2015 AP Chemistry free response 2f

During the dehydration experiment, Ethan gas and unreacted ethanol passed through the tube into the water. The ethine was quantitatively collected as a gas, but the unreacted ethanol was not. Explain this observation in terms of the intermolecular forces between water and each of the two gases.

Just to be clear what they're talking about, we can go to the original setup that they gave us at the beginning of this problem. What they're saying is that through this, when they warm up the reactants, the catalyst and the ethanol in here, some of that ethanol gets converted into ethine, but not all of it does. In fact, part of this experiment, we saw that we don't have a perfect yield; we have a 60-point-something percent yield.

So you have a combination of ethanol and ethine gas going through this tube. It cools down, then it goes through this water, and they're saying it looks like we're only seeing, or we're primarily seeing, the ethine gas here. How come we're not seeing the ethanol gas?

The reason—and I'll just paraphrase it right now, and then I'll write it down—is that the ethanol gas is much more dissolvable in the water because ethanol is a polar molecule. Water is a polar solvent, so it's going to dissolve much better in water than the ethine, which isn't a polar molecule.

So let me write this down. The ethine will kind of bubble through, while the ethanol can actually dissolve. So let's write this down.

So explain this observation in terms of intermolecular forces between water and each of the two gases. We could write ethanol: ethanol is polar, so it dissolves in water much better than ethine, which is non-polar.

We could say something like this: ethanol and water will have hydrogen bonds. You could even diagram it out if you like. The ethanol is right over here, so you have your oxygen, and then you have your hydrogen, and then you have your C2H5. This side over here is going to be partially negative; this is going to be partially positive. Oxygen is more electronegative than carbon; the difference is less than between oxygen and hydrogen, but this is also going to be partially positive, maybe not as partially positive as on this side right over here.

When you have water molecules, if this is a water molecule right over here with partially positive charges and partially negative charges, you're going to have the hydrogen bonds. So the ethanol is going to dissolve much better.

The ethene isn't polar and will only have induced dipole forces acting on it. Let me write this: ethine is not polar, and so I could say it will only have induced dipole interactions. Maybe I could say dipole because the water is polar.

Even though ethane is a symmetric molecule and has that double bond, it has no net polarity. There are parts of the ethane molecule that are going to be a little bit more negative than others, in particular when you look at the carbons over here. They're a little bit more electronegative than the hydrogens.

So, ethane is not polar, so we'll only have induced dipole, or I guess we could also say just dipole interactions with polar water. This is why ethine won't dissolve as well and bubbles through, while ethanol dissolves. Ethine bubbles through because it doesn't have as strong interactions with the water.