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-- sometimes called the van der
Waal's interactions.

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00:00:20 --> 00:00:26
And we saw that we could make a
molecule between two inert gas

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atoms, like argon two or
xenon two,

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00:00:30 --> 00:00:35
by virtue of these dispersion
interactions,

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where the instantaneous charge
in one atom or molecule produces

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a dipole.
That dipole then induces a

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dipole in the neighboring
molecule, and the result is a

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stabilization,
an attraction.

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And we saw, last time,
we had a generic form for the

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interaction potential for the
dispersion interactions.

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This Lennard-Jones interaction
potential.

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We talked about that in detail.
And one of the parameters in

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that Lennard-Jones potential was
this quantity epsilon,

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which actually was the well
depth.

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And I just wanted to talk a few
moments, here,

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about what determines what the
well depth is,

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what the strength of that
interaction is.

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And the bottom line is,
what determines that is the

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polarizability of the atoms or
the molecules that are

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interacting.
And we give the symbol alpha to

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the polarizability.
And what that is,

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is a measure of the ease with
which a charge distribution can

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be distorted.
That is the polarizability of

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the molecule.
That is a term you will hear in

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future courses quite a lot.
And, in general,

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the polarizability goes up with
the number of electrons present.

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As you go from --
And you can see on the side

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slides, here.
As you go from helium to argon

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to xenon, and let me fix my
pointer.

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As we go from helium to argon
to xenon, this polarizability

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goes up because the number of
electrons are going up.

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If the number of electrons are
going up, that means we have

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electrons in outer shells,
they are farther away from the

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nucleus.
They are more easily distorted.

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As this polarizability goes up,
helium, argon and xenon,

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that well depth for the
Lennard-Jones potential is going

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up, 0.085, 0.996 and 1.8.
And here, on this diagram,

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I actually show you what the
shape of the Lennard-Jones

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potential is.
Sometimes when I draw it on the

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board, it is not so accurate.
But you can see this

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Lennard-Jones potentially
actually has this repulsive wall

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that goes up really very
steeply.

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And, likewise,
on the next slide,

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you see some interactions for
at least one molecule here,

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helium, nitrogen,
and argon.

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You can also have these
dispersion interactions,

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and you do have them,
between molecules.

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So, the polarizability is going
up.

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Helium, nitrogen,
argon, the well depth is going

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up.
Again, these are the

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Lennard-Jones potentials for
those inert gases and for the

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interactions between two
nitrogen atoms,

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dispersion interaction.
All atoms and molecules have

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these dispersion interactions.
It is just that often times the

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dispersion interactions can be
so weak compared to some other

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interactions,
which we are going to look at

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today, that we don't even think
about them.

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But they are actually there.
But what I want to do now is to

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talk about what happens as we
lower the temperature even

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further.
In other words,

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last time we were talking about
the deviation from the inert gas

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law as we lowered the
temperature.

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And what we said is that it is
these dispersion interactions

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that are the microscopic origin
for the deviation from the inert

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gas law, which is a macroscopic
law, as you lower the

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temperature.
But we also know that if you

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lower the temperature enough,
that the gas condenses,

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the gas liquefies.
And it is these dispersion

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interactions that are also
responsible for the condensation

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of these gases.
But now, let's talk about,

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a little bit more deeply,
what exactly is going on as we

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lower the temperature close to
the liquification point.

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Let me draw a Lennard-Jones
potential again.

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And now I am going to do it for
two nitrogen molecules.

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Here is nitrogen,
here is nitrogen,

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two separated nitrogens as a
function of r,

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the distance between the two
nuclei.

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This is our zero of
interaction.

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And, as I said from that slide,
this bond association energy

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measured from the bottom of the
well is 0.79 kilojoules per

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mole, the bottom of the well,
where the molecules actually

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can never be.
But that is the well depth.

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Now, let's think about this.
At 300 degrees Kelvin,

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what is the average energy of
the molecules?

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Well, the average energy is
three-halves RT,

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as we saw.
And, if I substitute in this

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temperature, I am going to get
something on the order of 3.7

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kilojoules per mole,
if this has two significant

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figures.
3.7 kilojoules per mole.

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Well, let me do the following.
Let me draw 3.7 kilojoules per

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mole, here, on this
Lennard-Jones potential.

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When I do that,
and I am going to start here at

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the bottom of the well,
and I draw 3.7 kilojoules per

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mole, well, that is somewhere
way up here, compared to the

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interaction energy.
At 300 degrees Kelvin,

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the bottom line is that those
two nitrogen molecules have

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enough kinetic energy to totally
ignore this interaction energy.

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They are way up here.
If they have 3.7 kilojoules per

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mole of energy,
they are not going to stick

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together.
They are not going to condense.

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They are actually really going
to ignore this small interaction

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energy.
They are just going to fly

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apart.
And they do.

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You have a gas.
However, let's lower the

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temperature now.
Say we lower the temperature to

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100 degrees Kelvin.
When we do that and calculate

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the average energy at
degrees Kelvin,

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that is on the order of 1.2 or
so kilojoules per mole average

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energy.
1.2 kilojoules per mole,

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we are somewhere here.
And now, the relative energies

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between these nitrogen molecules
are much lower.

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And it is beginning to be
comparable to this well depth.

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And those nitrogen molecules
now are deviating from the inert

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gas law.
They are kind of hanging around

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each other.
They don't hit the walls as

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often, the walls of the vessel
that they are in,

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because they are having an
attractive interaction with

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their neighboring nitrogen
molecule.

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And so, we are deviating,
here, from the inert gas law.

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The pressure is not as large if
you are doing it under constant

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volume conditions.
And then, say we lower the

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temperature to 77 degrees
Kelvin, which is actually the

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boiling point of nitrogen to
make liquid nitrogen.

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00:10:00 --> 00:10:06
Well, at 77 degrees Kelvin,
the kinetic energy is 0.96

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00:10:06 --> 00:10:11
kilojoules per mole.
And now, in this case,

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we are fairly comparable to the
well depth here at 0.96

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kilojoules per mole.
What happens is the gas

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condenses.
The interaction energy between

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those two nitrogen molecules is
on the order of the kinetic

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energy.
And then these molecules stick

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together, the gas condenses,
and you have a liquid.

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That is the origin,
here, of this temperature

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dependence and the relevance to
the microscopic interactions

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between the molecules.
You can understand that.

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And then vice versa,
if you start to raise the

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temperature, the molecules start
to fly apart.

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They ignore this interaction.
Their energy is greater than

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that attractive interaction.
And so what you can see in the

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macroscopic boiling points is
the vestiges of this microscopic

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interaction energy.
I think I am going to put the

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center screen down.

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If you look at --

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-- helium, neon,
argon, krypton,

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and xenon.
What we said is that this

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polarizability increases as we
increase the number of

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electrons.
We see that the well depth

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increases.
Therefore, the boiling point

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increases as we go down the
inert gas column.

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You can see how that
macroscopic boiling point is a

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reflection of what is happening
on the microscopic scale between

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two individual molecules.
You can also see that in this

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plot or in this chart.
Here we have molecules,

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hydrogen, nitrogen,
and oxygen.

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The polarizability alpha is
also increasing as we go down.

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Correspondingly,
the well depth is increasing.

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Correspondingly,
that boiling point is also

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getting larger and larger.
That macroscopic quantity,

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then, reflects the change in
the individual interaction

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energies between the molecules,
the microscopic quantity.

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But it also turns out that the
shape of the molecules are

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important.
For example,

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let's take this molecule,
C five H twelve.

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There are several ways I can
draw a skeletal structure for C

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five H twelve.
One way is to make it pentane,

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making a linear molecule.
And another way is to make this

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two, two dimethylpropane,

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carbon in the center,
some methyl groups around that

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center carbon.
It turns out that the boiling

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point of pentane is 309 degrees
Kelvin.

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The boiling point of
2,2-dimethylpropane is

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degrees Kelvin.
These are two molecules that

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have the same number of atoms,
i.e., the same number of

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electrons, should have the same
polarizability.

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However, one has a higher
boiling point than the other.

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And the reason for this is
because of the different shapes

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of these molecules.
In the case of propane,

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if we have an instantaneous
fluctuation in our charge

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distribution,
it is going to be essentially

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along a line.
That instantaneous fluctuation

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is essentially kind of rod-like
because of the skeletal nature

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of the pentane.
So, one end is a little

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positive, one end is a little
negative, the other way around.

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That is going to induce a
dipole in a neighboring pentane

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molecule.
And then they are going to

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align, positive-negative here,
positive-negative there.

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But then, in the case of
dimethylpropane,

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you are also going to have this
charge fluctuation.

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But in the case of propane,
here, this is a more

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spherical-looking molecule.
And so, the charge fluctuation

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is not going to be so rod-like.
But, nevertheless,

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this charge fluctuation,
induced dipole here,

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is going to induce another
dipole in a neighboring

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molecule, and you are going to
have an attractive interaction.

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00:15:40 --> 00:15:43
But, in the case of the
2,2-dimethylpropane,

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this is a much more spherical
distribution than this.

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In this case,
these two dipoles are not as

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close together as they are in
the case of pentane,

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meaning that the interaction
energy, here,

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between the dimethylpropane,
is going to be less than it is

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in the case of the pentane.
And, again, that is reflected

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00:16:08 --> 00:16:11
in the macroscopic boiling
points.

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00:16:11 --> 00:16:15
The boiling point of pentane is
larger than that of

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00:16:15 --> 00:16:20
dimethylpropane because that
microscopic interaction energy

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is larger for propane because
the induced dipoles can get

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closer together.
So, the shape is also important

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00:16:29 --> 00:16:33
in determining these boiling
points, these energies of

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interactions.
That is going to take care of

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00:16:38 --> 00:16:44
our discussion of molecules or
discussion of the induced dipole

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00:16:44 --> 00:16:49
- induced dipole interaction
energy where we are talking

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00:16:49 --> 00:16:55
about molecules that do not have
permanent dipole moments.

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00:16:55 --> 00:17:01
But now we are going to turn to
molecules with permanent dipole

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00:17:01 --> 00:17:05
moments, such as HCl.
And, of course,

220
00:17:05 --> 00:17:09
in these molecules,
the dispersion interaction is

221
00:17:09 --> 00:17:14
also taking place.
It is just that that is going

222
00:17:14 --> 00:17:19
to be weak compared to the
interaction between two

223
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permanent dipoles.
Here we have one HCl molecule,

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00:17:23 --> 00:17:27
permanent dipole.
It is going to then,

225
00:17:27 --> 00:17:31
in a collection of HCl
molecules, attract another HCl

226
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molecule.
And that HCl molecule is going

227
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to align in the opposite
direction.

228
00:17:39 --> 00:17:43
The alignment of those two
dipoles is going to lower the

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00:17:43 --> 00:17:45
energy.
That is the attractive

230
00:17:45 --> 00:17:48
interaction.
And then you might say,

231
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you get this attractive
interaction, but you also have

232
00:17:52 --> 00:17:57
now this repulsive interaction
between the two chlorines and

233
00:17:57 --> 00:18:02
the two hydrogens.
Doesn't this all cancel out?

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00:18:02 --> 00:18:06
And the answer is no.
And that is because of this.

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00:18:06 --> 00:18:11
The positive end of one
molecule and the negative end of

236
00:18:11 --> 00:18:17
the other, this distance and
this distance on the opposite

237
00:18:17 --> 00:18:22
end are actually closer than the
positive charges.

238
00:18:22 --> 00:18:26
The yellow distance,
here, is smaller than the

239
00:18:26 --> 00:18:31
distance between the two
hydrogens, is smaller than the

240
00:18:31 --> 00:18:36
distance between the two
chlorines.

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00:18:36 --> 00:18:40
And so, it is this attractive
interaction that wins out,

242
00:18:40 --> 00:18:44
actually.
When you put those two dipoles

243
00:18:44 --> 00:18:49
together, the repulsion actually
is there, but it is the

244
00:18:49 --> 00:18:54
attractive interaction that wins
out, and the whole system is

245
00:18:54 --> 00:18:59
stabilized because the distance
between the unlike charges is

246
00:18:59 --> 00:19:05
smaller than the distance
between the like charges.

247
00:19:05 --> 00:19:11
Now, it also turns out that we
have a functional form for the

248
00:19:11 --> 00:19:16
dipole-dipole attractive
interaction.

249
00:19:16 --> 00:19:23
And that attractive interaction
turns out to be a minus one over

250
00:19:23 --> 00:19:30
r cubed dependence.
And this is exact.

251
00:19:30 --> 00:19:34
You can actually derive this.
You can show that this is the

252
00:19:34 --> 00:19:39
interaction between two
permanent dipoles is one over r

253
00:19:39 --> 00:19:42
cubed.
The quantity on the top,

254
00:19:42 --> 00:19:46
mu, that is not a reduced mass,
this time.

255
00:19:46 --> 00:19:50
If we go back to when we
started talking about dipole

256
00:19:50 --> 00:19:54
moments, this is the dipole
moment of the molecule.

257
00:19:54 --> 00:20:00
We have to use our symbols for
multiple quantities.

258
00:20:00 --> 00:20:04
So, that is the dipole moment.
We are talking about

259
00:20:04 --> 00:20:07
dipole-dipole attractive
interactions.

260
00:20:07 --> 00:20:11
And that attractive interaction
looks like this,

261
00:20:11 --> 00:20:14
minus one over r cubed.

262
00:20:14 --> 00:20:18
Now, you might say,
where is the repulsive part of

263
00:20:18 --> 00:20:21
this interaction?
In the case of the

264
00:20:21 --> 00:20:25
Lennard-Jones potential,
remember we had two parts,

265
00:20:25 --> 00:20:30
the attractive part and the
repulsive part.

266
00:20:30 --> 00:20:34
The repulsive part was one over
r to the 12.

267
00:20:34 --> 00:20:38
The attractive part was one
over r to the 6.

268
00:20:38 --> 00:20:42
But it turns out that for
dipole-dipole interaction,

269
00:20:42 --> 00:20:46
we do not have a general form
for the repulsive part,

270
00:20:46 --> 00:20:49
unlike induced dipole - induced
dipole.

271
00:20:49 --> 00:20:53
So, the best we can do,
in general, is to tell you that

272
00:20:53 --> 00:20:59
the dipole-dipole attractive
interaction is one over r cubed.

273
00:20:59 --> 00:21:04
But I want to compare this one
over r cubed to the attractive

274
00:21:04 --> 00:21:09
interaction due to the induced
dipole - induced dipole

275
00:21:09 --> 00:21:12
interaction.
You see that the dipole-dipole

276
00:21:12 --> 00:21:17
interaction is what we call
longer range than the induced

277
00:21:17 --> 00:21:22
dipole - induced dipole.
What I mean by that is that the

278
00:21:22 --> 00:21:26
value of r here,
the distance between the two

279
00:21:26 --> 00:21:32
dipoles, can be larger.
In the case of a longer range

280
00:21:32 --> 00:21:36
interaction, it can be larger
and still have some non-zero

281
00:21:36 --> 00:21:39
quantity for the interaction
potential.

282
00:21:39 --> 00:21:41
For example,
if you are out here,

283
00:21:41 --> 00:21:44
if this value of r,
you can just see with the

284
00:21:44 --> 00:21:48
shorter range interaction,
one over r to the 6,

285
00:21:48 --> 00:21:52
that the attractive
interaction is only the

286
00:21:52 --> 00:21:56
difference between the pink
curve and the black curve,

287
00:21:56 --> 00:21:59
which is zero.
Whereas, with a longer range

288
00:21:59 --> 00:22:04
interaction, we have more
attractive interaction.

289
00:22:04 --> 00:22:08
This is more negative.
That energy difference is

290
00:22:08 --> 00:22:11
larger.
That is what we mean by a

291
00:22:11 --> 00:22:15
longer range interaction.
And I want to just compare,

292
00:22:15 --> 00:22:20
in this diagram here,
the strength of the dispersion

293
00:22:20 --> 00:22:25
interaction to that of the
permanent dipole-dipole

294
00:22:25 --> 00:22:30
interaction by comparing the
interaction between two argon

295
00:22:30 --> 00:22:35
atoms to those between two HCl
molecules.

296
00:22:35 --> 00:22:39
Argon only has the dispersive
interaction between it because

297
00:22:39 --> 00:22:42
it does not have a dipole
moment.

298
00:22:42 --> 00:22:47
And you can see that this well
depth, here, is one kilojoule

299
00:22:47 --> 00:22:51
per mole, roughly speaking.
But in the case of HCl,

300
00:22:51 --> 00:22:54
that well depth,
here, is three kilojoules per

301
00:22:54 --> 00:22:59
mole because that has a
permanent dipole.

302
00:22:59 --> 00:23:03
And, in both cases,
HCl and argon-argon,

303
00:23:03 --> 00:23:08
we are talking about roughly
the same number of electrons,

304
00:23:08 --> 00:23:10
not exactly,
but roughly,

305
00:23:10 --> 00:23:14
meaning the polarizability is
roughly the same.

306
00:23:14 --> 00:23:19
The dispersion interaction
energy is roughly the same for

307
00:23:19 --> 00:23:24
HCl as it is for argon,
but the difference is that HCl

308
00:23:24 --> 00:23:27
has that permanent dipole
moment.

309
00:23:27 --> 00:23:30
Therefore, the deeper well
depth, therefore,

310
00:23:30 --> 00:23:35
in HCl, the higher boiling
point, 239 degrees Kelvin as

311
00:23:35 --> 00:23:43
opposed to 87 Kelvin for argon.
What is on this slide is just

312
00:23:43 --> 00:23:49
an example of how that
dipole-dipole interaction energy

313
00:23:49 --> 00:23:56
varies with the dipole moment.
The larger the dipole,

314
00:23:56 --> 00:24:02
of course, the larger the
interaction energy.

315
00:24:02 --> 00:24:05
Here, I show you several
molecules that all have,

316
00:24:05 --> 00:24:08
again, roughly the same number
of atoms.

317
00:24:08 --> 00:24:12
They have the same mass,
they roughly have the same

318
00:24:12 --> 00:24:15
number of electrons,
so they roughly have the same

319
00:24:15 --> 00:24:19
polarizability.
The induced dipole - induced

320
00:24:19 --> 00:24:22
dipole is the same,
but what is changing here,

321
00:24:22 --> 00:24:25
as I go down,
is the dipole moment.

322
00:24:25 --> 00:24:28
For propane,
dipole moment really small,

323
00:24:28 --> 00:24:32
0.1 debye.
Dimethyl ethe,r 1.3.

324
00:24:32.371 --> 2.7.
Acetaldehyde,

325
2.7. --> 00:24:34


326
00:24:34.387 --> 3.9.
Acetonitrile,

327
3.9. --> 00:24:36


328
00:24:36 --> 00:24:42
Dipole moment increases.
The boiling point increases

329
00:24:42 --> 00:24:48
because that attractive
interaction is increasing.

330
00:24:48 --> 00:24:53
It scales roughly as the dipole
moment squared.

331
00:24:53 --> 00:25:00
The dipole-dipole interaction
is stronger.

332
00:25:00 --> 00:25:04
Now, I just want to briefly
then remind you about one other

333
00:25:04 --> 00:25:09
attractive interaction that we
have talked about before.

334
00:25:09 --> 00:25:14
And that is between two ions.
There we are talking about the

335
00:25:14 --> 00:25:18
Coulomb interaction energy,
where the dependence is minus

336
00:25:18 --> 00:25:22
one over r.
That is the longest range

337
00:25:22 --> 00:25:25
interaction.
Again, you can see this way out

338
00:25:25 --> 00:25:28
here.
If I choose this value of r,

339
00:25:28 --> 00:25:33
the one over six interaction
term would give me a

340
00:25:33 --> 00:25:37
very small value for the
attractive interaction,

341
00:25:37 --> 00:25:41
--
-- because it is one over r to

342
00:25:41 --> 00:25:43
the six.
You take a large number for r

343
00:25:43 --> 00:25:47
and raise it to the 6 power and
put in the denominator.

344
00:25:47 --> 00:25:50
You are going to have a small
value for U of r.

345
00:25:50 --> 00:25:54
The one over r to the three
gives you some

346
00:25:54 --> 00:25:57
interaction energy,
but one over r gives you a lot

347
00:25:57 --> 00:26:02
of attractive interaction.
It is the longest range.

348
00:26:02 --> 00:26:05
And you can also see here,
in a moment,

349
00:26:05 --> 00:26:09
that it is going to be the
strongest interaction.

350
00:26:09 --> 00:26:13
What I am just doing is
comparing several molecules or

351
00:26:13 --> 00:26:17
atoms.
Here is argon 2 which has only

352
00:26:17 --> 00:26:20
the dispersive interaction.
Here is the plot,

353
00:26:20 --> 00:26:25
well depth minus one kilojoule.
Then, there is the HCl

354
00:26:25 --> 00:26:30
interaction energy that has the
dispersion interaction in it,

355
00:26:30 --> 00:26:36
but also the dipole-dipole
permanent interaction.

356
00:26:36 --> 00:26:40
It is minus three kilojoules.
And then we are talking about

357
00:26:40 --> 00:26:44
chlorine two.
This is a covalent bond.

358
00:26:44 --> 00:26:49
This is no longer a
Lennard-Jones potential energy,

359
00:26:49 --> 00:26:53
but this well depth,
here, is minus 200 kilojoules

360
00:26:53 --> 00:26:55
per mole.
And then, finally,

361
00:26:55 --> 00:27:00
this ionic interaction between
potassium and chlorine,

362
00:27:00 --> 00:27:06
look at how strong that is,
minus 450 kilojoules per mole.

363
00:27:06 --> 00:27:11
So, these are the relative
strengths here of these

364
00:27:11 --> 00:27:18
interaction energies.
That is all I that want to say

365
00:27:18 --> 00:27:23
about these kinds of
intermolecular interactions,

366
00:27:23 --> 00:27:30
where we are dealing with some
kind of dipole.

367
00:27:30 --> 00:27:34
But before we move on,
I want to talk about one other

368
00:27:34 --> 00:27:39
kind of intermolecular
interaction potential.

369
00:27:39 --> 00:28:00


370
00:28:00 --> 00:28:05
And that is something called
hydrogen bonding.

371
00:28:05 --> 00:28:15


372
00:28:15 --> 00:28:20
All right.
A final intermolecular

373
00:28:20 --> 00:28:26
interaction, hydrogen bonding.
Hydrogen bonding,

374
00:28:26 --> 00:28:33
here, occurs between a hydrogen
atom that is attached to an

375
00:28:33 --> 00:28:39
electronegative atom and another
molecule in the gas or in the

376
00:28:39 --> 00:28:43
solution.
The first requirement is that

377
00:28:43 --> 00:28:50
you have hydrogen attached to a
very electronegative atom.

378
00:28:50 --> 00:28:56
Hydrogen bonding occurs for
hydrogens attached to oxygen,

379
00:28:56 --> 00:29:02
nitrogen, and chlorine.
Those are all electronegative

380
00:29:02 --> 00:29:05
atoms.
Water is a good example.

381
00:29:05 --> 00:29:12
In the case of water-- You have
hydrogen bonded to this oxygen.

382
00:29:12 --> 00:29:16
This oxygen is really very
electronegative.

383
00:29:16 --> 00:29:22
What that oxygen does is it
pulls those electrons away from

384
00:29:22 --> 00:29:25
the hydrogen.
This hydrogen is kind of

385
00:29:25 --> 00:29:32
partially unshielded.
It is partially deshielded.

386
00:29:32 --> 00:29:38
And then, there is a lot of
electron density here on this

387
00:29:38 --> 00:29:42
oxygen.
Well, because that hydrogen is

388
00:29:42 --> 00:29:48
partially deshielded and because
it is really small,

389
00:29:48 --> 00:29:54
this hydrogen actually will
kind of see the oxygen atoms on

390
00:29:54 --> 00:30:01
a neighboring water molecule.
And since this is kind of

391
00:30:01 --> 00:30:05
partially negative,
this hydrogen will interact

392
00:30:05 --> 00:30:10
with one of the lone pairs on
this oxygen and will form a

393
00:30:10 --> 00:30:14
bond.
This is a little bit partially

394
00:30:14 --> 00:30:18
positive, this is a little bit
partially negative,

395
00:30:18 --> 00:30:22
and the result,
here, is a bond between the

396
00:30:22 --> 00:30:27
hydrogen and the oxygen.
And that bond is on the order

397
00:30:27 --> 00:30:34
of 20 to 60 kilojoules per mole.
This bond is not a covalent

398
00:30:34 --> 00:30:36
bond.
A covalent bond is

399
00:30:36 --> 00:30:39
kilojoules per mole.
This is 10% of it,

400
00:30:39 --> 00:30:43
but it is still a very
important quantity or a very

401
00:30:43 --> 00:30:47
important phenomenon,
this hydrogen bonding.

402
00:30:47 --> 00:30:51
The hydrogen bonding is
certainly responsible for the

403
00:30:51 --> 00:30:56
peculiar properties of water,
as you will learn more about in

404
00:30:56 --> 00:31:00
5.60, but it is also responsible
for the unique shape,

405
00:31:00 --> 00:31:05
oftentimes, of biological
molecules.

406
00:31:05 --> 00:31:10
The helix in DNA owes its
structure to hydrogen bonding.

407
00:31:10 --> 00:31:16
The hydrogen bonding is what
makes trees stand upright.

408
00:31:16 --> 00:31:21
The long cellulose molecules in
trees are actually bonded

409
00:31:21 --> 00:31:27
together by hydrogen bonding.
Nylon owes its strength to

410
00:31:27 --> 00:31:33
hydrogen bonding.
And hydrogen bonding is also

411
00:31:33 --> 00:31:40
responsible for whether or not
you have a bad hair day.

412
00:31:40 --> 00:31:47
As an example of that,
I want you to look at the slide

413
00:31:47 --> 00:31:53
up there on the walls.
What you see is a protein

414
00:31:53 --> 00:32:00
molecule.
This is the structure of hair.

415
00:32:00 --> 00:32:05
The molecules that make up the
strands of your hair look like

416
00:32:05 --> 00:32:07
this.
It is a polymer.

417
00:32:07 --> 00:32:12
Well, it is a polypeptide.
This unit right in here,

418
00:32:12 --> 00:32:17
carbon-oxygen bound to
carbon-hydrogen bound to

419
00:32:17 --> 00:32:20
nitrogen-hydrogen is the
peptide.

420
00:32:20 --> 00:32:24
It is repeated.
You see the next unit over?

421
00:32:24 --> 00:32:26
CO-CH-NH.
CO-CH-NH.

422
00:32:26 --> 00:32:32
That keeps repeating.
That is the repeat unit.

423
00:32:32 --> 00:32:36
And, of course,
this carbon-hydrogen right

424
00:32:36 --> 00:32:41
there, you can see it has a line
there, indicating that it is

425
00:32:41 --> 00:32:46
bonded to something.
And it is bonded to something.

426
00:32:46 --> 00:32:51
If that carbon-hydrogen is
bound to another hydrogen,

427
00:32:51 --> 00:32:56
then you have a polypeptide
which has been made from an

428
00:32:56 --> 00:33:02
amino acid that you might know
of as glycine.

429
00:33:02 --> 00:33:07
If that carbon is bound to a CH
three group,

430
00:33:07 --> 00:33:13
then that peptide was made from
an amino acid that you might

431
00:33:13 --> 00:33:18
know of as alanine.
And if it is bound to a CH two

432
00:33:18 --> 00:33:23
S H group,
well, that was an amino acid

433
00:33:23 --> 00:33:28
known as cysteine.
But what I want you to notice

434
00:33:28 --> 00:33:35
here is that these hydrogens on
the nitrogen --

435
00:33:35 --> 00:33:38
That nitrogen is an electron
negative atom.

436
00:33:38 --> 00:33:42
And that hydrogen,
if your hair is wet,

437
00:33:42 --> 00:33:46
it is actually hydrogen bonded
to a water molecule.

438
00:33:46 --> 00:33:51
Here is that hydrogen bond.
And then in the next strand

439
00:33:51 --> 00:33:57
over, this oxygen is hydrogen
bonded to a water molecule when

440
00:33:57 --> 00:34:02
your hair is wet.
And the bottom line is that

441
00:34:02 --> 00:34:08
when your hair is wet,
you have the strands of your

442
00:34:08 --> 00:34:12
hair that kind of slip by each
other.

443
00:34:12 --> 00:34:16
There is no registry of one
strand to another,

444
00:34:16 --> 00:34:21
because each strand has this
coating, here,

445
00:34:21 --> 00:34:26
of water molecules due to
hydrogen bonding.

446
00:34:26 --> 00:34:32
Suppose you take your hair and
put it in a very contorted

447
00:34:32 --> 00:34:38
configuration like this.
And then you drive off the

448
00:34:38 --> 00:34:41
water molecules,
you dry your hair.

449
00:34:41 --> 00:34:47
What happens is that these
water molecules then leave,

450
00:34:47 --> 00:34:53
the hydrogen bonds are broken.
This is not such a strong bond,

451
00:34:53 --> 00:34:58
20 to 60 kilojoules per mole,
those hydrogen bonds are

452
00:34:58 --> 00:35:02
broken.
And then this hydrogen on this

453
00:35:02 --> 00:35:07
nitrogen looks around and sees
the oxygen with its lone pairs

454
00:35:07 --> 00:35:11
on this strand,
and so you form a hydrogen bond

455
00:35:11 --> 00:35:14
between this strand and this
strand.

456
00:35:14 --> 00:35:19
And so now the strands of your
hair are in registry with each

457
00:35:19 --> 00:35:22
other.
They are actually stronger.

458
00:35:22 --> 00:35:27
And they do tell you not to
brush your hair when it is wet,

459
00:35:27 --> 00:35:33
because it isn't so strong.
Well, it is not so strong

460
00:35:33 --> 00:35:39
because these water molecules
are insulating each one of these

461
00:35:39 --> 00:35:42
strands.
And when it is dry these two

462
00:35:42 --> 00:35:48
strands are bond to each other.
They are in registry with each

463
00:35:48 --> 00:35:51
other.
And so, if you do this right,

464
00:35:51 --> 00:35:56
and you then let go of the
contorted configuration,

465
00:35:56 --> 00:36:01
which I am having trouble
doing, those strands are in

466
00:36:01 --> 00:36:08
registry with each other now.
And you have a good hair day.

467
00:36:08 --> 00:36:13
So, that is the importance of
hydrogen bonding.

468
00:36:13 --> 00:36:19
I curled my hair just for this
demo, a little asymmetric,

469
00:36:19 --> 00:36:22
here.
[LAUGHTER] But now,

470
00:36:22 --> 00:36:26
if you are as fortunate,
or unfortunate,

471
00:36:26 --> 00:36:33
depending on your preference,
to have naturally curly hair,

472
00:36:33 --> 00:36:40
then what you have are a lot of
sulfur-sulfur bonds.

473
00:36:40 --> 00:36:49
You have a lot of cysteine
peptide groups.

474
00:36:49 --> 00:36:58
What happens there is this.
On the CH groups,

475
00:36:58 --> 00:37:02
you have CH two sulfur H.

476
00:37:02 --> 00:37:07
And on the next strand over you
have S, CH two,

477
00:37:07 --> 00:37:11
and CH bonded to nitrogen,
bonded to a CO.

478
00:37:11 --> 00:37:17
And you actually have a
covalent bond between these two

479
00:37:17 --> 00:37:21
sulfurs, here.
This is a strong bond.

480
00:37:21 --> 00:37:27
The strands of your polymers in
your hair are in registry all of

481
00:37:27 --> 00:37:32
the time.
And if you want to make your

482
00:37:32 --> 00:37:36
natural curly hair straight you
have to do drastic things like

483
00:37:36 --> 00:37:40
use drastic chemicals to break
this sulfur-sulfur bond.

484
00:37:40 --> 00:37:43
You can do it,
but it is not easy.

485
00:37:43 --> 00:37:48
Likewise, if you have naturally
straight hair and you want to

486
00:37:48 --> 00:37:51
curl it and make it
semi-permanently curly,

487
00:37:51 --> 00:37:55
then you have to build in the
sulfur-sulfur bonds.

488
00:37:55 --> 00:37:58
And you have to do,
again, some rather drastic

489
00:37:58 --> 00:38:05
chemistry to make that happen.
Hydrogen bonding is important,

490
00:38:05 --> 00:38:12
especially if you go to do
anything in biologically-

491
00:38:12 --> 00:38:16
related sciences,
you will see that.

492
00:38:16 --> 00:38:22
Now, I am going to change
topics here.

493
00:38:22 --> 00:38:30


494
00:38:30 --> 00:38:36
I am going to change topics,
and we are going to talk a

495
00:38:36 --> 00:38:42
little bit about thermodynamics
in preparation to get up to

496
00:38:42 --> 00:38:48
chemical equilibrium so that
Professor Cummins can come in

497
00:38:48 --> 00:38:55
next Wednesday and start talking
about acid-base equilibrium.

498
00:38:55 --> 00:39:00
He is great.
You will love him.

499
00:39:00 --> 00:39:04
We are going to review some
thermodynamics today.

500
00:39:04 --> 00:39:08
I am going to go kind of
quickly because some of this I

501
00:39:08 --> 00:39:12
think you know,
but I want to make sure

502
00:39:12 --> 00:39:15
everybody is on the same page.
First of all,

503
00:39:15 --> 00:39:19
bond energies.
We talked about bond energies

504
00:39:19 --> 00:39:23
as delta E sub D.
And we measured it from the

505
00:39:23 --> 00:39:27
bottom of the well.
And I told you a few days ago,

506
00:39:27 --> 00:39:32
I lied to you.
The measured energies are

507
00:39:32 --> 00:39:35
really from v equals zero,
and they are.

508
00:39:35 --> 00:39:39
But what I am going to do is
change my language.

509
00:39:39 --> 00:39:43
Instead of talking about
energies, I am going to talk

510
00:39:43 --> 00:39:47
about enthalpies.
I am going to talk about delta

511
00:39:47 --> 00:39:51
Hs rather than delta Es.
The reason I am going to do

512
00:39:51 --> 00:39:56
this is because it is easier for
us to measure a bond enthalpy

513
00:39:56 --> 00:40:00
than a bond energy.
And that has to do with the

514
00:40:00 --> 00:40:04
fact that we usually make
measurements in bulk under

515
00:40:04 --> 00:40:09
constant pressure conditions.
And that is the quantity that

516
00:40:09 --> 00:40:12
comes out.
The relationship between delta

517
00:40:12 --> 00:40:15
H and delta E is this.
Delta H is equal to delta E

518
00:40:15 --> 00:40:18
plus delta PV.

519
00:40:18 --> 00:40:22
This is a
relationship that you will learn

520
00:40:22 --> 00:40:25
about in a lot of detail in
5.60, in Chemical

521
00:40:25 --> 00:40:29
Thermodynamics.
At this point,

522
00:40:29 --> 00:40:32
we are going to take it as a
given.

523
00:40:32 --> 00:40:38
For gases, delta H differs on
the order of 1% to 2% from delta

524
00:40:38 --> 00:40:39
E.
It is not much,

525
00:40:39 --> 00:40:44
but if you are doing some
precise calculation,

526
00:40:44 --> 00:40:48
you need to be aware of that.
For liquids and solids,

527
00:40:48 --> 00:40:54
delta H and delta E are really
the same for all intrinsic

528
00:40:54 --> 00:40:57
purposes.
The delta PV term is really

529
00:40:57 --> 00:41:02
very small.
And, in thermodynamics,

530
00:41:02 --> 00:41:06
since we are most always
looking at changes in energy,

531
00:41:06 --> 00:41:10
we need what we call standard
states.

532
00:41:10 --> 00:41:13
And we are going to put a
nought, here,

533
00:41:13 --> 00:41:18
on all of our delta Hs to
designate the standard state.

534
00:41:18 --> 00:41:22
And our standard state that
your book uses,

535
00:41:22 --> 00:41:26
and will use,
refers really to the pressure.

536
00:41:26 --> 00:41:33
And the pressure is one bar.
And one bar is equal to 10^5

537
00:41:33 --> 00:41:37
Pascal.
That is equal to 10^5 kilograms

538
00:41:37 --> 00:41:43
per meter second squared.
The delta Hs we are going to

539
00:41:43 --> 00:41:47
talk about are also,
just about all of them,

540
00:41:47 --> 00:41:51
measured at 298.15 degrees
Kelvin.

541
00:41:51 --> 00:41:57
Delta H does depend on
temperature, but we actually are

542
00:41:57 --> 00:42:04
not going to look at that in the
next few days.

543
00:42:04 --> 5.60.
You are going to do that in

544
5.60. --> 00:42:07


545
00:42:07 --> 00:42:12
Our delta Hs are going to be
delta Hs at 298.15 degrees

546
00:42:12 --> 00:42:15
Kelvin.
On the first slide here,

547
00:42:15 --> 00:42:20
I show you a bunch of bond
enthalpies for CH bonds.

548
00:42:20 --> 00:42:25
And, of course,
those bond enthalpies are a

549
00:42:25 --> 00:42:30
little bit different,
depending on what molecule you

550
00:42:30 --> 00:42:33
have.
But they are not that

551
00:42:33 --> 00:42:36
different.
And so, what is often done,

552
00:42:36 --> 00:42:40
and your book does this,
is that somebody goes and

553
00:42:40 --> 00:42:45
calculates the average of the
bond energies for lots of CH

554
00:42:45 --> 00:42:50
bonds and lots of molecules and
they prepare a table that looks

555
00:42:50 --> 00:42:53
like this.
This is the mean bond enthalpy.

556
00:42:53 --> 00:42:56
And they have CH,
CC, carbon-carbon.

557
00:42:56 --> 00:43:01
But these are average bond
enthalpies.

558
00:43:01 --> 00:43:04
Now, why are bond enthalpies
important to us?

559
00:43:04 --> 00:43:08
Well, they are important
because they determine the

560
00:43:08 --> 00:43:13
enthalpy of a chemical reaction.
If the bonds are stronger in

561
00:43:13 --> 00:43:18
the products than in the
reactants, that is going to give

562
00:43:18 --> 00:43:22
us an exothermic reaction.
If the bonds are stronger in

563
00:43:22 --> 00:43:27
the reactants than the products,
that gives us an endothermic

564
00:43:27 --> 00:43:31
reaction.
And so let's look at this

565
00:43:31 --> 00:43:34
reaction.
This is an important reaction.

566
00:43:34 --> 00:43:36
This is the oxidation of
glucose.

567
00:43:36 --> 00:43:40
This is a reaction very
exothermic, minus 

568
00:43:40 --> 00:43:44
kilojoules per mole.
It is a reaction that is being

569
00:43:44 --> 00:43:48
carried out in every cell of
your body as we speak.

570
00:43:48 --> 00:43:53
It is the reaction that is
providing the energy to maintain

571
00:43:53 --> 00:43:56
your body temperature,
the energy to move your

572
00:43:56 --> 00:44:03
muscles, the energy to repair
tissue, and the energy to think.

573
00:44:03 --> 00:44:06
Important reaction.
This is the reason why we eat.

574
00:44:06 --> 00:44:10
This is the reason why we
breathe, this is the reason why

575
00:44:10 --> 00:44:13
we exhale, and this is the
reason why we pee.

576
00:44:13 --> 00:44:15
[LAUGHTER]

577
00:44:15 --> 00:44:23


578
00:44:23 --> 00:44:27
What do we need to do to
calculate the enthalpy for this

579
00:44:27 --> 00:44:30
reaction?
We have to figure out how much

580
00:44:30 --> 00:44:35
energy is required to break all
of the bonds of the reactants

581
00:44:35 --> 00:44:38
because that is how much energy
we put in.

582
00:44:38 --> 00:44:43
And then we have to figure out
then how much energy we get back

583
00:44:43 --> 00:44:47
when we form the product bonds.
Bottom line is,

584
00:44:47 --> 00:44:51
the enthalpy necessary to break
all of the bonds,

585
00:44:51 --> 00:44:54
you can calculate,
is 12,452 kilojoules per mole.

586
00:44:54 --> 00:44:59
That number comes from using
these average bond energies I

587
00:44:59 --> 00:45:04
told you about.
We can then get back some

588
00:45:04 --> 00:45:07
energy, minus 15,000,
approximately,

589
00:45:07 --> 00:45:12
when we form some new bonds.
That number comes from those

590
00:45:12 --> 00:45:17
average bond enthalpies.
The difference between these

591
00:45:17 --> 00:45:22
two energy levels is the
exothermicity of the reaction.

592
00:45:22 --> 00:45:25
Now, what did we do to get the
enthalpy?

593
00:45:25 --> 00:45:31
Well, what I did is took the
bond enthalpies of each bond of

594
00:45:31 --> 00:45:36
the reactants and summed them.
Then, I took the bond

595
00:45:36 --> 00:45:40
enthalpies for each one of the
products, summed them,

596
00:45:40 --> 00:45:44
and subtracted the two to get
the enthalpy of the reaction.

597
00:45:44 --> 00:45:47
I want you to notice something
here, important.

598
00:45:47 --> 00:45:49
This is reactants minus
products.

599
00:45:49 --> 00:45:52
In a moment,
I am going to show you another

600
00:45:52 --> 00:45:55
way to calculate the enthalpy
for a reaction.

601
00:45:55 --> 00:46:00
And it is going to be products
minus reactants.

602
00:46:00 --> 00:46:03
You have to know this.
But you also see that the

603
00:46:03 --> 00:46:07
calculated enthalpy is not the
experimental enthalpy.

604
00:46:07 --> 00:46:10
Why?
Because we use the average bond

605
00:46:10 --> 00:46:13
enthalpies.
We did not use the exact bond

606
00:46:13 --> 00:46:18
enthalpies because if we had to
use the exact bond enthalpies,

607
00:46:18 --> 00:46:23
can you imagine the size of the
table of data that we would have

608
00:46:23 --> 00:46:26
to have?
We would have to have a bond

609
00:46:26 --> 00:46:31
enthalpy for every bond for
every known molecule.

610
00:46:31 --> 00:46:34
That is a lot.
What are we going to do,

611
00:46:34 --> 00:46:37
then?
Is there a more accurate way to

612
00:46:37 --> 00:46:41
do that?
Yes, with knowing the absolute

613
00:46:41 --> 00:46:45
bond enthalpies.
But that is too much data.

614
00:46:45 --> 00:46:47
Is there some other way to do
it?

615
00:46:47 --> 00:46:50
Yes.
We are going to use heats of

616
00:46:50 --> 00:46:53
formation.
A heat of formation,

617
00:46:53 --> 00:46:57
delta H nought with an F as a
subscript.

618
00:46:57 --> 00:47:02
The heat of formation is the

619
00:47:02 --> 00:47:08
enthalpy of a reaction that
forms one mole of a compound

620
00:47:08 --> 00:47:14
from the pure elements in their
most stable form in their

621
00:47:14 --> 00:47:17
standard state.
For example,

622
00:47:17 --> 00:47:21
here is water.
We are forming one mole of

623
00:47:21 --> 00:47:25
water from its elements,
hydrogen and oxygen.

624
00:47:25 --> 00:47:31
The enthalpy for this reaction
is defined as the heat of

625
00:47:31 --> 00:47:36
formation of water.
Why is this the heat of

626
00:47:36 --> 00:47:39
formation of water?
Well, because we are forming

627
00:47:39 --> 00:47:44
one mole, and that is important,
from the elements that make up

628
00:47:44 --> 00:47:46
water.
What elements are they?

629
00:47:46 --> 00:47:49
They are hydrogen.
But notice that this hydrogen

630
00:47:49 --> 00:47:52
is H two.
It is not hydrogen atoms

631
00:47:52 --> 00:47:56
because H two is the most
stable form of hydrogen.

632
00:47:56 --> 00:48:00
Oxygen is O two,
not oxygen atoms because this

633
00:48:00 --> 00:48:05
is the most stable form of
oxygen at bar pressure.

634
00:48:05 --> 00:48:08
Look at this here.
What is the heat of formation

635
00:48:08 --> 00:48:12
of oxygen?
Well, the enthalpy change for

636
00:48:12 --> 00:48:16
this reaction is zero.
That is the heat of formation

637
00:48:16 --> 00:48:17
of oxygen.
Why?

638
00:48:17 --> 00:48:22
Because we are forming one mole
of oxygen from the elements in

639
00:48:22 --> 00:48:27
their most stable form.
For elements like oxygen,

640
00:48:27 --> 00:48:30
hydrogen, nitrogen,
chlorine, two

641
00:48:30 --> 00:48:34
in the gas phase,
those all have heats of

642
00:48:34 --> 00:48:39
formation that are equal to
zero.

643
00:48:39 --> 00:48:42
And then, finally,
here is the expression or the

644
00:48:42 --> 00:48:45
reaction that gives us one mole
of glucose.

645
00:48:45 --> 00:48:49
The enthalpy for this reaction
is the heat of formation of

646
00:48:49 --> 00:48:51
glucose.
We get it from its elements,

647
00:48:51 --> 00:48:55
hydrogen, oxygen,
and look at the elemental form,

648
00:48:55 --> 00:49:00
the most stable form of the
element carbon is graphite.

649
00:49:00 --> 00:49:02
Is there an 18.0-something
exam?

650
00:49:02 --> 00:49:03
Yes?
Okay.

651
00:49:03 --> 49:06
See you on Monday.