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2015 AP Chemistry free response 2d and e

Lewis electron-dot diagram and bond angles for ethanol. From 2015 AP Chemistry free response 2d and 2e. 

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  • blobby green style avatar for user rojas.moey
    For the answer to e, why is it not just 104.5 degrees? Isn't the electron geometry the only thing that matters in determining bond angles?
    (8 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      Yes, electron geometry is what determines bond angles.
      The theoretical bond angle is 109.5 °, but the actual bond angle is 104.5 ° because of lone pair-bond pair repulsions.
      The examiners can't expect you to memorize every bond angle, but you should be able to predict theoretical angles of 90°, 109.5°, 120°, or 180°.
      I would have answered "109.5 °", but they should accept any value between 104.5° and 109.5°.
      (12 votes)
  • male robot hal style avatar for user Kirk
    At - Sal mentions a tetrahedral shape. I really don't understand what it is. Please explain to me. What is a tetrahedral shape?
    (2 votes)
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  • blobby green style avatar for user DC83
    In C2H4 why are there 4 electrons between the two Carbon bonds? I thought there would only be 2?
    (3 votes)
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    • mr pants purple style avatar for user Ryan W
      Carbon basically always follows the octet rule, this means it wants to have valence 8 electrons around it.

      In C2H4 each carbon is going to be bonded to 2 hydrogens, and to the other carbon, but this only accounts for 6 electrons.
      So in order for each carbon to follow the octet rule, there has to be a second bond between the carbons, meaning there is 4 electrons total between the carbons.
      (7 votes)
  • blobby green style avatar for user mar2611002
    Why did they write the ethanol like that at ?
    (4 votes)
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  • aqualine seedling style avatar for user Diana Valle
    Why can't the hydrogen be bonded at the opposite side of carbon? Do the lone electron pairs always have to be side by side? I predicted the molecule shape would be linear because I counted the lone electron pairs on opposite sides, and the hydrogen on the opposite side of carbon, not next to carbon, i.e. instead of carbon, hydrogen, and two lone electron pairs, I counted carbon, lone electron, hydrogen, lone pair electron, so carbon and hydrogen would be 180 apart. Please explain!
    (2 votes)
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    • mr pants purple style avatar for user Ryan W
      You need to start thinking in terms of 3D space with this. There is no "opposite side" here, the oxygen is sp3 hybridised so the four electron regions around it have tetrahedral geometry and the bond angle is approximately 109.5 degrees, as that is the furtherest that the 4 groups can be around a central atom. The lone pairs repel one another a little bit more which pushes in the C-O-H bond angle so the appropriate answer would be approx 109.5 or just less than 109.5

      Again, you need to consider how these bonds look in 3 dimensions, they aren't all 90 degrees apart and flat. A molecular model kit can show you why it isn't linear but it isn't exactly easy to show on paper.
      (5 votes)
  • duskpin ultimate style avatar for user Mohit
    I dont understand why is the angle predicted to be only above 104.5 degrees and not below it......
    It could be that the lone pair of oxygen could repel the C2H5 MORE than H atoms in water molecules, in which case, bond angle would be less than 104.5
    But the lone pair could as well may not be able to repel C2H5 AS MUCH AS H atoms , in which case, the bond angle would be a little more than 104.5

    The correct angle would have to be experimentally determined or maybe calculated upon the basis of electronegetivities of H and C2H5....
    is it not correct way to think?
    (3 votes)
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    • piceratops ultimate style avatar for user Matthew Chen
      Because in a "normal" tetrahedral molecule, that is, made out of the same atoms, e.g. CCl4, the angle would be 109.5. However, in this example there are 2 lone pairs, and it has already been proven that lone pairs repel more than atoms do, so it would be slightly less than 109.5, and not more.
      (1 vote)
  • leaf green style avatar for user Dennis Chamberlin
    I didn't ask my question very clearly. I guess I'm just wondering why we are interested in the shape of just part of the molecule. I do see that this smaller part of the molecular mass is tetrahedral.
    (2 votes)
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  • blobby green style avatar for user romasharma1998
    Difference between dipole moment and polarization of a bond ?
    (1 vote)
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  • hopper cool style avatar for user archi31
    Do we just have to memorize these approximate degree values for questions like (e)? Or is there some trick to approximate them linked to VSEPR?
    (1 vote)
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  • piceratops ultimate style avatar for user wanglx123456789
    At , why did Sal draw it into that way, one H popping in, another popping out? What does this mean?
    (1 vote)
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    • leaf red style avatar for user Richard
      Think back to VSEPR theory which tries to explain the geometries of molecules. The oxygen in ethanol has four electron groups, but only 2 bonds. Which means it has tetrahedral electron geometry, but bent molecular geometry. The bent structure containing the bonds to other atoms is usually drawn with the bonds in the plane of the page with the bonds drawn as straight lines to make it look 2D. But we exist in 3D space which means the molecule is also 3D so you can imagine if you rotate that molecule around the oxygen atom you have one bond point away from you and one pointing towards you. In chemistry we represent a bond pointing away from you(or into the plane of the page) as a dashed wedge, and a bond pointing towards you(or our of the plane of the page) as a solid wedge. So basically they're drawn like that sometimes to emphasize the 3D nature of molecules. Hope that helps.
      (2 votes)

Video transcript

- [Voiceover] The Lewis electron-dot diagram for C_two H_four is shown below in the box on the left. In the box on the right, complete the Lewis electron-dot diagram for C_two H_five OH or ethanol, by drawing in all of the electron pairs. So as they said, this right over here, this is the Lewis electron-dot diagram for ethene and they want us to fill in all of the, they want us to draw in all the electron pairs for ethanol. And what we could do, and I'll do it in a way where we can see which electron comes from which atom, but they're not even asking you to do that. But in blue, I'm going to make the electrons from the hydrogens. So each hydrogen, you can think of it as contributing one electron to each pair, and that's what's forming the covalent bonds. So say this hydrogen is going to contribute this electron, this hydrogen can contribute this electron, this hydrogen can contribute this electron, this hydrogen can contribute that electron, this hydrogen can contribute that electron, this hydrogen can contribute this electron right over there. And then, let's see. So now let's think about the carbons. And if you were actually taking the EP test, you wouldn't, probably you don't even have markers around, but I'm gonna do it in different colors just so you can see it. So each carbon has four valence electrons that it can contribute for covalent bonding. So this carbon over here, that's one, two, three, and then four. And now this carbon over here can contribute one, two, three, and four. And now we think about the oxygen. So oxygen's interesting. It's going to have two lone pairs. So it's gonna have one lone pair. I can do it there. One lone pair like that. And then, it's going to have, it's going to form this bond by contributing one electron to this pair and one electron to this pair right over there. So each of these pairs represent a covalent bond. This one I could have drawn a little bit lower, but I think you get the idea. Those, I have drawn in all of the electron pairs. And I'll just think about part e. What is the approximate value of the carbon-oxygen-hydrogen bond angle in the ethanol molecule. So, in the ethanol, they're really saying, so the ethanol molecule, they want to know the bond angle. You have the oxygen bonded to a hydrogen bonded to the C_two H_five. So I'll just write that as, C_two, I'll just write C_two H_five, like that. And they want to know what is approximately this bond angle going to be. And the important thing to realize is, when you form these bonds with oxygen, it's going to have pretty close to a tetrahedral shape. Why? Because the oxygen has these two lone pairs. So these two lone pairs are forming the other parts, the other, the other, I guess you could say points of the tetrahedral shape. And so if we were talking about just water, so, water, if we were talking about a water molecule right over here, and I'm not doing a good job of drawing it. You could draw one electron pair there and then the other electron pair would be here in the back. Actually, I could, let me draw it like this. Let me draw, I could draw it like this. If we were talking about water, I could draw one hydrogen popping out. I could draw the other hydrogen popping in. And then one electron pair is over here and one electron pair is over here. This bond angle over here in water, this is, I guess, a semi-useful thing to know in general. You could kind of eyeball, and say, oh that looks like a little bit more than 100 degrees, and you'd be right. This is approximately 100 point four point five degrees. And this is actually not a perfect tetrahedral shape. It gets distorted because these lone pairs of electrons are repelling each other and making these two get in a little bit closer together with each other. And if you have a pure tetrahedral shape. So you have something in he middle. Well, let me just, if you had a pure tetrahedral shape like this. So let me, so let me draw it like this. So something popping out. You have something, something going in. And then you have two things like this. That's one way to think about a tetrahedron. There's others. Then the bond angle between all of these if it's a more, I guess you could say, symmetric tetra, a non-distorted tetrahedral shape, is 109.5 degrees. And this is a reasonable, a reasonably useful number to know, obviously for this question as well. I don't know if you could, if this is obvious. Let me actually draw the tetra-hed, let me connect the tetrahedron. So, you could draw it like that, and then that would be the other side right there. I don't know if that helps. Let me draw another one. So, if I were to draw a tetrahedron and if this was transparent, you have a molecule, or you have an atom in the middle, and then you have the four bonds. One, two, three, four. I could say four bonds or lone pairs. Then, let me make sure you see the one in the center, then the angle here is approximately 109.5 degrees. So this is going to be tetrahedral, we have these lone pairs that are going to be repelling each other a little bit. So you're going to be someplace in the neighborhood, of, I don't know, around where water is or more pure tetrahedral shape. So I would say your bond angle is going to be, I don't know, between 104 and 110 degrees. 104 to 110 degrees. In fact, I would, I would estimate that it's going to be more than 104.5, because these, well, I just, I won't try to dig too much into it. They really just want us to approximate, approximate the value. So you could give anything in this range would be a suitable, suitable answer.