2015 AP Chemistry free response 2d and e

- [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.