GOB Chemistry

Learn the toughest concepts covered in your GOB - General, Organic, and Biological Chemistry class with step-by-step video tutorials and practice problems.

BONUS: Lab Techniques and Procedures

Chromatography uses differences in molecular attraction to separate components within a mixture. 

Mixture Separation 




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in this video, we take a look at chromatography. Now here. We're gonna say this technique involves the separation of components and here we're talking about really the separation of a solid from a liquid within a mixture because off a difference in molecular attraction, we're gonna say in the process, a mixture is spotted on a silica plate and the progress of the components on that plate is based on their affinity to the liquid, which is the solvent or the plate itself. So these are the two forces at work that are driving are solids up this silica plate. Now we have There's two phases involved in the first phase. This represents the silica plate, which holds the mixture. So we're going to say that this is our stationary phase and then we have our second phase, which represents to solve in itself, which moves up the silica plate by capital. A reaction. Okay, so you already moves by capillary action. So this is our mobile phase. Now, the way this process works is I have my mixture, right. So let's say a mixture is here. Alright, So here goes my mixture in here and so I take like a bit of this mixture, and I spotted on this silica plate here. Now here is our solvent, which represents our mobile face here. It's customary to have it like behalf Ah, polar solvent and half a non polar solvent. So here the polar part will be ethanol, so we'll say it's 50% ethanol and then 50% heck sane, which is are non polar. Of course, you can mess around with the with the, uh, percentages of both. You could make the solvent more polar by decreasing the percentage of Heche saying and increasing the percentage of ethanol. What's gonna happen over time is by capital a reaction. The water. The solvent portion is going to get basically soaked up by the silica plate and move up the silica plate so you see how it's moving up, and as it's moving up, it's dragging these dots with it. Now these dots represent different components within that reaction mixture, and here we have again to forces at work. We have the attraction that the solid mixture, spots or dots have for the plate itself, and also we have the attraction that could potentially have with the solvent as it's moving up. We're gonna say here for the dots. We're gonna say if the attraction for the plate is greater than that of the solvent, that means that we're gonna have low movement off the dock. It's not gonna move very far up because it's more attracted to the plate which wants it to stay there and the solvents not strong enough to drag it up. But if the solvent phase has greater attraction for the solid for the mixture dots than the plate, then there's gonna be hive movement. So they're gonna move high up the plate. So what's happening here is a solvent is dragging these dots up. And here we've got different different spots being produced. Now, look, you see how they're different colors. That's because these dots here represent the different components that were within that mixture. We've actually separated them from each other. It looks like one of them looks like it was mawr more polar, so it's stuck to the polar plate more. And then here these guys who got dragged up by the solvent and the solvent was 50% ethanol, 50%. Heck saying they kind of like balance each other out, so this would be less polar up here. Now we're gonna say here this line that we've drawn here, this is the starting position, and from there we can measure out how far the other dots have moved. We're gonna say the distance traveled by the components is a method we can we can use toe find what's called our our f value. So this helps identify the components within that mixture. You would be given an R F chart with different values for different components, and then you have to just match them up with one another. So the way it works is you would say, Here your value is distance traveled by the component that docked, divided by the distance, traveled by the solvent. So let's say we're trying to figure out the r F values for the blue dots and the red dots at the end. This is after it's been taken out of the German led to do let dry. So if we're looking at the blue dots, we're gonna say their position to and the solvent went all the way up to position for up here, slut equals half or 0.5, and then for the red dot we were a position three and solvent up to four. Again, that's 0.75 Okay, so here we look at an R F chart and see, based on the values given what could be the potential components that were within my mixture. So when you get to this lab, this is the approach you have to take. Remember the key things about the stationary phase in the mobile phase. Again, you can play around with the polarity of the solvent, and in that way you can either have the more polar component of the mixture. Get driven up with it or you could have them or non polar component getting driven up with it. The plane itself is always polar because silica itself, because it's made up of silica gel silica gel is a polar gel, so the plate itself is always polar. It's a solvent that you can play around to making either more polar or less polar than the plate. So knowing these things helps us to separate separate the different solid components within the mixture

The mixture that is spotted on the silica gel plate has its movement based on its affinity to either the solvent or to the plate itself.