10_09 Sublimation And Fusion
So we've looked at face changes that involve gases. And we've looked at the boiling point we talked to vaporization, which is, you know going into the vapor phase now, we're kind of going to look at phase changes that involve solid state and there's, two terms here, sublimation and fusion. That may not be familiar with you, so we're going to find those as we continue on. So the liquid solid equilibrium terms that we see are this one, which we're familiar with freezing is a phase change that goes. From the liquid state to the solid state. So you put your water in the freezer.
We call it that for reason, and it freezes okay and turns into ice and the opposite process and everybody calls it melting, but it is equally correct - cause it call it fusion that process going from the solid state to the liquid state. Now, if you take an ice water mixture, and you hover, it right at its melting point, or it's freezing point, which happen to be the same temperature. Okay, it will not melt or freeze at that. Temperature it's at equilibrium. So some water molecules might go into liquid phase. But every time they do another water molecule will go into the solid phase.
So it's, an equilibrium established there at that temperature. Okay. So what is that temperature? The melting point in the freezing point, which are the same temperature? We just use the language differently, depending on whether we're putting heat in and moving it towards melting or put pulling heat out and moving it towards freezing. So that's. The temperature at which you are going to be at that solid liquid equilibrium phase boundary.
Okay. So if you have a right at that melting point or freezing point, then it will neither go towards melting nor towards freezing. It will maintain an equilibrium between those two.
Now, when we say that water freezes at zero degrees, Celsius, we're, actually, giving what's called the normal melting point. Talk about the normal melting point or the normal freezing point. Just like we talked about the normal boiling.
Point being this is the temperature water boils at when we have one atmosphere pressure. So when the external pressure is one, this fear water will freeze at zero degrees, Celsius. It will melt at zero degrees Celsius. Now, how does it know what it's supposed to do melt or freeze we'll? Talk about that as we proceed, but that is what the normal melting point or the normal freezing point means now just like we talked about heat of vaporization, the amount of energy you have to put in to vaporize something. We can talk about heat of fusion, the amount of energy you need to put in to break attractions to get them melted. Remember in the solid state.
What you have is the molecules with a nice connection between each other, and they're, not moving past each other. They have very tight attractions there. When we turn it into the liquid phase, and we add some thermal energy, those molecules speed up and now can slide past each other they're, still touching, but they're able to move past each other. So you've broken. Those attractions just enough to allow them to migrate around each other and still touch each other.
So it takes some energy to do that. You have to break those attractions and that's, what heat of fusion is. So here once you that's, the definition, the energy required, and it's generally reported in moles. So you'll, look at values, and I'll say, kilojoules per mole, the amount of energy required to melt a mole of the solid. She is you taking a mole of the solid, and you're, turning it into a mole of the. Liquid, and you can look at tables of values for this.
Now we had defined heat of vaporization. And now we see heat of fusion. You need to know that heat of vaporization is always much larger than the heat of fusion it's, not as sometimes thing. Now, why would that be let's go back to our picture on the light board here of our molecules to go from the solid state to the liquid state, they're still, very close together? Okay?
So we're melting it here, and it takes Delta H of fusion to do that amount of. Energy, but when we go from the liquid phase, where they're still pretty close to each other, but they're allowed to move past each other, and we go to the gas phase now, they're, very, very far apart, and they have no attraction for each other whatsoever. To do this processes, Delta H of vaporization. It takes a lot more energy to break the in track interactions, entirely than it does just to break them partially. So we're only breaking them partially enough for them to slide pass each other. But when. We go to the gas phase.
These guys are buzzing about totally independent of each other and have no attraction so whatsoever. So that's, why as to why heat of vaporization must always be larger than the heat of fusion all right? So that is kind of a statement of what the picture is being drawn the molecules and liquid states are still fairly closely. Packed, they're, still attracted to each other somewhat when I say, they're, larger. This diagram here shows us indeed how much larger. So what you see in. Red on the image, there is the heat of vaporization.
And what you see in blue is the heat of fusion, the melting energy. So it is not just a little bigger. It is a lot more energy to break those intro interactions, entirely than it is just to break them partially. So I want you to see that picture?
So you can keep in mind visually that that is indeed the case now we're going to talk about an interaction from the solid straight to the vapor phase without going through the melting phase. There is. This process that can take place this process is called sublimation. So sublimation is a conversion of a solid directly to its vapor phase by passing the liquid phase entirely. Now, different substances do sublimation at different proper means in different conditions. Okay, you can actually make ice turn into vapor directly under certain conditions. But that's.
What sublimation is now the opposite of sublimation. And some people may have heard the term sublimation and not have heard the term deposition. It's, not all that commonly used even in the chemistry world, but deposition is the reverse of sublimation, so it's going from the solid state during from the gas phase directly to the solid phase. And that is a process of deposition.
Well, we'll talk about that more. Now, there is an energy associated with this if there's an energy associated with melting. And there is an energy associated with going into the gas phase. Okay, there is an energy associated with going from here straight to here. And. Bypassing that all together all right so that would be the molar heat of sublimation Delta, H sub as it's abbreviated there. Now, something that we haven't gotten into right now.
But I want to mention when you're talking about these enthalpies, whether you do something in multi steps or you're doing it in one fell swoop all together. The energies have to be the same. So if I take and break the interactions completely in one fell swoop it's going to take a certain amount of energy to do that if I.
Partially do it and then go the rest of the way it should make sense, and it takes exactly the same amount of energy to do that as well. So it's going to be true that the sublimation is going to be the sum of these two little pieces. So the Delta H fusion would be going from here. All the way to here in one-step Delta, H of sublimation would be the sum of those two energies, right? There may or may not have to do any problems where that comes into play, but I do want you to understand the concept for.
Sure of those relationships between them, this is always smaller than this one. This is the sum of those. So would this always be smaller than this one? Yes, would this always be smaller than this one? Yes, then we see the relationship between those all right now. I want to kind of finish with this diagram here, and I'm going to kind of draw out those three boxes here, because what I want you to be able to do is I want you to be able to just know how to interact between them and the language associated. With going from a solid to a liquid to a gas.
Now this process takes place as the temperature increases we're breaking those intermolecular attractions as we add thermal energy, and we go up. So you ought to be able to take this image as part of your learning outcomes, and you ought to be able to define all of those relationships between that. So as you take a solid, and you turn it into a liquid what's that called well, that's melting or fusion. But if you take a liquid, and you turn it into solid, then. That's freezing okay, so we know that that's the name of those processes if you take a liquid, and you turn it into a gas. It is vaporization. It could do it slowly like evaporation or all at once like in boiling.
And then we can go from a gas to a liquid. And that is condensation and I'll just run. It Co ND. But we know the definition of that.
And then we have just learned that you can go solid all the way to a gas. And that is called sublimation now where you would have seen this in your everyday world, Might be in the substance that we call dry ice. All right, you can go to I think certain hardware stores and purchase dry ice is solid carbon dioxide, co2 solid, and under normal conditions of atmospheric pressure. This dry ice will go straight to a gas. Now we see why it's called that it's freezing as a solid it's, very, very cold.
But it doesn't go into the liquid state. So go straight to a gas, it's really wonderful to pack foods in dry ice when you have to keep them cold over a trip because. You're, not going to end up having them sloshing around in water and getting all wet and as water melts, or the ice melts to form water. This will just be evaporating away into the gas phase and I, just used the word wrong at night, he'll be subliming into the gas phase, and you're, not going to get that liquid mess it's going to happen. And then the opposite is true is we can take a gas and turn it into this by dropping the temperature. This is deposition.
So that is a connection of all the terms. Associated with this, and we know that terms, and we know the energies associated with each one of those processes.