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Current time:0:00Total duration:9:30

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 and 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 set up 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 cold 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 I haven't labeled yet 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 into 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 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 you're plotting temperature versus time with temperature on that 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 so it gets vaporized up here you'll see this temperature register in the thermometer and then it will condense back down into the liquid phase since this side is so cool and there you'll be collecting these drops of hexane okay 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 floss to show that this is a brand new flask for collecting pure toluene you have 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 would 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 first compound that we have here is acetone which looks like this the next one is cyclohexane it 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 till 81 degrees or it'll hit a plateau 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'll affect what happens during the distillation process well ideally what you'd want to see in your graph of temperature versus time it's like what we've seen in the past you'd hope that what would come out something that looks like an increase in temperature followed by plateau at 78 degrees followed by another sharp increase and finally a plateau at a hundred 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 ink at the beginning but instead here you have this slope why is this 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 that 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 are 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 would hope 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 set up that will allow us to effectively do multiple distillation at once this is known as fractional distillation if you look at this picture in the right the only thing different between this picture and 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'd want to completely fill this column and how does that affect what goes on during distillation if you had your two compounds again 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 well 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 condensation and 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 a 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