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

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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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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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Well, at 77 degrees Kelvin,
the kinetic energy is 0.96

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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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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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00:16:08,000 --> 00:16:11,000
in the macroscopic boiling
points.

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00:16:11,000 --> 00:16:15,000
The boiling point of pentane is
larger than that of

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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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in determining these boiling
points, these energies of

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interactions.
That is going to take care of

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our discussion of molecules or
discussion of the induced dipole

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00:16:44,000 --> 00:16:49,000
- induced dipole interaction
energy where we are talking

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about molecules that do not have
permanent dipole moments.

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00:16:55,000 --> 00:17:01,000
But now we are going to turn to
molecules with permanent dipole

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00:17:01,000 --> 00:17:05,000
moments, such as HCl.
And, of course,

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in these molecules,
the dispersion interaction is

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also taking place.
It is just that that is going

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to be weak compared to the
interaction between two

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permanent dipoles.
Here we have one HCl molecule,

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00:17:23,000 --> 00:17:27,000
permanent dipole.
It is going to then,

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in a collection of HCl
molecules, attract another HCl

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00:17:31,000 --> 00:17:36,000
molecule.
And that HCl molecule is going

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to align in the opposite
direction.

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00:17:39,000 --> 00:17:43,000
The alignment of those two
dipoles is going to lower the

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00:17:43,000 --> 00:17:45,000
energy.
That is the attractive

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00:17:45,000 --> 00:17:48,000
interaction.
And then you might say,

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you get this attractive
interaction, but you also have

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00:17:52,000 --> 00:17:57,000
now this repulsive interaction
between the two chlorines and

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00:17:57,000 --> 00:18:02,000
the two hydrogens.
Doesn't this all cancel out?

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00:18:02,000 --> 00:18:06,000
And the answer is no.
And that is because of this.

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00:18:06,000 --> 00:18:11,000
The positive end of one
molecule and the negative end of

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00:18:11,000 --> 00:18:17,000
the other, this distance and
this distance on the opposite

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00:18:17,000 --> 00:18:22,000
end are actually closer than the
positive charges.

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00:18:22,000 --> 00:18:26,000
The yellow distance,
here, is smaller than the

236
00:18:26,000 --> 00:18:31,000
distance between the two
hydrogens, is smaller than the

237
00:18:31,000 --> 00:18:36,000
distance between the two
chlorines.

238
00:18:36,000 --> 00:18:40,000
And so, it is this attractive
interaction that wins out,

239
00:18:40,000 --> 00:18:44,000
actually.
When you put those two dipoles

240
00:18:44,000 --> 00:18:49,000
together, the repulsion actually
is there, but it is the

241
00:18:49,000 --> 00:18:54,000
attractive interaction that wins
out, and the whole system is

242
00:18:54,000 --> 00:18:59,000
stabilized because the distance
between the unlike charges is

243
00:18:59,000 --> 00:19:05,000
smaller than the distance
between the like charges.

244
00:19:05,000 --> 00:19:11,000
Now, it also turns out that we
have a functional form for the

245
00:19:11,000 --> 00:19:16,000
dipole-dipole attractive
interaction.

246
00:19:16,000 --> 00:19:23,000
And that attractive interaction
turns out to be a minus one over

247
00:19:23,000 --> 00:19:30,000
r cubed dependence.
And this is exact.

248
00:19:30,000 --> 00:19:34,000
You can actually derive this.
You can show that this is the

249
00:19:34,000 --> 00:19:39,000
interaction between two
permanent dipoles is one over r

250
00:19:39,000 --> 00:19:42,000
cubed.
The quantity on the top,

251
00:19:42,000 --> 00:19:46,000
mu, that is not a reduced mass,
this time.

252
00:19:46,000 --> 00:19:50,000
If we go back to when we
started talking about dipole

253
00:19:50,000 --> 00:19:54,000
moments, this is the dipole
moment of the molecule.

254
00:19:54,000 --> 00:20:00,000
We have to use our symbols for
multiple quantities.

255
00:20:00,000 --> 00:20:04,000
So, that is the dipole moment.
We are talking about

256
00:20:04,000 --> 00:20:07,000
dipole-dipole attractive
interactions.

257
00:20:07,000 --> 00:20:11,000
And that attractive interaction
looks like this,

258
00:20:11,000 --> 00:20:14,000
minus one over r cubed.

259
00:20:14,000 --> 00:20:18,000
Now, you might say,
where is the repulsive part of

260
00:20:18,000 --> 00:20:21,000
this interaction?
In the case of the

261
00:20:21,000 --> 00:20:25,000
Lennard-Jones potential,
remember we had two parts,

262
00:20:25,000 --> 00:20:30,000
the attractive part and the
repulsive part.

263
00:20:30,000 --> 00:20:34,000
The repulsive part was one over
r to the 12.

264
00:20:34,000 --> 00:20:38,000
The attractive part was one
over r to the 6.

265
00:20:38,000 --> 00:20:42,000
But it turns out that for
dipole-dipole interaction,

266
00:20:42,000 --> 00:20:46,000
we do not have a general form
for the repulsive part,

267
00:20:46,000 --> 00:20:49,000
unlike induced dipole - induced
dipole.

268
00:20:49,000 --> 00:20:53,000
So, the best we can do,
in general, is to tell you that

269
00:20:53,000 --> 00:20:59,000
the dipole-dipole attractive
interaction is one over r cubed.

270
00:20:59,000 --> 00:21:04,000
But I want to compare this one
over r cubed to the attractive

271
00:21:04,000 --> 00:21:09,000
interaction due to the induced
dipole - induced dipole

272
00:21:09,000 --> 00:21:12,000
interaction.
You see that the dipole-dipole

273
00:21:12,000 --> 00:21:17,000
interaction is what we call
longer range than the induced

274
00:21:17,000 --> 00:21:22,000
dipole - induced dipole.
What I mean by that is that the

275
00:21:22,000 --> 00:21:26,000
value of r here,
the distance between the two

276
00:21:26,000 --> 00:21:32,000
dipoles, can be larger.
In the case of a longer range

277
00:21:32,000 --> 00:21:36,000
interaction, it can be larger
and still have some non-zero

278
00:21:36,000 --> 00:21:39,000
quantity for the interaction
potential.

279
00:21:39,000 --> 00:21:41,000
For example,
if you are out here,

280
00:21:41,000 --> 00:21:44,000
if this value of r,
you can just see with the

281
00:21:44,000 --> 00:21:48,000
shorter range interaction,
one over r to the 6,

282
00:21:48,000 --> 00:21:52,000
that the attractive
interaction is only the

283
00:21:52,000 --> 00:21:56,000
difference between the pink
curve and the black curve,

284
00:21:56,000 --> 00:21:59,000
which is zero.
Whereas, with a longer range

285
00:21:59,000 --> 00:22:04,000
interaction, we have more
attractive interaction.

286
00:22:04,000 --> 00:22:08,000
This is more negative.
That energy difference is

287
00:22:08,000 --> 00:22:11,000
larger.
That is what we mean by a

288
00:22:11,000 --> 00:22:15,000
longer range interaction.
And I want to just compare,

289
00:22:15,000 --> 00:22:20,000
in this diagram here,
the strength of the dispersion

290
00:22:20,000 --> 00:22:25,000
interaction to that of the
permanent dipole-dipole

291
00:22:25,000 --> 00:22:30,000
interaction by comparing the
interaction between two argon

292
00:22:30,000 --> 00:22:35,000
atoms to those between two HCl
molecules.

293
00:22:35,000 --> 00:22:39,000
Argon only has the dispersive
interaction between it because

294
00:22:39,000 --> 00:22:42,000
it does not have a dipole
moment.

295
00:22:42,000 --> 00:22:47,000
And you can see that this well
depth, here, is one kilojoule

296
00:22:47,000 --> 00:22:51,000
per mole, roughly speaking.
But in the case of HCl,

297
00:22:51,000 --> 00:22:54,000
that well depth,
here, is three kilojoules per

298
00:22:54,000 --> 00:22:59,000
mole because that has a
permanent dipole.

299
00:22:59,000 --> 00:23:03,000
And, in both cases,
HCl and argon-argon,

300
00:23:03,000 --> 00:23:08,000
we are talking about roughly
the same number of electrons,

301
00:23:08,000 --> 00:23:10,000
not exactly,
but roughly,

302
00:23:10,000 --> 00:23:14,000
meaning the polarizability is
roughly the same.

303
00:23:14,000 --> 00:23:19,000
The dispersion interaction
energy is roughly the same for

304
00:23:19,000 --> 00:23:24,000
HCl as it is for argon,
but the difference is that HCl

305
00:23:24,000 --> 00:23:27,000
has that permanent dipole
moment.

306
00:23:27,000 --> 00:23:30,000
Therefore, the deeper well
depth, therefore,

307
00:23:30,000 --> 00:23:35,000
in HCl, the higher boiling
point, 239 degrees Kelvin as

308
00:23:35,000 --> 00:23:43,000
opposed to 87 Kelvin for argon.
What is on this slide is just

309
00:23:43,000 --> 00:23:49,000
an example of how that
dipole-dipole interaction energy

310
00:23:49,000 --> 00:23:56,000
varies with the dipole moment.
The larger the dipole,

311
00:23:56,000 --> 00:24:02,000
of course, the larger the
interaction energy.

312
00:24:02,000 --> 00:24:05,000
Here, I show you several
molecules that all have,

313
00:24:05,000 --> 00:24:08,000
again, roughly the same number
of atoms.

314
00:24:08,000 --> 00:24:12,000
They have the same mass,
they roughly have the same

315
00:24:12,000 --> 00:24:15,000
number of electrons,
so they roughly have the same

316
00:24:15,000 --> 00:24:19,000
polarizability.
The induced dipole - induced

317
00:24:19,000 --> 00:24:22,000
dipole is the same,
but what is changing here,

318
00:24:22,000 --> 00:24:25,000
as I go down,
is the dipole moment.

319
00:24:25,000 --> 00:24:28,000
For propane,
dipole moment really small,

320
00:24:28,000 --> 00:24:32,000
0.1 debye.
Dimethyl ethe,r 1.3.

321
00:24:32,371 --> 00:00:02,700
Acetaldehyde,

322
00:24:34,387 --> 00:00:03,900
Acetonitrile,

323
00:24:36,000 --> 00:24:42,000
Dipole moment increases.
The boiling point increases

324
00:24:42,000 --> 00:24:48,000
because that attractive
interaction is increasing.

325
00:24:48,000 --> 00:24:53,000
It scales roughly as the dipole
moment squared.

326
00:24:53,000 --> 00:25:00,000
The dipole-dipole interaction
is stronger.

327
00:25:00,000 --> 00:25:04,000
Now, I just want to briefly
then remind you about one other

328
00:25:04,000 --> 00:25:09,000
attractive interaction that we
have talked about before.

329
00:25:09,000 --> 00:25:14,000
And that is between two ions.
There we are talking about the

330
00:25:14,000 --> 00:25:18,000
Coulomb interaction energy,
where the dependence is minus

331
00:25:18,000 --> 00:25:22,000
one over r.
That is the longest range

332
00:25:22,000 --> 00:25:25,000
interaction.
Again, you can see this way out

333
00:25:25,000 --> 00:25:28,000
here.
If I choose this value of r,

334
00:25:28,000 --> 00:25:33,000
the one over six interaction
term would give me a

335
00:25:33,000 --> 00:25:37,000
very small value for the
attractive interaction,

336
00:25:37,000 --> 00:25:41,000
--
-- because it is one over r to

337
00:25:41,000 --> 00:25:43,000
the six.
You take a large number for r

338
00:25:43,000 --> 00:25:47,000
and raise it to the 6 power and
put in the denominator.

339
00:25:47,000 --> 00:25:50,000
You are going to have a small
value for U of r.

340
00:25:50,000 --> 00:25:54,000
The one over r to the three
gives you some

341
00:25:54,000 --> 00:25:57,000
interaction energy,
but one over r gives you a lot

342
00:25:57,000 --> 00:26:02,000
of attractive interaction.
It is the longest range.

343
00:26:02,000 --> 00:26:05,000
And you can also see here,
in a moment,

344
00:26:05,000 --> 00:26:09,000
that it is going to be the
strongest interaction.

345
00:26:09,000 --> 00:26:13,000
What I am just doing is
comparing several molecules or

346
00:26:13,000 --> 00:26:17,000
atoms.
Here is argon 2 which has only

347
00:26:17,000 --> 00:26:20,000
the dispersive interaction.
Here is the plot,

348
00:26:20,000 --> 00:26:25,000
well depth minus one kilojoule.
Then, there is the HCl

349
00:26:25,000 --> 00:26:30,000
interaction energy that has the
dispersion interaction in it,

350
00:26:30,000 --> 00:26:36,000
but also the dipole-dipole
permanent interaction.

351
00:26:36,000 --> 00:26:40,000
It is minus three kilojoules.
And then we are talking about

352
00:26:40,000 --> 00:26:44,000
chlorine two.
This is a covalent bond.

353
00:26:44,000 --> 00:26:49,000
This is no longer a
Lennard-Jones potential energy,

354
00:26:49,000 --> 00:26:53,000
but this well depth,
here, is minus 200 kilojoules

355
00:26:53,000 --> 00:26:55,000
per mole.
And then, finally,

356
00:26:55,000 --> 00:27:00,000
this ionic interaction between
potassium and chlorine,

357
00:27:00,000 --> 00:27:06,000
look at how strong that is,
minus 450 kilojoules per mole.

358
00:27:06,000 --> 00:27:11,000
So, these are the relative
strengths here of these

359
00:27:11,000 --> 00:27:18,000
interaction energies.
That is all I that want to say

360
00:27:18,000 --> 00:27:23,000
about these kinds of
intermolecular interactions,

361
00:27:23,000 --> 00:27:30,000
where we are dealing with some
kind of dipole.

362
00:27:30,000 --> 00:27:34,000
But before we move on,
I want to talk about one other

363
00:27:34,000 --> 00:27:39,000
kind of intermolecular
interaction potential.

364
00:28:00,000 --> 00:28:05,000
And that is something called
hydrogen bonding.

365
00:28:15,000 --> 00:28:20,000
All right.
A final intermolecular

366
00:28:20,000 --> 00:28:26,000
interaction, hydrogen bonding.
Hydrogen bonding,

367
00:28:26,000 --> 00:28:33,000
here, occurs between a hydrogen
atom that is attached to an

368
00:28:33,000 --> 00:28:39,000
electronegative atom and another
molecule in the gas or in the

369
00:28:39,000 --> 00:28:43,000
solution.
The first requirement is that

370
00:28:43,000 --> 00:28:50,000
you have hydrogen attached to a
very electronegative atom.

371
00:28:50,000 --> 00:28:56,000
Hydrogen bonding occurs for
hydrogens attached to oxygen,

372
00:28:56,000 --> 00:29:02,000
nitrogen, and chlorine.
Those are all electronegative

373
00:29:02,000 --> 00:29:05,000
atoms.
Water is a good example.

374
00:29:05,000 --> 00:29:12,000
In the case of water-- You have
hydrogen bonded to this oxygen.

375
00:29:12,000 --> 00:29:16,000
This oxygen is really very
electronegative.

376
00:29:16,000 --> 00:29:22,000
What that oxygen does is it
pulls those electrons away from

377
00:29:22,000 --> 00:29:25,000
the hydrogen.
This hydrogen is kind of

378
00:29:25,000 --> 00:29:32,000
partially unshielded.
It is partially deshielded.

379
00:29:32,000 --> 00:29:38,000
And then, there is a lot of
electron density here on this

380
00:29:38,000 --> 00:29:42,000
oxygen.
Well, because that hydrogen is

381
00:29:42,000 --> 00:29:48,000
partially deshielded and because
it is really small,

382
00:29:48,000 --> 00:29:54,000
this hydrogen actually will
kind of see the oxygen atoms on

383
00:29:54,000 --> 00:30:01,000
a neighboring water molecule.
And since this is kind of

384
00:30:01,000 --> 00:30:05,000
partially negative,
this hydrogen will interact

385
00:30:05,000 --> 00:30:10,000
with one of the lone pairs on
this oxygen and will form a

386
00:30:10,000 --> 00:30:14,000
bond.
This is a little bit partially

387
00:30:14,000 --> 00:30:18,000
positive, this is a little bit
partially negative,

388
00:30:18,000 --> 00:30:22,000
and the result,
here, is a bond between the

389
00:30:22,000 --> 00:30:27,000
hydrogen and the oxygen.
And that bond is on the order

390
00:30:27,000 --> 00:30:34,000
of 20 to 60 kilojoules per mole.
This bond is not a covalent

391
00:30:34,000 --> 00:30:36,000
bond.
A covalent bond is

392
00:30:36,000 --> 00:30:39,000
kilojoules per mole.
This is 10% of it,

393
00:30:39,000 --> 00:30:43,000
but it is still a very
important quantity or a very

394
00:30:43,000 --> 00:30:47,000
important phenomenon,
this hydrogen bonding.

395
00:30:47,000 --> 00:30:51,000
The hydrogen bonding is
certainly responsible for the

396
00:30:51,000 --> 00:30:56,000
peculiar properties of water,
as you will learn more about in

397
00:30:56,000 --> 00:31:00,000
5.60, but it is also responsible
for the unique shape,

398
00:31:00,000 --> 00:31:05,000
oftentimes, of biological
molecules.

399
00:31:05,000 --> 00:31:10,000
The helix in DNA owes its
structure to hydrogen bonding.

400
00:31:10,000 --> 00:31:16,000
The hydrogen bonding is what
makes trees stand upright.

401
00:31:16,000 --> 00:31:21,000
The long cellulose molecules in
trees are actually bonded

402
00:31:21,000 --> 00:31:27,000
together by hydrogen bonding.
Nylon owes its strength to

403
00:31:27,000 --> 00:31:33,000
hydrogen bonding.
And hydrogen bonding is also

404
00:31:33,000 --> 00:31:40,000
responsible for whether or not
you have a bad hair day.

405
00:31:40,000 --> 00:31:47,000
As an example of that,
I want you to look at the slide

406
00:31:47,000 --> 00:31:53,000
up there on the walls.
What you see is a protein

407
00:31:53,000 --> 00:32:00,000
molecule.
This is the structure of hair.

408
00:32:00,000 --> 00:32:05,000
The molecules that make up the
strands of your hair look like

409
00:32:05,000 --> 00:32:07,000
this.
It is a polymer.

410
00:32:07,000 --> 00:32:12,000
Well, it is a polypeptide.
This unit right in here,

411
00:32:12,000 --> 00:32:17,000
carbon-oxygen bound to
carbon-hydrogen bound to

412
00:32:17,000 --> 00:32:20,000
nitrogen-hydrogen is the
peptide.

413
00:32:20,000 --> 00:32:24,000
It is repeated.
You see the next unit over?

414
00:32:24,000 --> 00:32:26,000
CO-CH-NH.
CO-CH-NH.

415
00:32:26,000 --> 00:32:32,000
That keeps repeating.
That is the repeat unit.

416
00:32:32,000 --> 00:32:36,000
And, of course,
this carbon-hydrogen right

417
00:32:36,000 --> 00:32:41,000
there, you can see it has a line
there, indicating that it is

418
00:32:41,000 --> 00:32:46,000
bonded to something.
And it is bonded to something.

419
00:32:46,000 --> 00:32:51,000
If that carbon-hydrogen is
bound to another hydrogen,

420
00:32:51,000 --> 00:32:56,000
then you have a polypeptide
which has been made from an

421
00:32:56,000 --> 00:33:02,000
amino acid that you might know
of as glycine.

422
00:33:02,000 --> 00:33:07,000
If that carbon is bound to a CH
three group,

423
00:33:07,000 --> 00:33:13,000
then that peptide was made from
an amino acid that you might

424
00:33:13,000 --> 00:33:18,000
know of as alanine.
And if it is bound to a CH two

425
00:33:18,000 --> 00:33:23,000
S H group,
well, that was an amino acid

426
00:33:23,000 --> 00:33:28,000
known as cysteine.
But what I want you to notice

427
00:33:28,000 --> 00:33:35,000
here is that these hydrogens on
the nitrogen --

428
00:33:35,000 --> 00:33:38,000
That nitrogen is an electron
negative atom.

429
00:33:38,000 --> 00:33:42,000
And that hydrogen,
if your hair is wet,

430
00:33:42,000 --> 00:33:46,000
it is actually hydrogen bonded
to a water molecule.

431
00:33:46,000 --> 00:33:51,000
Here is that hydrogen bond.
And then in the next strand

432
00:33:51,000 --> 00:33:57,000
over, this oxygen is hydrogen
bonded to a water molecule when

433
00:33:57,000 --> 00:34:02,000
your hair is wet.
And the bottom line is that

434
00:34:02,000 --> 00:34:08,000
when your hair is wet,
you have the strands of your

435
00:34:08,000 --> 00:34:12,000
hair that kind of slip by each
other.

436
00:34:12,000 --> 00:34:16,000
There is no registry of one
strand to another,

437
00:34:16,000 --> 00:34:21,000
because each strand has this
coating, here,

438
00:34:21,000 --> 00:34:26,000
of water molecules due to
hydrogen bonding.

439
00:34:26,000 --> 00:34:32,000
Suppose you take your hair and
put it in a very contorted

440
00:34:32,000 --> 00:34:38,000
configuration like this.
And then you drive off the

441
00:34:38,000 --> 00:34:41,000
water molecules,
you dry your hair.

442
00:34:41,000 --> 00:34:47,000
What happens is that these
water molecules then leave,

443
00:34:47,000 --> 00:34:53,000
the hydrogen bonds are broken.
This is not such a strong bond,

444
00:34:53,000 --> 00:34:58,000
20 to 60 kilojoules per mole,
those hydrogen bonds are

445
00:34:58,000 --> 00:35:02,000
broken.
And then this hydrogen on this

446
00:35:02,000 --> 00:35:07,000
nitrogen looks around and sees
the oxygen with its lone pairs

447
00:35:07,000 --> 00:35:11,000
on this strand,
and so you form a hydrogen bond

448
00:35:11,000 --> 00:35:14,000
between this strand and this
strand.

449
00:35:14,000 --> 00:35:19,000
And so now the strands of your
hair are in registry with each

450
00:35:19,000 --> 00:35:22,000
other.
They are actually stronger.

451
00:35:22,000 --> 00:35:27,000
And they do tell you not to
brush your hair when it is wet,

452
00:35:27,000 --> 00:35:33,000
because it isn't so strong.
Well, it is not so strong

453
00:35:33,000 --> 00:35:39,000
because these water molecules
are insulating each one of these

454
00:35:39,000 --> 00:35:42,000
strands.
And when it is dry these two

455
00:35:42,000 --> 00:35:48,000
strands are bond to each other.
They are in registry with each

456
00:35:48,000 --> 00:35:51,000
other.
And so, if you do this right,

457
00:35:51,000 --> 00:35:56,000
and you then let go of the
contorted configuration,

458
00:35:56,000 --> 00:36:01,000
which I am having trouble
doing, those strands are in

459
00:36:01,000 --> 00:36:08,000
registry with each other now.
And you have a good hair day.

460
00:36:08,000 --> 00:36:13,000
So, that is the importance of
hydrogen bonding.

461
00:36:13,000 --> 00:36:19,000
I curled my hair just for this
demo, a little asymmetric,

462
00:36:19,000 --> 00:36:22,000
here.
[LAUGHTER] But now,

463
00:36:22,000 --> 00:36:26,000
if you are as fortunate,
or unfortunate,

464
00:36:26,000 --> 00:36:33,000
depending on your preference,
to have naturally curly hair,

465
00:36:33,000 --> 00:36:40,000
then what you have are a lot of
sulfur-sulfur bonds.

466
00:36:40,000 --> 00:36:49,000
You have a lot of cysteine
peptide groups.

467
00:36:49,000 --> 00:36:58,000
What happens there is this.
On the CH groups,

468
00:36:58,000 --> 00:37:02,000
you have CH two sulfur H.

469
00:37:02,000 --> 00:37:07,000
And on the next strand over you
have S, CH two,

470
00:37:07,000 --> 00:37:11,000
and CH bonded to nitrogen,
bonded to a CO.

471
00:37:11,000 --> 00:37:17,000
And you actually have a
covalent bond between these two

472
00:37:17,000 --> 00:37:21,000
sulfurs, here.
This is a strong bond.

473
00:37:21,000 --> 00:37:27,000
The strands of your polymers in
your hair are in registry all of

474
00:37:27,000 --> 00:37:32,000
the time.
And if you want to make your

475
00:37:32,000 --> 00:37:36,000
natural curly hair straight you
have to do drastic things like

476
00:37:36,000 --> 00:37:40,000
use drastic chemicals to break
this sulfur-sulfur bond.

477
00:37:40,000 --> 00:37:43,000
You can do it,
but it is not easy.

478
00:37:43,000 --> 00:37:48,000
Likewise, if you have naturally
straight hair and you want to

479
00:37:48,000 --> 00:37:51,000
curl it and make it
semi-permanently curly,

480
00:37:51,000 --> 00:37:55,000
then you have to build in the
sulfur-sulfur bonds.

481
00:37:55,000 --> 00:37:58,000
And you have to do,
again, some rather drastic

482
00:37:58,000 --> 00:38:05,000
chemistry to make that happen.
Hydrogen bonding is important,

483
00:38:05,000 --> 00:38:12,000
especially if you go to do
anything in biologically-

484
00:38:12,000 --> 00:38:16,000
related sciences,
you will see that.

485
00:38:16,000 --> 00:38:22,000
Now, I am going to change
topics here.

486
00:38:30,000 --> 00:38:36,000
I am going to change topics,
and we are going to talk a

487
00:38:36,000 --> 00:38:42,000
little bit about thermodynamics
in preparation to get up to

488
00:38:42,000 --> 00:38:48,000
chemical equilibrium so that
Professor Cummins can come in

489
00:38:48,000 --> 00:38:55,000
next Wednesday and start talking
about acid-base equilibrium.

490
00:38:55,000 --> 00:39:00,000
He is great.
You will love him.

491
00:39:00,000 --> 00:39:04,000
We are going to review some
thermodynamics today.

492
00:39:04,000 --> 00:39:08,000
I am going to go kind of
quickly because some of this I

493
00:39:08,000 --> 00:39:12,000
think you know,
but I want to make sure

494
00:39:12,000 --> 00:39:15,000
everybody is on the same page.
First of all,

495
00:39:15,000 --> 00:39:19,000
bond energies.
We talked about bond energies

496
00:39:19,000 --> 00:39:23,000
as delta E sub D.
And we measured it from the

497
00:39:23,000 --> 00:39:27,000
bottom of the well.
And I told you a few days ago,

498
00:39:27,000 --> 00:39:32,000
I lied to you.
The measured energies are

499
00:39:32,000 --> 00:39:35,000
really from v equals zero,
and they are.

500
00:39:35,000 --> 00:39:39,000
But what I am going to do is
change my language.

501
00:39:39,000 --> 00:39:43,000
Instead of talking about
energies, I am going to talk

502
00:39:43,000 --> 00:39:47,000
about enthalpies.
I am going to talk about delta

503
00:39:47,000 --> 00:39:51,000
Hs rather than delta Es.
The reason I am going to do

504
00:39:51,000 --> 00:39:56,000
this is because it is easier for
us to measure a bond enthalpy

505
00:39:56,000 --> 00:40:00,000
than a bond energy.
And that has to do with the

506
00:40:00,000 --> 00:40:04,000
fact that we usually make
measurements in bulk under

507
00:40:04,000 --> 00:40:09,000
constant pressure conditions.
And that is the quantity that

508
00:40:09,000 --> 00:40:12,000
comes out.
The relationship between delta

509
00:40:12,000 --> 00:40:15,000
H and delta E is this.
Delta H is equal to delta E

510
00:40:15,000 --> 00:40:18,000
plus delta PV.

511
00:40:18,000 --> 00:40:22,000
This is a
relationship that you will learn

512
00:40:22,000 --> 00:40:25,000
about in a lot of detail in
5.60, in Chemical

513
00:40:25,000 --> 00:40:29,000
Thermodynamics.
At this point,

514
00:40:29,000 --> 00:40:32,000
we are going to take it as a
given.

515
00:40:32,000 --> 00:40:38,000
For gases, delta H differs on
the order of 1% to 2% from delta

516
00:40:38,000 --> 00:40:39,000
E.
It is not much,

517
00:40:39,000 --> 00:40:44,000
but if you are doing some
precise calculation,

518
00:40:44,000 --> 00:40:48,000
you need to be aware of that.
For liquids and solids,

519
00:40:48,000 --> 00:40:54,000
delta H and delta E are really
the same for all intrinsic

520
00:40:54,000 --> 00:40:57,000
purposes.
The delta PV term is really

521
00:40:57,000 --> 00:41:02,000
very small.
And, in thermodynamics,

522
00:41:02,000 --> 00:41:06,000
since we are most always
looking at changes in energy,

523
00:41:06,000 --> 00:41:10,000
we need what we call standard
states.

524
00:41:10,000 --> 00:41:13,000
And we are going to put a
nought, here,

525
00:41:13,000 --> 00:41:18,000
on all of our delta Hs to
designate the standard state.

526
00:41:18,000 --> 00:41:22,000
And our standard state that
your book uses,

527
00:41:22,000 --> 00:41:26,000
and will use,
refers really to the pressure.

528
00:41:26,000 --> 00:41:33,000
And the pressure is one bar.
And one bar is equal to 10^5

529
00:41:33,000 --> 00:41:37,000
Pascal.
That is equal to 10^5 kilograms

530
00:41:37,000 --> 00:41:43,000
per meter second squared.
The delta Hs we are going to

531
00:41:43,000 --> 00:41:47,000
talk about are also,
just about all of them,

532
00:41:47,000 --> 00:41:51,000
measured at 298.15 degrees
Kelvin.

533
00:41:51,000 --> 00:41:57,000
Delta H does depend on
temperature, but we actually are

534
00:41:57,000 --> 00:42:04,000
not going to look at that in the
next few days.

535
00:42:04,000 --> 00:00:05,600
You are going to do that in

536
00:42:07,000 --> 00:42:12,000
Our delta Hs are going to be
delta Hs at 298.15 degrees

537
00:42:12,000 --> 00:42:15,000
Kelvin.
On the first slide here,

538
00:42:15,000 --> 00:42:20,000
I show you a bunch of bond
enthalpies for CH bonds.

539
00:42:20,000 --> 00:42:25,000
And, of course,
those bond enthalpies are a

540
00:42:25,000 --> 00:42:30,000
little bit different,
depending on what molecule you

541
00:42:30,000 --> 00:42:33,000
have.
But they are not that

542
00:42:33,000 --> 00:42:36,000
different.
And so, what is often done,

543
00:42:36,000 --> 00:42:40,000
and your book does this,
is that somebody goes and

544
00:42:40,000 --> 00:42:45,000
calculates the average of the
bond energies for lots of CH

545
00:42:45,000 --> 00:42:50,000
bonds and lots of molecules and
they prepare a table that looks

546
00:42:50,000 --> 00:42:53,000
like this.
This is the mean bond enthalpy.

547
00:42:53,000 --> 00:42:56,000
And they have CH,
CC, carbon-carbon.

548
00:42:56,000 --> 00:43:01,000
But these are average bond
enthalpies.

549
00:43:01,000 --> 00:43:04,000
Now, why are bond enthalpies
important to us?

550
00:43:04,000 --> 00:43:08,000
Well, they are important
because they determine the

551
00:43:08,000 --> 00:43:13,000
enthalpy of a chemical reaction.
If the bonds are stronger in

552
00:43:13,000 --> 00:43:18,000
the products than in the
reactants, that is going to give

553
00:43:18,000 --> 00:43:22,000
us an exothermic reaction.
If the bonds are stronger in

554
00:43:22,000 --> 00:43:27,000
the reactants than the products,
that gives us an endothermic

555
00:43:27,000 --> 00:43:31,000
reaction.
And so let's look at this

556
00:43:31,000 --> 00:43:34,000
reaction.
This is an important reaction.

557
00:43:34,000 --> 00:43:36,000
This is the oxidation of
glucose.

558
00:43:36,000 --> 00:43:40,000
This is a reaction very
exothermic, minus

559
00:43:40,000 --> 00:43:44,000
kilojoules per mole.
It is a reaction that is being

560
00:43:44,000 --> 00:43:48,000
carried out in every cell of
your body as we speak.

561
00:43:48,000 --> 00:43:53,000
It is the reaction that is
providing the energy to maintain

562
00:43:53,000 --> 00:43:56,000
your body temperature,
the energy to move your

563
00:43:56,000 --> 00:44:03,000
muscles, the energy to repair
tissue, and the energy to think.

564
00:44:03,000 --> 00:44:06,000
Important reaction.
This is the reason why we eat.

565
00:44:06,000 --> 00:44:10,000
This is the reason why we
breathe, this is the reason why

566
00:44:10,000 --> 00:44:13,000
we exhale, and this is the
reason why we pee.

567
00:44:13,000 --> 00:44:15,000
[LAUGHTER]

568
00:44:23,000 --> 00:44:27,000
What do we need to do to
calculate the enthalpy for this

569
00:44:27,000 --> 00:44:30,000
reaction?
We have to figure out how much

570
00:44:30,000 --> 00:44:35,000
energy is required to break all
of the bonds of the reactants

571
00:44:35,000 --> 00:44:38,000
because that is how much energy
we put in.

572
00:44:38,000 --> 00:44:43,000
And then we have to figure out
then how much energy we get back

573
00:44:43,000 --> 00:44:47,000
when we form the product bonds.
Bottom line is,

574
00:44:47,000 --> 00:44:51,000
the enthalpy necessary to break
all of the bonds,

575
00:44:51,000 --> 00:44:54,000
you can calculate,
is 12,452 kilojoules per mole.

576
00:44:54,000 --> 00:44:59,000
That number comes from using
these average bond energies I

577
00:44:59,000 --> 00:45:04,000
told you about.
We can then get back some

578
00:45:04,000 --> 00:45:07,000
energy, minus 15,000,
approximately,

579
00:45:07,000 --> 00:45:12,000
when we form some new bonds.
That number comes from those

580
00:45:12,000 --> 00:45:17,000
average bond enthalpies.
The difference between these

581
00:45:17,000 --> 00:45:22,000
two energy levels is the
exothermicity of the reaction.

582
00:45:22,000 --> 00:45:25,000
Now, what did we do to get the
enthalpy?

583
00:45:25,000 --> 00:45:31,000
Well, what I did is took the
bond enthalpies of each bond of

584
00:45:31,000 --> 00:45:36,000
the reactants and summed them.
Then, I took the bond

585
00:45:36,000 --> 00:45:40,000
enthalpies for each one of the
products, summed them,

586
00:45:40,000 --> 00:45:44,000
and subtracted the two to get
the enthalpy of the reaction.

587
00:45:44,000 --> 00:45:47,000
I want you to notice something
here, important.

588
00:45:47,000 --> 00:45:49,000
This is reactants minus
products.

589
00:45:49,000 --> 00:45:52,000
In a moment,
I am going to show you another

590
00:45:52,000 --> 00:45:55,000
way to calculate the enthalpy
for a reaction.

591
00:45:55,000 --> 00:46:00,000
And it is going to be products
minus reactants.

592
00:46:00,000 --> 00:46:03,000
You have to know this.
But you also see that the

593
00:46:03,000 --> 00:46:07,000
calculated enthalpy is not the
experimental enthalpy.

594
00:46:07,000 --> 00:46:10,000
Why?
Because we use the average bond

595
00:46:10,000 --> 00:46:13,000
enthalpies.
We did not use the exact bond

596
00:46:13,000 --> 00:46:18,000
enthalpies because if we had to
use the exact bond enthalpies,

597
00:46:18,000 --> 00:46:23,000
can you imagine the size of the
table of data that we would have

598
00:46:23,000 --> 00:46:26,000
to have?
We would have to have a bond

599
00:46:26,000 --> 00:46:31,000
enthalpy for every bond for
every known molecule.

600
00:46:31,000 --> 00:46:34,000
That is a lot.
What are we going to do,

601
00:46:34,000 --> 00:46:37,000
then?
Is there a more accurate way to

602
00:46:37,000 --> 00:46:41,000
do that?
Yes, with knowing the absolute

603
00:46:41,000 --> 00:46:45,000
bond enthalpies.
But that is too much data.

604
00:46:45,000 --> 00:46:47,000
Is there some other way to do
it?

605
00:46:47,000 --> 00:46:50,000
Yes.
We are going to use heats of

606
00:46:50,000 --> 00:46:53,000
formation.
A heat of formation,

607
00:46:53,000 --> 00:46:57,000
delta H nought with an F as a
subscript.

608
00:46:57,000 --> 00:47:02,000
The heat of formation is the

609
00:47:02,000 --> 00:47:08,000
enthalpy of a reaction that
forms one mole of a compound

610
00:47:08,000 --> 00:47:14,000
from the pure elements in their
most stable form in their

611
00:47:14,000 --> 00:47:17,000
standard state.
For example,

612
00:47:17,000 --> 00:47:21,000
here is water.
We are forming one mole of

613
00:47:21,000 --> 00:47:25,000
water from its elements,
hydrogen and oxygen.

614
00:47:25,000 --> 00:47:31,000
The enthalpy for this reaction
is defined as the heat of

615
00:47:31,000 --> 00:47:36,000
formation of water.
Why is this the heat of

616
00:47:36,000 --> 00:47:39,000
formation of water?
Well, because we are forming

617
00:47:39,000 --> 00:47:44,000
one mole, and that is important,
from the elements that make up

618
00:47:44,000 --> 00:47:46,000
water.
What elements are they?

619
00:47:46,000 --> 00:47:49,000
They are hydrogen.
But notice that this hydrogen

620
00:47:49,000 --> 00:47:52,000
is H two.
It is not hydrogen atoms

621
00:47:52,000 --> 00:47:56,000
because H two is the most
stable form of hydrogen.

622
00:47:56,000 --> 00:48:00,000
Oxygen is O two,
not oxygen atoms because this

623
00:48:00,000 --> 00:48:05,000
is the most stable form of
oxygen at bar pressure.

624
00:48:05,000 --> 00:48:08,000
Look at this here.
What is the heat of formation

625
00:48:08,000 --> 00:48:12,000
of oxygen?
Well, the enthalpy change for

626
00:48:12,000 --> 00:48:16,000
this reaction is zero.
That is the heat of formation

627
00:48:16,000 --> 00:48:17,000
of oxygen.
Why?

628
00:48:17,000 --> 00:48:22,000
Because we are forming one mole
of oxygen from the elements in

629
00:48:22,000 --> 00:48:27,000
their most stable form.
For elements like oxygen,

630
00:48:27,000 --> 00:48:30,000
hydrogen, nitrogen,
chlorine, two

631
00:48:30,000 --> 00:48:34,000
in the gas phase,
those all have heats of

632
00:48:34,000 --> 00:48:39,000
formation that are equal to
zero.

633
00:48:39,000 --> 00:48:42,000
And then, finally,
here is the expression or the

634
00:48:42,000 --> 00:48:45,000
reaction that gives us one mole
of glucose.

635
00:48:45,000 --> 00:48:49,000
The enthalpy for this reaction
is the heat of formation of

636
00:48:49,000 --> 00:48:51,000
glucose.
We get it from its elements,

637
00:48:51,000 --> 00:48:55,000
hydrogen, oxygen,
and look at the elemental form,

638
00:48:55,000 --> 00:49:00,000
the most stable form of the
element carbon is graphite.

639
00:49:00,000 --> 00:49:02,000
Is there an 18.0-something
exam?

640
00:49:02,000 --> 00:49:03,000
Yes?
Okay.

641
00:49:03,000 --> 00:49:06,000
See you on Monday.