Announcements
Exam #3: Thursday, Dec. 6th, 7:00-8:15pm (Conflict:
5:15-6:30pm, sign up outside of 101 CA)
No calculators allowed
Exam rooms posted on-line
Lon-capa HW Remaining
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Homework 8 – Type 1 due tonight, 12/4 by 7pm
Review Questions for Exam III due tomorrow, 12/5 by 10pm
Online ICES Evaluations
Bring clickers to Thursday lecture!
Intermolecular Forces
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Intramolecular forces: forces within a molecule
(bonds)
Intermolecular forces: forces between
molecules
Help us to explain some physical properties of
molecules like:

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Melting/freezing point
Boiling point
Solubility
Intermolecular Forces
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Intermolecular forces are interactions that hold
molecules together (“stickiness” between
molecules)
The stronger the interactions, the more energy
it will take to “break them up” to go from solid
 liquid or liquid  gas

Resulting in higher melting/freezing and boiling
points for that substance
London Dispersion Forces
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Interaction type that all molecules have
But it is the only type of intermolecular
interaction found in nonpolar molecules and
noble gases
Key point to remember: LDF is due to random,
uneven electron distribution (electron to proton
attraction)
How LDF occurs

Something happens in a molecule that causes
an attraction to another molecule
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Random momentary dipole can induce a dipole
on a neighboring atom
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Causes random momentary dipole
Which can then induce a dipole on another atom,
which can induce another atom, etc.
Start to have interactions between molecules
due to momentary dipoles; these interactions
are London Dispersion Forces
London Dispersion Forces
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+
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Ar
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-
+
-
eee- -ee
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Ar
e-e
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+
e-
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ee-e Ar ee- - - eee
-
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eAr ee- - ee
e-
London Dispersion Forces

The more electrons there are on a molecule,
the stronger the LDF

more energy is required to break up the interaction
to go from solid  liquid or liquid  gas

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Good way to remember is the larger the molar
mass, the larger the LDF
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More energy = higher temperature
since larger mass means more electrons!
At lower temperatures, momentary dipoles are
more likely (molecules are closer together)
London Dispersion Forces

Example: Cl2, I2, Br2
Clicker #1
Which of the following has the strongest
London dispersion forces?
A) Ne
B) N2
C) O2
D) All of the above have the same amount of
London dispersion forces.
Dipole-Dipole Interactions

Occurs with polar molecules
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The more polar the molecule, the greater the
attraction between the molecules
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Have permanent dipoles within the molecule
Greater difference in electronegativity in the bonds
Tend to be stronger than LDF (when
comparing molecules of same size) but not as
strong as bonding within molecule itself
Dipole-Dipole Interactions
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Example: OF2
Fluorine has greater electronegativity than oxygen
Results in interactions between partial positive and
partial negative regions of neighboring molecules
 These are dipole-dipole interactions!
Dipole-Dipole Interactions
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Example: OF2, SF2, SeF2, TeF2
Electron density maps (blue = partial positive charge,
red = partial negative charge)
As difference in electronegativity increases, density of
electrons shifts to more electronegative element
(fluorine)
Hydrogen Bonding

Particularly strong dipole-dipole is hydrogen bonding

Special type of dipole-dipole

Only occurs when H is directly connected to N, O, or F
 N-H, O-H, H-F are all very polar bonds (difference in
electronegativity between N, O, F and H is very
large)
 N, O, F atoms are rather small atoms so they can
approach other molecules more closely, resulting in
a much stronger dipole-dipole interaction
Hydrogen Bonding

Example: H2O
+
-
+
Clicker #2
How many of the following interactions does
both CH3CH2OH and CH3OCH3 exhibit?
London dispersion
dipole-dipole
hydrogen bonding
A) 0
B) 1
C) 2
D) 3
Hydrogen Bonding
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Example: CH3CH2OH versus CH3OCH3
Ethanol has hydrogen bonding
Dimethyl ether does not
Ion-Ion Interactions
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Occur between ionic compounds
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Between metals and non-metals
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Result of transfer of electrons
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Very strong interactions!
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Which is why ionic compounds have such
high melting points and boiling points
Ranking Interactions
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Strongest intermolecular forces are ion-ion interactions
Next strongest are dipole-dipole, with hydrogen
bonding being strongest type of dipole-dipole
Weakest are LDF
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If you are ranking molecules with only LDF, look at molar
mass (number of electrons)
Ion-ion > H bonding > dipole-dipole > LDF
Determining Physical Properties
Ionic or covalent bonding?
Covalent
Ionic
Polar or nonpolar?
Ion-ion
Interactions*
Polar
Nonpolar
N-H, O-H, or H-F?
*also have LDF!
Yes
No
Hydrogen
Bonding*
Dipoledipole*
LDF
compare
molar masses
Determining Physical Properties

When ranking substances to determine properties like
melting/freezing point and boiling point:
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Stronger interactions: higher melting/freezing and
boiling points (most likely a solid at room
temperature)
Weaker interactions: lower melting/freezing and
boiling points (most likely a gas at room
temperature)
General Solubility
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During the lectures about solutions, we talked
about solutes and solvents
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Solute – substance being dissolved
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Solvent – substance that is doing the dissolving
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How does polarity help us determine solubility?
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Like dissolves like!
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Polar solute will dissolve well in a polar solvent
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Example: NaCl
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Very polar, will dissolve in polar solvent (like water)
Will not dissolve well in nonpolar solvents (like olive oil, or
CCl4)
Nonpolar solute will dissolve well in a nonpolar
solvent
Example: Br2

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Will not dissolve well in water (water is polar)
Will dissolve well in CH4, CCl4, etc.
Clicker #3
How many of the following would be soluble in
CCl4?
PCl5
IF5
BF3
A) 0
B) 1
C) 2
D) 3
Determining Polarity
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First: draw Lewis structure for molecule
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PLACE ALL VALENCE ELECTRONS BEFORE ADDING
MULTIPLE BONDS!!
Second: determine geometry (how atoms and lone
pairs are arranged around central atom)
Third: determine shape of molecule (how does
molecule look with just atoms, not including lone pairs)
Fourth: Ask yourself - is the molecule symmetrical?
Nonpolar or polar: check
electronegativity of atoms
Always polar