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2015 AP Chemistry free response 2f

Solubility of ethanol and ethene gas in water. From 2015 AP Chemistry free response 2f. 

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  • blobby green style avatar for user P
    Are these induced dipole dipole interactions of ethene like regular dipole dipole interactions but weaker due to less electronegativity difference between the carbon and hydrogen or is it like a London dispersion force?
    (11 votes)
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    • blobby green style avatar for user Ryan
      London dispersion forces are induced dipole-induced dipole forces. This is a dipole (water)-induced dipole (ethene) force because all non-polar molecules can have induced dipoles, but water is a dipole because of its constituents' relative electronegativities.

      Also, I believe a dipole moment is similar to a moment of inertia in the way that it is charge times radius just as the moment of inertia is mass times radius. Increasing the radius separates the charge better and increases the effective charge at each pole because there is less resistive force from the opposing pole.
      (5 votes)
  • blobby green style avatar for user Stephano Paraskeva
    During this experiment, why when the ethanol is heated does it break into c2h4 I thought you couldnt break bonds through just heating or that heating/boiling just releases molecules as they are?

    What occurs in this experiment?
    (3 votes)
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  • piceratops sapling style avatar for user Gaurvi Chauhan
    In the water molecule, why does oxygen have 4 electrons represented on the top of it () even though it already has two bonds with hydrogen?
    (2 votes)
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  • leaf green style avatar for user penguinzrkool49
    how would you know which molecules are polar or non-polar on a test like this? i'm sure you're not calculating electronegativity for each molecule, so how do you know for sure? he didn't explain that, just kind of said it.
    (3 votes)
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    • primosaur ultimate style avatar for user Matthew
      I know this was asked a year ago but I'll answer it anyway just incase anyone else has this question. The polarity of a molecule can be predicted two ways: symmetry and electronegativity differences. Symmetry can be determined from looking at a Lewis dot structure: if you can cut the molecule in half and it's the same on both sides, it's non-polar. If you can't, it's polar. This is the way you will be determing polarity on the exam. The other way is by electronegativity differences between atoms; you can't possibly be expected to memorize the electronegativity values of every atom (although it's easy to memorize the values for atoms common in covalent bonds, such as chlorine or carbon) so you usually won't have to use this method during the exam to determine if a molecule is polar or not (unless they gave you a Periodic Table with electronegativity values on it, which I doubt would happen.) However, this is something you should know. Differences in electronegativity can determine polarity or not. The rule of thumb that I learned (your teacher or textbook may disagree) is that if the electronegativity difference is below 0.5 between atoms the molecule is nonpolar; if it's above 0.5 its either polar or ionic. Hope that helped!
      (2 votes)
  • aqualine ultimate style avatar for user Eddy
    How do you know if certain elements in a compound are partially negative or partially positive?
    (2 votes)
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Video transcript

- [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.