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Simple and fractional distillations

Video transcript
Have you ever wondered why vodka is such a strong alcoholic drink compared to other beverages? That's because they use a process known as distillation once or even multiple times to increase the concentration of ethanol in the drink. Today, we'll be talking about how distillation works. You can do this in your organic chemistry lab, and let's take a look at the setup I've drawn here. First, in green, you have what's called the distilling flask. This is where you put in your mixture of compounds that you want to separate out. Next, in orange, you have your oil bath. You want to use an oil bath, because oil won't evaporate when you heat it up, and it's good for maintaining a constant temperature throughout this process. Shown up there in red, you have the thermometer. You need to be able to measure what temperature your compounds are boiling out at. And shown in yellow is the condenser. With a condenser, water has to cycle in and then out. This keeps the condenser cool, and the reason the condenser needs to be cool is because distillation involves a series of vaporizations and condensations. So initially what happens is, you have some liquid in your distilling flask, and as it gets heated up, it turns into a gas. Then, because the condenser is so cool, it'll condense down the distilling flask, ultimately landing here in the pink receiving flask as a liquid. This also needs to be kept cold. So what I've shown here in blue is the ice bath. And again, it's kept cold for the exact same reasons. You want this whole right side of the setup to be cold, so that the liquid can readily condense back into its pure form. There's one more tiny thing we haven't labeled yet, and that's this. This is a vacuum adapter. Why would you need a vacuum in distillation, you might ask. Isn't it enough to just heat it up really hot to get it to evaporate to a gas? Well, no actually, because sometimes you have compounds that have very, very high boiling points. If you have such a high boiling point, it can be difficult to distill. But lowering the pressure of the entire system makes it easier to vaporize substances, because at lower pressure there isn't much of a force pushing back down on the liquid, which makes it so much easier for it to vaporize upwards into the gas phase. Let's take a two-component mixture. The first thing you have is hexane, and I'll show that here in the flask. The second thing you have is toluene. This is a pretty conjugated aromatic ring, which is why it has a higher boiling point, and I'll also show that here in the flask. How do we monitor what's going on during a distillation? Usually, you'd want to plot this out in the form of a graph. We're plotting temperature versus time, with temperature on the y-axis and time on the x-axis. And what happens first is, initially, you're just heating up the system. So the temperature's rising slowly but surely. When you approach the boiling point of hexane, around 68 degrees, you see a plateau. Why is it that you see a plateau? Let's quickly review what happens during a phase change. The temperature stays constant. What you'll see here as the hexane is going from liquid to gas is it gets vaporized up here. You'll see this temperature register in the thermometer, and then it'll condense back down into the liquid phase since this side is so cool, and there you'll be collecting these two drops of hexane. What happens after we've collected that flask? What you'll see again is an increase in temperature. And make sure to switch out your receiving flask. You'll see me magically change the color of the flask to show that this is a brand new flask for collecting pure toluene. You have this toluene now at 110 degrees, again hitting a plateau, because once more that represents a phase change going from liquid to gas, then condensing back into liquid again and dripping into this new receiving flask. So ultimately what we've collected are these two flasks, one with toluene and one with hexane. And there we were able to do a pretty successful distillation. What happens if instead you have a three-component mixture? Well, this works pretty much the same as a two-component mixture, except that you'll see more plateaus in your graph of temperature versus time, which I'll draw here on the side. The first compound that we have here is acetone, which looks like this. The next one is cyclohexane, which has a slightly higher boiling point. And lastly, you have acetic acid. This has a higher boiling point than the other compounds, because it's capable of hydrogen bonding, meaning there's more forces between the acetic acid molecules, making it harder to pull them apart into the gas phase. So again, let's look at our graph. First, what you might see again is a slight increase at the beginning when you're just heating up your flask. But as soon as you hit 56 degrees, you'll see that plateau as acetone vaporizes and condenses. Then, you'll see the temperature increase again until 81 degrees, where it'll hit a plateau, and that represents cyclohexane vaporizing and condensing. Lastly, you'll see another increase. And finally, you'll be able to get acetic acid. Let's take our final example and answer the question, how is it that they get vodka and other drinks to be so strong. To produce strong drinks, this is the kind of distillation they might need to do. As you can see, ethanol and water, their boiling points aren't too different from each other, only 22 degrees Celsius. Do you think that will affect what happens during the distillation process? Well ideally, what you'd want to see in your graph of temperature versus time is like what we've seen in the past. You'd hope that what would come out is something that looks like an increase in temperature, followed by a plateau at 78 degrees, followed by another sharp increase, and finally a plateau at 100 degrees. However, that's not really what happens. Instead what you get is something that looks kind of like this purple graph. You might get this increase at the beginning, but instead here you have this slope. Why is this is a problem? Well, before you might have been able to get the pure ethanol, meaning the pink line, and pure water separate. But here, they're kind of mixing into this purple. So anything you see between these two points isn't really pure ethanol or pure water, it's some mixture of the two. And if you're still getting a mixture, it means your alcohol still isn't very strong. So how can we fix this problem? You might think, what happens if I distill the compound again? If you were to distill this compound again, the next time you might get something that looks kind of like this orange line. This would be a little bit flatter, maybe it would get a little bit steeper than before, but it still wouldn't be very pure. And if you kept distilling it over and over again, you might eventually reach what you had hoped to get in the beginning, that ideal blue and pink separation. But just doing a simple distillation multiple times can be extremely time consuming, so is there another way we can do it? There's actually another setup that will allow us to effectively do multiple distillations at once. This is known as fractional distillation. If you look at this picture in the right, the only thing different between this picture in the picture on the left is that you have a fractionating column here. This column can be filled with a number of substances, such as beads or other things, but just for fun I'm going to fill it with some stars. So you see, you want to completely fill this column. And how does that affect what goes on during distillation? If you had your two compounds again-- let's just say that you have purple representing two compounds in here-- what happens is that instead of just going straight up, vaporizing once, and then condensing again, it will go up, vaporize, and then maybe condense onto one of these stars. And from there, it'll vaporize again then condense into another of these stars. So as it goes up and up past the packaging material, it's going through so many vaporizations and condensations so that when it finally does reach the receiving flask, it'll be much purer than what you would get through just one simple distillation. So let's summarize what we've talked about today. We talked about how you would set up a simple distillation and how that's great for separating out compounds with pretty big boiling point differences, say a difference bigger than 25 to 30 degrees Celsius, and fractional distillation, which is great for separating out compounds with smaller differences in boiling point.