Lec 3. Last Gas(p) and Condensed Phases
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Separation processMan pageCollisionMaterials scienceUraniumChemical plantGasLecture/ConferenceComputer animation
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Setzen <Verfahrenstechnik>FoodCarbon dioxideZellmigrationSpring (hydrology)ChemistryCoalAreaLecture/Conference
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WaterGolgi apparatusHydrogenLithiumSodiumMoleculeChemical propertyFunctional groupIcePhase (waves)Atomic numberCarbon dioxideGasLecture/Conference
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WaterCarbon monoxideDipol <1,3->WeinfehlerHyperpolarisierungHemoglobinCobaltoxideSymptomWine tasting descriptorsCO-VergiftungLactitolLecture/Conference
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Wine tasting descriptorsLactitolMethylbutan <2->ZigarettenschachtelPentaneLecture/Conference
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TanningPentaneDipol <1,3->IsomerNitrosamineContainment buildingQuartzMethylbutan <2->DimethylpropanCrystalWine tasting descriptorsLecture/Conference
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ZigarettenschachtelMan pageHexagonal crystal systemWine tasting descriptorsSystemic therapyPilot experimentComputer animation
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QuartzPedosphäreLecture/Conference
Transcript: English(auto-generated)
00:06
The lecture today is The Last Gasp and Condensed Phases. Recall we were talking about gases, and we were talking about purifying uranium
00:25
for various uses. And this is a much more efficient way. Chemists love this. Very energy efficient.
00:41
Boy, you shine a laser light at these gases, and the uranium 235 absorbs the light, and the uranium 238 doesn't. And so you ricochet them off. Usually you ionize them or you do something. And then you sweep everything aside,
01:02
and then you have what you want. Separated the material. Unfortunately, there was a collision between science and policy here, because these kind of plants can be made very, very tiny.
01:20
And so you could hide them anywhere. And the government decided, maybe we don't really want to do this and popularize this technology, because it'll be very hard to keep track of who's doing what. And science and technology always has this tension.
01:42
They made a new kind of bird flu to see how it worked. It's very contagious. There's a lot of controversy about whether they should publish how they made it, so that somebody not quite as bright can copy that and make it.
02:03
Of course, scientists would like to publish everything. We believe in freedom of information. But you have to be a little cautious sometimes. So here's the summary of gases. They're the simplest form of matter to understand.
02:23
That's why we have these simple, dimple equations that model gases fairly well. And an ideal gas is like a mathematical circle. It's something we invent to stretch out the behavior of real gases to all possible temperatures
02:43
and pressures, from 0 to infinity. And it can't possibly be true in all circumstances for a real gas. And separating gases from one another is hard work, because the atoms or molecules just
03:00
move around randomly. And you're trying to impose order on them. And they don't have any brains. So you have to do all the controlling. If you decide that after you read the NOAA report that recently came out that said that it's
03:22
6 degrees Fahrenheit warmer this spring than average, and that lots of migratory birds appear to be confused, coming back too soon, and then there's no food for them to eat, and then they stand around, moving to the wrong areas where they didn't normally
03:42
go because it's too hot where they used to go, that type of thing. And you say, I got an idea. Let's try to put the genie back in the bottle and capture all that CO2. That is going to be a lot of work. And work in chemistry is synonymous with using energy.
04:05
And the question is, where are you going to get that energy if you're going to try to do something like that? You can't get it by burning more coal. We have the kinetic theory of gases.
04:21
Starts from four assumptions. Know the four assumptions. Whenever you have a theory, you have to know what the assumptions of the theory are, if any. Helium gas, because of the Maxwell-Boltzmann distribution,
04:41
helium atoms go much, much faster than argon, krypton, or xenon. In fact, they go so fast that they achieve escape velocity. And so every helium atom you let into the atmosphere eventually leaves the Earth's gravitational field,
05:02
and we leave a streamer of them behind in space. But helium is ever so important to have around. It's a non-renewable resource, and it's extremely important. You wouldn't have any MRI without liquid helium.
05:22
And so we should not be filling balloons and letting them go up as if there's no tomorrow. We should be conserving it, like we should be conserving a lot of things that we're using up too quickly.
05:41
The atmosphere, when you look up and you see the waning gibbous, and it looks like it goes on forever, because you're looking through it. It doesn't go on forever. It basically goes nowhere. This part here is the atmosphere,
06:03
this little thin layer. It's not as big as people think, and you can change its composition by belching all kinds of things into it. And then you're going to be standing around saying, why do we do that?
06:21
Well, the answer in the early days was, nobody knew it was a problem. That's not the case now. What's happening now is a case of colossal inertia and the prisoner's dilemma problem being played out.
06:44
OK, let's talk about condensed phases. First of all, never, ever write PV equals nRT, unless you're talking about an ideal gas, or trying to approximate a real gas as an ideal gas.
07:02
This equation of state is only for gases. There are no simple equations of state for liquids or solids, because liquids or solids have some order, and they change a lot depending on what the material is.
07:21
In fact, intermolecular forces play a big role in the behavior of liquids and solids, because the atoms are close together. They're bumping into each other constantly, or they're locked in place in a solid lattice, and they're locked in place by forces.
07:42
The only forces that matter for atoms and molecules are electrical forces. Gravity, in particular, has no role to play on the atomic scale, because the atoms don't have much mass.
08:00
So if you think that a liquid condenses because the atoms see each other and attract each other, like the Earth attracts the moon, that's not even remotely applicable. Yes.
08:28
Yeah. It's extremely difficult to compress a liquid. You can always compress anything, even a solid, if you squeeze on it enough.
08:42
You can crush it. Sometimes you get a different behavior. If you take solid hydrogen, hydrogen is in the first column above lithium, sodium, and all the others. All those other ones are metallic, but solid hydrogen is not metallic.
09:06
But if you really, really, really squeeze on it, it collapses. It goes down, and then it actually conducts electricity. So it becomes a different material entirely.
09:23
And that's true, as we'll see when we talk about phases. The same molecule or same group of atoms can behave differently depending on the phase. Solid CO2 is completely different than liquid CO2
09:42
or gaseous CO2, same with ice, water, and steam. They have different properties. And this is what I said. The forces are electrical, so the strongest electrical force is a charge, and then a dipole, and then those fluctuating dipoles
10:03
that I described to you. When we talked about the van der Waals A term for attraction, we said the chance that all the electrons would be perfectly symmetrical at all times is very tiny. And so there have to be fluctuations.
10:20
And once there's a fluctuation, then anybody around the electrons can move toward the positive side, and there's a momentary attraction. They attract momentarily. I knew I was paying attention to language too closely when
10:42
I came back on a transatlantic flight from England that was landing in San Francisco, and the pilot said, we will be landing momentarily in San Francisco. And my heart started beating, and I started to get up, because I thought if we're landing momentarily,
11:00
I might not have time to get off the plane, because landing momentarily means OK. It doesn't mean landing in a moment. It means landing for a moment.
11:20
But it's been misused so much. I think if you look it up now, they accept either usage. But when I talk about momentarily, having charge, I mean momentarily. OK. Liquids can flow, although sometimes they
11:42
flow very slowly. You may have to be extremely patient to figure out that something is a liquid. You may have to watch it very, very closely over a long time. And eventually, liquids assume the shape of the container.
12:02
And we have a measure of resistance to flow called viscosity. I think you all know qualitatively what it is. Something like gasoline is not very viscous. So if you carry it around in a bucket, it's very likely to slosh over the side of the bucket.
12:25
Nobody usually carries around buckets of gasoline now, because that's very bad for the air. But when I was a kid, we would have gasoline, and that's how you cleaned your bike chain, is you dumped it in gasoline, and you went back and forth.
12:42
And then you oiled the chain, got all the gunk off it. And you did that. You were 11, and there was nobody around, no adult supervision. And it was quite a bit of gas, so you could get burned.
13:00
But you knew that, and so you didn't light any matches, and you didn't play any games. OK? The kids who did that weren't around. Those of us still around were cautious. So viscosity, if we look at it,
13:21
viscosity, I guess, will give us a clue, some kind of clue, not the ultimate clue, but some kind of clue about the kind of strength of intermolecular forces in liquids. And hydrogen bonding, in particular,
13:42
can lead to very viscous solutions. Carbohydrates, sugars, have lots of hydroxyl groups that can make hydrogen bonds. And therefore, if we pour a ton of sugar into water
14:00
and make syrup, we get a much more viscous solution. And then we can boil it up and pour it into molds and harden it, and then we have candy and various things like that. OK, let's predict the viscosity of something.
14:24
My goodness, now look at these structures. So intimidating. They are not. Chemists get lazy because they have to draw structures all the time. And if you have to draw a C, H, H, ow, my wrist
14:45
is killing me. And it's taking forever. It's taking so long to draw the molecule that it's like reading a novel letter by letter. You go so slowly, if you read slowly,
15:02
that you can't follow the plot because you're taking too long to read it and you're forgetting what happened. So we have a shorthand, and in the shorthand, whenever we change direction like that, that means there's a carbon. And if there's one line, there's
15:22
a single bond between them. And we don't bother drawing the two hydrogens on this carbon or the two hydrogens on this carbon or the three hydrogens on the end because we know the carbon will make four single bonds. We just leave it all off so that we can get on with it.
15:44
And when you take organic chemistry, you'll be drawing all kinds of things like this very quickly. And if you become an organic chemist, you'll be able to draw these things so fast, it's like signing your name because you'll have to draw so many of them to see what's going on.
16:03
But for now, I'll just tell you that the first one is 1-propanol. If the OH is in the middle, it's called isopropanol or isopropyl alcohol. That's often used to sterilize things. But this is 1-propanol.
16:22
And then we have this with two hydroxyls and then this with three hydroxyls. But they all have three carbons. And all we're changing is the number of hydroxyl groups.
16:41
That does increase the molar mass. But you never, ever say, gee, the molecule on the right is heavier. Never. It has nothing to do with it. Don't use terms that refer to gravity.
17:02
It has to do with hydrogen bonding and electrical forces. The more opportunities we have for hydrogen bonding, and hydrogen bonding can only occur when a hydrogen is on what atoms?
17:25
Oxygen, nitrogen, or fluorine. And fluorine doesn't occur that often. So oxygen and nitrogen are the ones you want to think about. If you see a hydrogen on an oxygen or a nitrogen,
17:42
and then there's another oxygen or a nitrogen around, you say, my goodness, that could hydrogen bond for sure. And that would stick the two molecules together. And then if I were trying to pour them, and they were tending to stick together, they aren't going to pour very quickly,
18:01
because they have to unstick to start moving. And the more I have them stuck, the longer that's going to take. And therefore, we would guess that one propanol, which has one hydroxyl group, is the least viscous. And that propylene glycol, which you probably
18:23
have seen as an antifreeze ingredient, for example, miscible with water, lowers the freezing point so that the water in your radiator doesn't freeze when you go up the right wood
18:41
and then break, shatter the radiator. And then finally, glycerol, which is very viscous. And if we look at room temperature, these are approximate numbers, then one propanol is 1.94 millipascal seconds.
19:02
That's also known as centipoise. That's just the units of viscosity. 48.6, and then about 1,000. You don't use this one then to add to your car radiator. This one is about right.
19:21
They'll both lower the freezing point, but one of them will be a lot more inconvenient to work with, because it'll take you 50 minutes to pour it in. And the customer wants to get moving.
19:41
And then, of course, they add dyes so that you know if you spill it that you should clean it up. And you don't want to be spilling chemicals around. And if they're colorless, which these are, you can't tell where they are, and so you leave them. So they add dyes, usually some green.
20:05
And then they add other colors to the other fluids in your car. The guy can call you up and troubleshoot it, saying, what's the color? And you say, well, it's this red stuff, aha. Then he knows while your power steering is out or your transmission fluid or whatever.
20:21
If you add colors, though, you have to be sure whatever you add doesn't change the properties of the liquid and what it's supposed to do. No good having a brightly colored liquid that doesn't work as a power steering fluid. This experiment has been going since 1930.
20:42
Bitumen, which is also known as pitch, hence the phrase pitch black. That's as black as this stuff. It turns out that it is a liquid. And the proof is you pour it into a funnel.
21:03
You heat it up, pour it into a funnel, let it cool. And then you watch it for 80 years. We can't do experiments these days like that very well because the funding agency comes back in three years
21:22
and says, what have you done? And if you say, well, I've drunk a lot of coffee and I've been watching this thing and it's barely moving, then unless you're a physicist, they say we are going to scram that experiment. Sometimes in physics, you have to sit around to watch for a rare event.
21:44
But you don't want too many people doing that because it's hard to distinguish whether they're actually working. This has dropped eight experiments. Sorry, eight drops have fallen here. And you become the steward of the experiment.
22:02
And then when Dr. Mainstone dies, they will have to appoint somebody else to take it over and keep watching. So this becomes one of your tasks is to not do anything, don't touch it, keep at the same temperature, and be sure you're there
22:22
when the drop actually falls because that's very rare. Solids don't flow. Of course, you could say, how do we know? Maybe we should watch them even longer.
22:43
But it turns out solids don't flow because the molecules or atoms are locked into place in a 3D structure. And they can't move until you melt them.
23:01
Crystalline solids have a repeating structure. And everything is formed from this repeating structure, which we will call a unit cell. And we just move the unit cell in three directions.
23:21
And we reproduce the whole gigantic solid. So the unit cell is the smallest piece that has all the parts you need to make the entire solid. Sometimes the unit cell is quite big. In the case of macromolecules, sometimes it should be quite small.
23:41
In the case of something simple like rock salt, sodium chloride, the unit cell can be tiny. And the unit cell has to have the right stoichiometry. Therefore, if I have something like zinc oxide, one zinc atom and one oxygen atom, the unit cell
24:01
has to come out to be one to one. It could have four of one and four of the other. But it has to have the same stoichiometry as the entire material does because you just make the entire material by translating the unit
24:21
around in space, just like a cookie cutter. Boom, boom, boom, boom, boom. You just move it around. We'll talk more about that later. Amorphous solids don't have such a nice structure like that.
24:42
They could be very long molecules. Usually round molecules or atoms or ions can pack very efficiently, like eggs in a carton or oranges at the supermarket. And they, therefore, tend to form crystalline solids.
25:03
But if I have a material that is long like spaghetti, like snakes, and I heat it up to a liquid and the snakes all get wrapped around each other, and then I cool it down, they
25:22
didn't have time to reorganize into the best situation. They got trapped. They got stuck. Oh, this guy's through me. And now I've removed all the energy. And they're stuck like that. And they're all stuck in different places.
25:41
And therefore, I have a disordered solid. They have different properties, sometimes very interesting properties. And you can actually make molecules, but tend to show this behavior. Usually, you have a core, like a head,
26:02
and then you put on strings like Medusa's hair, snakes coming out. And you throw all these things in. And you get something that is basically an amorphous material that you can use in a car bumper, like a star polymer.
26:23
You specifically make a material to do that, that can move. You don't want a crystalline car bumper, because crystals tend to shatter when you hit them. You want something that's more like a rubber mallet.
26:41
So if you back up and you hit something, it's not a total disaster. Your bumper protects the car from damage. Gives a little bit. Usually, solids are denser than liquids. What is the major, major exception?
27:03
Water. Water is really a unique substance. And if you, I'm sure you've all done it, you leave something in the freezer and it's too full, or even if it's not too full,
27:22
if you freeze it wrong, if you put in a bottle that is closed at the top, what happens is the top will freeze. Now it's a solid and it can't move. Now the rest of it freezes, and it's got nowhere to go.
27:41
And the force is incredible. And so the thing just blows up, shatters. Yeah. Did it freeze?
28:05
It could be that it froze more slowly. You can freeze things if you do it carefully. For example, if you have a long, thin tube of liquid, like an NMR tube in my lab, and you
28:20
want to freeze it to store it, you don't just put it in the freezer. You put it in the freezer, but you put the bottom in an Eppendorf tube. And that has good thermal contact, and so it freezes from the bottom up. And then you're OK, because the liquid can still move.
28:43
What you don't want to do is trap the liquid and then have it decide. And so it can depend on the shape of the container, too, if it's going to be susceptible to cold damage. And engineers know that. That's why things have different shapes sometimes.
29:00
They know that fully well. This is a mock-up. Obviously, it would be hard to take a photo like this. So what they did is they turned an inappropriately sized iceberg upside down, pretended that was on the bottom. But it's realistic.
29:21
It's realistic. Now you can see why the Titanic crashes. Because here's what you see. And so you say, well, I'll sail way over here, and I'll be safe, far away from it. And underwater, if you don't have sonar, and you can't see
29:41
it, it's clear, there it is. And then you run your ship into it. And if you're unlucky, it's like opening a can of sardines. The unsinkable ship, you rip it along the whole length. They had all these different compartments. That's why they thought it couldn't sink. But you just rip a ton of them open, and it sinks.
30:04
It's not unsinkable. Nobody, as the ship comes off, and the champagne bottles crash against it, nobody ever says anymore, this ship is unsinkable. It seemed to be bad luck to attempt fate.
30:25
OK. Oh, I did want to point out one thing on this. If we have icebergs that melt, they don't raise the level of the ocean much, because they're already underwater.
30:42
And therefore, the ocean's already gone up when they fall off, if they actually melt. But if we have ice on land, as on Antarctica, and then that melts, and that runs in, then the ocean level goes up a lot.
31:04
Not a small amount, a lot. As in, you can go bathing here, which wouldn't be good. OK. The strongest forces are between ions.
31:22
And if we have an ionic solid, it will typically have a very high melting point, because we have an actual positively charged ion, and a negatively charged ion, and they're stuck together. And boy, it takes a lot of energy to pry those guys apart and get them
31:42
to move around independently. You probably have never seen a molten salt, period. You can take flames, and you can try to take table salt and heat it like crazy, and nothing happens.
32:00
It just shows a brilliant yellow light, but it doesn't melt. If you want to melt it, you have to get it much, much, much, much, much, much hotter, which you shouldn't do on your own. Magnesium oxide has twice the charge.
32:22
We can model it as magnesium 2 plus ions, and oxide 2 minus ions. And therefore, with twice the charge, the Coulombic energy is much, much bigger. And we would expect that magnesium oxide, then,
32:40
would have a much higher melting point than sodium chloride. And in fact, if we look them up, sodium chloride melts at 801 Celsius, whereas magnesium oxide melts at 2852 Celsius.
33:04
And if we take it to the next level and look at thorium oxide, where the thorium is probably best modeled as a plus 4 ion, and 2 minus ions. That's 3390 degrees C.
33:26
Sometimes, the trend suddenly changes. You have a high melting point, high melting point, suddenly low melting point. If you see that, you just say, I'll bet that last one is not an ionic solid.
33:44
I'll bet it's not really. The best model is ions on a lattice. It's doing something else. Something happened. So you can judge whether something is or is not an ionic material that way. You can also dissolve it in water
34:01
and see if it conducts electricity, if it's an electrolyte. Thorium oxide won't dissolve in water, however. So you can't test that one that way. You have to be sure that the salt dissolves. And that can be tricky. OK. The next strongest force is between ions and dipoles.
34:24
For example, between sodium ion and water. The sodium ion, when we do dissolve sodium chloride in water and make salt water, the sodium ion's not moving around on its own, because it's this big positive charge.
34:43
And so the negative part of the water tends to sidle up to it. It's attracted to it. So all the oxygens face toward the ion like that and back up to it. And they crowd around it.
35:01
But of course, they get hit by other waters. They may get knocked off. And at any time, you may have between four and six water molecules loosely bound to the sodium ion. For some metals, the first six waters that go on, that's it.
35:23
There's one here, one here, one here, one here, one here, one here. And they just basically ride around with that ion forever. Same six guys. This one exchanges, because it's only a plus one.
35:45
Your body transports sodium and potassium around. And it's very important to have the right concentration of sodium and potassium. If you have too much of one or not enough of another,
36:04
you may either be weak or your heart may stop beating suddenly, which has happened to people. If you eat a ton of salt, which most Americans do, you just put a tremendous load on your biochemistry.
36:23
Salt was hard to find historically. Plants don't have any salt. You put salt on plants, they die. And therefore, it was rare. So if you put out a big block of salt,
36:42
all the cows in the field come over and start licking the salt. And they need to have some salt. They need to find some, and it's hard to find. And therefore, you're going to like the taste of salt, because you could never get enough.
37:00
But now that you can get enough, it's like food. You have to be careful, because you're actually forcing your kidneys to get rid of salt all day long. Your kidneys are designed to not get rid of salt. They're designed to produce, get rid of water
37:20
and other waste, and keep the salt in your blood. That's what they're designed to do. Now, of course, just like a car, you can run your kidneys in reverse. But there's a difference. If you run your car forward all the time and drive it properly, it's going to last for a certain length of time,
37:44
because that's how it's meant to be driven. If you shift it into reverse every day and start going like this, like a madman down the freeway, you will find that the transmission doesn't last as long, because it's not meant to run that way.
38:03
And that's why health authorities are saying, don't eat so darn much salt. Keep track of it. Look at it. Processed foods have a ton of salt. Since prokaryotes need salt too,
38:23
we can make antibiotics just based on capturing salt. And here is one, monensin A. There's the structure.
38:40
Why is the structure interesting? Because, look, I've got all these oxygens, and then I've got these hydroxyls. So I've got all these negative things, like a little cavity. And then a positive ion, like sodium, comes along, and it gets trapped.
39:09
And now the bad acting bacteria that wanted to use that sodium to run its chemistry can't get it. Too bad, lights out.
39:22
They feed this stuff to cows by the tongue, as far as I can tell, which is not a good idea. OK, let's talk about dipole-dipole forces instead of ion-dipole, or ion-ion, or hydrogen bonding.
39:47
Dipole-dipole forces are between molecules that have a dipole. Therefore, the molecule can't be symmetrical. If the molecule is symmetrical, and the dipole's pointing this way, and the molecule is symmetrical,
40:04
then if I turn the molecule around, the dipole would have to turn around, since it follows the molecule. But if it's symmetrical, it's the same molecule. And therefore, the only thing that can be opposite itself is zero, and therefore, a symmetrical molecule
40:22
has zero dipole moment. Something like molecular nitrogen has no dipole moment. In liquid chloroform, then, these dipoles move around, and they have attractive.
40:41
In red, I'm sorry, it's kind of dim. And repulsive in blue forces, these two positive guys repel each other, but this negative guy's closer. On average, there's more attraction than repulsion,
41:00
because the molecules orient themselves so that that's the case. And therefore, this would probably have a higher boiling point than something that didn't have those forces. And finally, the weakest forces are dispersion forces.
41:22
Those are the ones that arise from a momentary charge fluctuation. The molecule itself may not have a dipole moment or a charge, but nevertheless, the electrons get off the molecular framework just
41:41
for an instant or two, and that's enough time for the other electrons, which can also move very quickly, to slosh over, and you get a momentary attraction. And you get umpteen, countless repeats of this, and on average, you get this stuff sticking together,
42:02
much more than if that couldn't happen. These will always occur, but if we have something like a charge or a dipole, basically that's so much bigger that it swamps the dispersion forces. So usually, we only talk about the dispersion forces
42:23
being the main thing. If there's no charge and there's no dipole and there's no hydrogen bonding, then we go looking for dispersion forces. And as I said before, you can go back to the material on gases, where we introduced the van der Waals parameter, A, for attraction,
42:43
and I explained that for neon in terms of these momentary charge fluctuations. Let's see if we can rationalize differences in boiling points by looking at these intermolecular forces. Here's a practice problem.
43:03
The two molecules, N2, which is harmless, and CO, which is poisonous, but unfortunately odorless,
43:21
are isoelectronic. They have the same number of electrons. Which should have the higher boiling point and why? Without looking it up qualitatively, who wants to guess?
43:41
CO, and my argument would be CO has a dipole moment, and if I only knew a little bit, I'd guess that because oxygen is more electronegative than carbon, that the oxygen side of the CO is negative,
44:00
and the carbon side of the CO is positive. And guess what? I'd be wrong, because in fact, our current Vice Chancellor for Research, Professor Hemminger, when he was a graduate student, proved that the carbon side is negative,
44:24
and the oxygen side is positive. And if you draw a correct Lewis structure with formal charges and a triple bond, you have to put the positive charge on carbon, on oxygen, and the negative charge on carbon. And when you look at how carbon monoxide poisons you,
44:43
it's the carbon side that sticks to the metal in hemoglobin, not the oxygen side. And that would make sense that the carbon side is negative, and the metal is a positive ion. Difference between carbon monoxide and oxygen,
45:03
oxygen comes on and then comes back off. Carbon monoxide comes on, and then that's it. And you die bright red like a big red tomato, because your hemoglobin is even redder than normal.
45:20
And that's when you find a body like that. It's bright red like a tomato. Then first of all, you don't stay in that enclosed space very long. And second of all, you check for carbon monoxide poisoning. What are the symptoms of carbon monoxide poisoning?
45:49
The first symptom is confusion. That's why it's important to be a clear thinker. Because if you aren't, you may not know you're confused.
46:03
If you're confused in an enclosed space for any reason, get outside. There could be a faulty water heater, could be anything. If you just stay there and say, gee, I'm confused. I can't think straight, but I think I'll just sit here.
46:22
Or you wait too long, and you're so confused you decide not to go out. Then you can die. Or if you happen to be asleep, that would be very unfortunate. OK. We look at this, and we say, yeah, N2 symmetrical.
46:42
It doesn't have a dipole moment. Carbon monoxide can and does. We would have guessed the wrong polarity. But anyway, it does have a dipole moment. And we expect the dipole-dipole forces to be stronger in carbon monoxide than the London dispersion forces in N2. And anyway, since they have the same number of electrons,
47:02
we expect the dispersion forces to be about the same. And if we look it up, nitrogen boils at 77 Kelvin. And carbon monoxide boils at 82.
47:21
Gee, it's not very reassuring, because it's not a very big difference. It's almost a coin toss. Well, that's because, in fact, the dipole moment of carbon monoxide is very small. It's about 0.12 debye. A molecule like KBr in the gas phase
47:41
would have a dipole moment of about 10 debye. And most molecules have dipole moments between 0 and 10. Therefore, this is on the very low end. Hydrogen fluoride is about 2. The debye is a unit used to characterize the dipole moment.
48:00
If you wonder what it is and you're curious, look it up. Look how much charge separation it corresponds to. How much positive charge? How much negative charge? How far are they apart? That's a good thing. To know if you want to characterize dipole moments. Now, we just talked about boiling points.
48:22
We talked about viscosity. But what about melting points? And here, the point I want to get across to you is that melting points are very, very much more complicated than boiling points. Therefore, if you're given a problem ever
48:40
on a standard exam and it's about melting points, yes, you look at intermolecular forces. But you look at a lot more than just that. Because solids have structure. And you have to know what the structure is, how the things are packing before you can be sure that it's all down
49:02
to the intermolecular forces. Shape matters a lot. Round things, or things like footballs even, can pack efficiently in a solid.
49:22
Funny shaped molecules like H2O can't pack as efficiently. And therefore, for a given amount of intermolecular forces, a solid made out of round things like beach balls will have a much higher melting point than you might expect.
49:43
So it's not just intermolecular forces, but it also has to do with packing efficiency in a solid lattice. That also influences things a lot in this particular case.
50:01
If you have a molecule that's round and that doesn't have much in the way of intermolecular forces, it'll have a very, very small liquid range. It'll take forever to melt. And then once it melts, the forces are pretty small. So then it just boils just shortly after.
50:23
And that means the liquid range is very low. Let's have a look. I drew these out. The long chain one's called n-pentane. One, two, three, four, five.
50:44
And then I just move one of these guys from here over to here. And I move the hydrogen from here over to there. And I have a branch. So this is a long like a snake.
51:02
This is a branched hydrocarbon. When they're doing cracking of gasoline, they want to make branched hydrocarbons. They don't want these guys. The car engine won't run very smoothly on those. So there's a whole chemistry of how to do that efficiently.
51:25
Unfortunately, it involves a lot of energy. The first thing you do with a barrel of oil is you heat the thing up like crazy. And that energy, to heat the barrel of oil up, to crack it to make kerosene, gasoline, propane,
51:45
et cetera. And then you're left with road cars, so-called asphalting, at the bottom. Takes a lot of energy. And as long as you've got lots of oil, you just take one barrel of oil and you burn it.
52:01
And the other barrel of oil, you crack it. And then you burn that too. You burn all of them in the end. But if you start running low, you're going to be in a world of hurt. If you just run low on energy, even if you have crude oil,
52:22
you'll be in trouble. And then what did I do here? Well, I did the same trick again. I moved one of these over here on this side. And then I put that hydrogen over here. And then I've got a CH3 here, CH3 here, CH3 here.
52:42
This is 2,2-dimethylpropane. I'm naming them correctly, but I don't expect you to know the names or anything. This is just for information. Don't be overwhelmed. I don't want to use an incorrect name, but I don't want you to memorize names of hydrocarbons.
53:01
OK, let's try a practice problem with these. Let's discuss the liquid ranges of these three molecules after looking up the data. Now, we'll have the data. We aren't guessing, but then you have to explain the data. Why is it that it's this way and not that way?
53:24
If you look up the melting points and boiling points, and these all have trivial names, n-pentane, isopentane, and neopentane. Now look at the melting point. The melting point of n-pentane is minus 129.8.
53:46
The melting point of isopentane is minus 159.9. This one melts at a lower temperature. It's got that tinker toy sticking out.
54:03
We could rationalize that, but it doesn't pack very well. It has an odd shape. Maybe these guys, like snakes, can kind of line up. We're guessing, but let's just guess for a while. And then look at the melting point of neopentane.
54:21
It's on a cold day in Canada, neopentane freezes. It's that much higher. Now that can't possibly be due to intermolecular forces because neopentane is symmetrical and doesn't have a dipole moment.
54:43
And they all have the same number of carbons and hydrogens. So they have the same number of electrons. And it's huge. It's almost 100 degrees different. And the point here is that this anomaly
55:02
is due to the fact that neopentane is round and can pack into a solid extremely efficiently. And so when you try to heat neopentane up, these round things jiggle around, but they can't go anywhere because there's no free spaces. There's no voids.
55:21
If I have an odd-shaped thing, it can't pack efficiently. There's a void. I heat it. It can move and squeeze through. Then these guys can start moving, and then guess what? It's melted. This one can't. They all have to go at once.
55:41
It takes a lot more energy to do that. But then look at the boiling point. This one's the highest, and that tends to be true, these long-chain ones. And this one is intermediate, and this one is now the lowest because it has no, once it's moving around in a liquid,
56:03
we aren't talking about packing. We're just talking about forces. And therefore, it boils very easily. Well, the important thing is, be careful not to assume melting points have everything to do with intermolecular forces. This particular problem has made the rounds for 30 years
56:29
because I was given that problem to do. And it's still around. And people still can't explain it if they haven't studied it.
56:43
OK. This is just what I was saying. I've given you the data again. Neither n-pentane or iso-pentane can pack that efficiently, but iso-pentane has smaller forces, so it has a lower melting point.
57:01
But neo-pentane can suddenly pack extremely efficiently, and that's why it has a much, much higher melting point. A very small liquid range. OK. Let's talk about crystalline solids. Let's start with two-dimensional.
57:21
We'll start with a load of circles. And we'll put them together like anybody's done with pennies. And there is a closest way you can get them to fit until they touch. And this is the way.
57:41
Bees have worked this out, of course. There's a hexagonal symmetry. There are six closest neighbors to any one central guy. And I can tile the entire plane with these hexagons.
58:02
I can tile the plane with triangles, the square hexagons. I can't with octagons. And if you look at a soccer ball, the soccer ball has facets with six sides.
58:22
But the soccer ball is round, curved. Therefore, I can tell you that not all the facets have six sides. Otherwise, it would be flat. And occasionally, if you look at it, you'll find one with five. And when you put one with five, it starts curving down.
58:42
And then if you know what you're doing, you can make something round with sixes and fives. And you can make all sorts of shapes with crystals with different facets, six, five, and so forth. It gets complicated. But they're very, very beautiful objects if you understand how they're put together.
59:00
They're much more interesting than you ever thought. Now, there's only one way to do this here. And you might think if I take marbles and I put them in, stack them up like eggs, there's only one way to do it as
59:21
well. That's what I thought. But surprisingly, that's not true. There are two ways to do it with spheres. Not one. There's two. And they're both as tightly packed as possible. Now, here's how it works.
59:44
On top of the red, I've put a layer of blue ones. You can see I've put them right in the divot. Here's the blue one on top of the three reds. You know that's how they would stack up. But then, interestingly,
01:00:00
are some holes in between. So there's a blue one here, there's a blue one here, but here there's a hole. So if I shine a flashlight through, the light comes through that hole. And there's a bunch of holes along there, where there's, sorry, along here, where
01:00:22
there are neither a red one nor a blue one. And it's every other layer. These are covered. These you can see through. Now I have a choice to put on a third layer. And the choice is, I can certainly put in this divot, there's three blue ones, I can
01:00:46
put another red one right on top of the other red one. In that case the light still shines through. Or I could cover the holes. In that case the light doesn't
01:01:03
shine through. They both have the same packing, same density, but they're different. Let's see. Now I put the red one on top of the other red one so we
01:01:21
can't see him. And now you can see, here's a blue one behind here, but here's a hole. I'm sorry, I should have picked a darker blue. Here's a hole, here's a hole, here's a hole, here's a hole. But if I do it, and this is called ABA,
01:01:41
to tell us that the third layer is the same as the first, just moved up. They're all in the same position, just a shorthand notation. But we can also put the third layer, I put it in yellow. Now it's lights out, no holes. And that's because
01:02:04
instead of lining up the yellow with the red, I moved it over one to cover the holes. The best way to do this is to get a bunch of round things and play around with it, and you'll see it quite quickly. And this structure is
01:02:22
called ABC, to show that the third layer is different from the other two. The next thing to ask yourself is, is this, have we covered all the possibilities? And it turns out for closest packing spheres we have. There
01:02:41
are no other possibilities. They're both close packed. They both can make the densest structures we can make with identical spheres. And the ABA structure has hexagonal symmetry. It's just like the 2D case, like what a honeybee might
01:03:06
put together. And that's called hexagonal closest pack, or the abbreviation is HCP. And the other one has cubic symmetry, and it's called
01:03:22
cubic closest pack, or CCP. Now here's a question a mathematician would ask you. Suppose I had a crystal made out of four-dimensional spheres. How many ways,
01:03:44
how many different ways are there to pack them? You may have to think about that a little. You probably can't imagine what a four-dimensional sphere looks like. Most people can't imagine the fourth dimension at all. Because for
01:04:02
one thing, a surgeon would love a fourth dimension. You know why? Because they could see inside you, everywhere. And they could probably hear everything
01:04:20
you were thinking. Just like when we look down, here's the two-dimensional circle talking. They can't see inside each other. Hey, how you doing? Fine, how are you? Then they go each other's way. They can't see inside if they've got an
01:04:40
opaque. But the guy in the third dimension looks down on them. He can see all their guts inside, everything they've eaten, everything. And if we had a fourth dimension, we could see all of us spread out and see everything. So a four-dimensional space is much cooler than what you might have thought
01:05:04
it might be. And a 4D sphere is much more complicated than you might have thought too. Okay, we'll pick this up next time with crystal and solid.
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