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PROFESSOR: Hi, I'm Jocelyn, and
today we're going to go

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over Fall 2009 Exam 2,
Problem number 5.

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As always, we're going to read
the problem first. Which

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compound do you expect to have
the higher boiling point: HF

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or ammonia?

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Justify your choice with an
explanation, using narrative

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or cartoons or both that makes
reference to the operative

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chemical bonding.

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So first, we are trying to
figure out which has a greater

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temperature of vaporization,
or boiling point; and we're

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looking at HF and ammonia.

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Now when we're talking about
boiling points, we're talking

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about breaking a certain type of
bond, and that type of bond

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is intermolecular bonds.

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We're not going to be breaking
the actual molecular bonds

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bonding hydrogen to fluorine
or nitrogen, we're breaking

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the bonds in between molecules
of HF or ammonia, and so what

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types of bond do we
have to look at?

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And Professor Sadoway, in class,
went over a couple of

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different types of
intermolecular bonds.

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And I'm going to write
them down in order

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of increasing strengths.

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So the weakest intermolecular
bonds we

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have are Van der Waals.

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The next is induced
dipole-dipole.

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Next we have dipole-dipole, and
finally a special type of

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dipole-dipole is the
hydrogen-bond, which is the

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strongest. So here we're
increasing strength.

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Van der Waals is an induced
dipole-dipole.

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That means that although your
atom or molecule does not have

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a net dipole, the electrons can
shift, and at some point

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will cause a difference in
polarity between different

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sides of the molecule.

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Induced dipole-dipole is
two different molecules

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interacting: one that has no net
dipole and one that does.

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Dipole-dipole: pretty
self-explanatory.

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You have two molecules that
have dipoles, and they are

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interacting.

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And then hydrogen-bond is when
you have hydrogen bonded with

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x, where x is nitrogen,
oxygen, and fluorine.

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We call that a hydrogen-bond,
because these are really,

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really strong dipole-dipole
interactions and cause much

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different behaviors in
molecules, and so we, kind of,

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separate it out at the subset.

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So now that we've reviewed
intermolecular bonds, let's go

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back to the problem.

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So we were asked to figure out
the difference, or which one

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has a higher vapor pressure, or
temperature of boiling, so

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that's a relative thing.

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And as we now know that we need
to look at intermolecular

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bonds, a good place to start is
with the Lewis structure,

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because that will tell us if
we have dipoles or not, and

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then we can see what the
strongest intermolecular bond

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is that we need to break to
boil these molecules.

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So starting with HF, we have
hydrogen bonded to fluorine.

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We know that fluorine has a much
higher electronegativity

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than hydrogen, so we're going
to have a net dipole.

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Well, we have a polarized
bond and a net

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dipole towards fluorine.

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This causes there to be a
partial positive charge on the

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hydrogen and a partial negative

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00:05:04,480 --> 00:05:07,760
charge on the fluorine.

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00:05:07,760 --> 00:05:10,830
So what is our strongest
intermolecular bond that we

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have for HF?

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Going back to our list of bonds,
we see that we have a

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dipole, so we might first think
about dipole-dipole.

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However, we need to remember
about hydrogen bonds, and here

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in HF, we have hydrogen bonded
to fluorine, and so that would

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be our strongest interaction.

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So let's write that down
to keep track.

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Now for ammonia, we have
nitrogen bonded to three

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hydrogens, and again we
have a difference in

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00:05:58,840 --> 00:06:03,380
electronegativity, and so each
of these bonds is polar.

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00:06:03,380 --> 00:06:08,940
And because of the asymmetric
geometry, we will have a net

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dipole, and each of these
have a partially --

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sorry, the hydrogens have a
partially positive charge,

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right, because they are less
electronegative, and the

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nitrogen has a partial
negative charge.

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00:06:25,240 --> 00:06:29,830
So, which is our highest or
strongest intermolecular bond

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00:06:29,830 --> 00:06:31,750
that we have here.

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00:06:31,750 --> 00:06:36,950
Again, we have a net dipole, and
so we have dipole-dipole

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00:06:36,950 --> 00:06:41,520
interactions, but because
hydrogen is bonded to one of

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the special three atoms, we
have hydrogen bonding, and

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that is the strongest. So, that
is what we care about.

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00:06:57,990 --> 00:07:03,540
So now, we're, kind of, at a
roadblock here; both have

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hydrogen bonding, which is the
strongest intermolecular bond.

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00:07:06,510 --> 00:07:10,060
So how do we decide which one
is actually going to be

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stronger than the other, and
therefore, have the higher

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00:07:12,710 --> 00:07:15,060
boiling point.

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00:07:15,060 --> 00:07:17,620
Well even though they're both
hydrogen bonds, they will have

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difference in strength, and
that's going to depend on the

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partial positive and partial
negative charges, because all

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of these interactions are due
to coulombic attraction.

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In fluorine, we know that
fluorine is the most

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electronegative element and has
a higher electronegativity

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00:07:37,920 --> 00:07:48,920
than nitrogen, so just to keep
track of our thought process

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here: this will cause the
partial positive on hydrogen

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to be larger in the HF than in
the ammonia, and so from this

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we can say that the hydrogen
bonding in HF will be stronger

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than the hydrogen bonding in
ammonia, and therefore, the t

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evaporation of HF will be
greater than the t--

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00:08:22,070 --> 00:08:26,304
the boiling point of ammonia.

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00:08:30,100 --> 00:08:33,040
So now we're going to
move on to Part B.