
The Process of Vaporization
The Energetics of Vaporization
Vapor Pressure and Dynamic Equilibrium
The Critical Point: The Transition to an Unusual Phase of
Matter
Sec 11.6 – Sublimation and Fusion
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▫
▫
Sublimation
Fusion
Energetics of Melting and Freezing
Comparing Solids, Liquids and Gases;
Introduction to Intermolecular Forces
10