2015 AP Chemistry free response questions
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2015 AP Chemistry free response 2f
- [Voiceover] During the dehydration experiment, ethene gas and unreacted ethanol passed through the tube into the water. The ethene 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. And 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. And what they're saying is is, when they warm up, they'll react, the catalyst and the ethanol in here. Some of that ethanol gets converted into ethene, but not all of it does. In fact, part of this experiment, we saw that we don't have a perfect, we have a 60-point-something percent yield. And so you have a combination of ethanol and ethene 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 ethene gas here. How come we're not seeing the ethanol gas? And 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 ethene, which isn't a polar molecule. So let me write this down. So the ethene will kind of bubble through, while the ethanol can actually dissolve. So let's write this down. So, whoops, having trouble, all right, there you go. So explain this observation in terms of intermolecular forces between water and each of the two gases. So we could write ethanol. Ethanol is polar, so it dissolves in water much better than ethene, which is nonpolar. And so we could say something like, so ethanol, ethanol and water will have hydrogen bonds, will have hydrogen bonds. 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, this 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. And so when you have water molecules, so if this is a water molecule right over here, where this was partially positive charges, partially negative charges, you're going to have the hydrogen bonds. Hydrogen bonds. And so the ethanol is going to dissolve much better. The ethene isn't polar and will only have induced dipole forces acting on it. So let me write this. Ethene, ethene is not polar. Is not polar. And so, or I could say, so will only have induced dipole interactions. Induced dipole. And maybe well I could say dipole, 'cause the water is polar, and even though ethene is a symmetric molecule and has that double bond, it has no net polarity. There are parts of the ethene 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. And so ethene is not polar, wo will only have induced dipole, or I guess we could also say just dipole interactions, interactions, interactions, with polar water. Polar water. And so this is why. So this is why ethene won't dissolve as well. Ethene won't dissolve as well and bubbles through. And bubbles through. Ethanol dissolves. Ethene bubbles through, 'cause it doesn't have strong interactions with the water.