This introductory physical chemistry course examines the connections between molecular properties and the behavior of macroscopic chemical systems.

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From the course by University of Minnesota

Statistical Molecular Thermodynamics

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This introductory physical chemistry course examines the connections between molecular properties and the behavior of macroscopic chemical systems.

From the lesson

Module 1

This module includes philosophical observations on why it's valuable to have a broadly disseminated appreciation of thermodynamics, as well as some drive-by examples of thermodynamics in action, with the intent being to illustrate up front the practical utility of the science, and to provide students with an idea of precisely what they will indeed be able to do themselves upon completion of the course materials (e.g., predictions of pressure changes, temperature changes, and directions of spontaneous reactions). The other primary goal for this week is to summarize the quantized levels available to atoms and molecules in which energy can be stored. For those who have previously taken a course in elementary quantum mechanics, this will be a review. For others, there will be no requirement to follow precisely how the energy levels are derived--simply learning the final results that derive from quantum mechanics will inform our progress moving forward. Homework problems will provide you the opportunity to demonstrate mastery in the application of the above concepts.

- Dr. Christopher J. CramerDistinguished McKnight and University Teaching Professor of Chemistry and Chemical Physics

Chemistry

We've reached the last members of the chemical menagerie.

Now that we've taken care of atoms and diatomic molecules, let's talk about

polyatomic molecules. Already shown you this slide, how is

energy stored in a molecule? There's electronic energy, which is both

potential and kinetic associated with the electrons, and then, there is kinetic

energy of the molecule itself. There's the translational energy, which

is movement of the entire molecule in space, comes from particle in a box

solutions. There's the rotational energy of the

molecule rotating about its center of mass that comes from the Rigid-rotator

equation. And finally, there is vibrational energy

associated with the nuclei moving relative to one another.

It's handy to divide up the various contributions to the kinetic energy that

comes from molecular motion into what are known as degrees of freedom.

So, if I want to specify completely the position in space of a molecule that has

n nuclei, I must need three n coordinates, right?

There's an x, a y and a z coordinate for each atom, and if there are n atoms,

that's 3 times n. So, we often will refer to that then as

3n degrees of freedom which dictate specification of every atomic position.

And degrees of freedom can be thought of as being subdivided into translational,

rotational, or vibrational components. And so, if I think about translation,

that's really the motion of the center of mass in three-dimensional space, and so,

there are three degrees of freedom. It can move in the x direction, it can

move in the y direction, and it can move in the z direction.

There's rotation, so that is movement about the center of mass.

And for a linear molecule, if I imagine a linear molecule laid out along some axis,

I can rotate this way. I guess that's end over end in a sense.

I can rotate this way, end over end in a different plane.

But, there is no rotation about the axis that's defined by the bond of the

molecule. So there's two ways to rotate a diatomic.

Once I am nonlinear, so that's really the, the issue there.

A diatomic molecule per force is linear. Two points determine a line.

once I go to three or more atoms, I could be linear.

There are examples of linear molecules with more than two atoms, in which case,

I still only have two ways to rotate, but once I'm nonlinear, now I can rotate

about any of the Cartesian axes, so that's three degrees of freedom.

Well, so all that's left then is vibrations, and when determines the

number of vibrational degrees of freedom by taking the total 3n and subtracting

out the translational, always 3 and the rotational, 2 for linear, 3 for

nonlinear. And that gives you these these values, 3n

minus 5 vibrations for a linear molecule, 3n minus 6 vibrations for a nonlinear

molecule. And so, when we sum them together

appropriately, always linear or always nonlinear, you always get 3n as you must.

The diatomics, which we dealt with up till now, of course only have our per

force linear and only have 3n minus 5 vibrations.

So let's, let's just remember that. 3 times 2 would be 6, minus 5 is 1.

One vibration. We looked at the vibration in a diatomic

molecule. In a polyatomic, there would be more.

So actually, let's pause here for a moment, and this is sort of trivial

arithmetic and and thinking about the physics, but maybe take a moment and

think about the numbers of degrees of freedom available in different molecules.

Well, let's take a look now at rotational energy levels in polyatomic molecules.

So it turns out, if the polyatomic molecule is indeed still linear, then,

exactly the same Schrodinger equation applies.

You get exactly the same solutions. Namely, that the allowed energy levels

are given by this expression, with these quantum numbers, and this degeneracy.

The only difference is the moment of inertia is defined by all of the atoms,

not just by two atoms, and so this is simply a generalization of the formula

for moment of inertia, namely that it's the sum over all the atoms, their mass

times the square of their distance from the center of mass.

Nonlinear molecules, things become a bit more complicated.

So actually, there are three rotational axes in a nonlinear molecule, and for

each of those axes, there is an associated moment of inertia.

There are categories of molecules and the categories are named depending on the

relationship between the moments of inertia.

So for certain special molecules, all three moments of inertia along the

different Cartesian directions are the same, such molecules are called spherical

tops. And so, an example of a spherical top

would be a baseball, that's not a molecule, but it's a macroscopic object.

Or, in the chemical arena, methane is a molecule with three moments of inertia

all equal to one another. The next step down, clearly, would be

instead of having all three be equal to one another, have two be equal to one

another, and so, such molecules are called symmetric tops.

And again, a macroscopic object that has that characteristic would be an American

football, and so its got one long axis. But otherwise, it looks pretty symmetric

and that gives rise to two identical moments of inertial, but one that's

different. and example in the case of a molecule

would be ammonia. So ammonia, two equal moments of

inertial. And then finally, obviously, the most

general possible case would be all three moments of inertial are different from

one another, that is called an asymmetric top.

Seems, sort of odd maybe to keep using the word tops since we've removed any

kind of agreement between moments of inertia, but that’s what the terminology

is. So an example of an asymmetric top would

be macroscopically, a boomerang and a molecule that looks a little bit like a

boomerang water. So water with its three atoms bent with

oxygen in the middle. When you take account of those those

different moments on inertia, you get complicated expressions for the energy

levels. I'm not going to show you actual

formulas. You can look up certain of them if you

really needed them, but for the moment, we are going to focus on the more simple

linear levels that provides enough basis to make progress.

Now, what about the vibrations in a polyatomic molecule?

So, we refer to the, the various vibrations in a molecule, a polyatomic,

now that there's more than one, as soon as you go past a diatom, you have more

than one vibration. So, each one of them has a, a

characteristic motion and that motion is called a normal mode.

So, the individual vibrations are also called the normal modes of the molecule.

And so as an example, water has three normal modes.

That is, it's non-linear, so the number of vibrational degrees of freedom will be

3n minus 6, 3 translations, 3 rotations, so 3 times 3 is 9 minus 6 is 3.

Three normal modes, three vibrational modes.

If we look at those modes, the actual vibrations themselves correspond to, and

they're shown here with their corresponding vibrational frequencies.

The lowest vibrational frequency is a bending motion and generations of

physical chemists have done this, so bear with me.

Imagine that my two fists are hydrogen atoms and my head is an oxygen atom.

The lowest frequency is a bend, so it consists of the molecule doing something

like this, just a little bit of a chicken move sort of, that's a bending motion.

There's also a symmetric stretching motion at 3,686 wave numbers.

And so, if we do the anthropomorphism of water again, that's the two hydrogen

atoms moving outward from the oxygen in concert with one another.

And then, finally, the last mode is the asymmetric stretch, and that's where one

molecule, one hydrogen atom moves in while the other moves out.

So here is the asymmetric stretch, and actually, the oxygen will move a bit so

it's kind it's like doing a little exercise in the middle of a lecture

video. Thank you for bearing with me for that,

it's a, it's a classic routine. Coming back to the to the energetics,

each of these normal modes act as an independent harmonic oscillator, so each

one contributes. The normal harmonic oscillator

contribution, but they've got associated with them these unique frequencies that

are shown over on the right-hand side. So, I would index for vibration 1,

Planck's constant times the vibrational frequency for vibration 1 times the

quantum number, that describes which level the first normal mode is sitting

in. And then I would add to that h times the

frequency for two times its quantum number plus a half, and so on, so I just

add them up. So what's the total energy then in a

polyatomic molecule, or indeed, in, in an atom or a diatomic for that matter?

Well, it's the sum of the energies overall the degrees of freedom.

So remember, that, for a molecule in a three-dimensional box, you would get this

from solving a particle in a box equation.

So there are quantum numbers and box lengths, side lengths required, as well

as a mass. Rotation for linear molecules is given by

this rotational energy formula and these quantum numbers, and I won't write down

as I said, the nonlinear, sometimes it has a nice form, sometimes less nice.

We'll deal with that if we need to . The vibrations are determined from the

quantum mechanical harmonic oscillator expression, these energy levels.

One associated with each vibration, and finally, the electronic energy, which has

a pretty simple form for the hydrogen atom.

Hydrogen atom, not that exciting. We won't be doing a whole lot more with

it. for diatomics, there's a number we can

associate it with, with it, DE. Otherwise, it's probably something we're

going to be looking up. And what I want to leave you with is a

feeling for the spacing of these energy levels.

How far apart are they? And that really matters in terms of how

you can store energy in a system? So there's a general trend in energy

spacing. And that is, that the electronic energy

spacing is much, much greater than the vibrational spacing, which itself is

greater than rotational, which itself is much, much greater than translational.

So let me, rather than just memorizing words, let's actually have a picture at

least to associate with that. So, shown here is a plot of two

electronic states of a system. So here's some low energy potential

associated with a, let's call it the ground state.

And then, somewhere up here is an excited electronic state, a long way up in

energy, this separation. I'll just, just dictate that.

We'll call that big. Big relative to what?

Everything else. So, within this potential is a series of

vibrational levels. So they are much more closely spaced, one

to another, than the bottom of the ground state electronic energy to the first

excited state electronic energy. And so, if I zoom in on the vibration

energy levels and move them over here. Now, I could see above each of the

vibrational levels the rotational spacing.

So the rotational levels are much closer to one another than the vibrational

levels are. And finally, if I zoom in, yet again,

just take this little red circle here, the first few rotational levels.

And expand those a great deal, then, on top of each rotational level, superdense

are the translational levels. So they're extremely close to one

another. And keeping in mind that spacing will be

useful as we proceed forward and think about what that means for how energy can

partition in to these various kinds of motion.

So, that wraps up what we're going to do in terms of studying the quantized energy

levels of atoms and molecules. And indeed, we're almost done with the

material for this week of the course. The next video, we'll try to provide a

sort of review summary picture of what we've talked about that's important so

far and that will complete Week 1.

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