2015 AP Chemistry free response 2 d e

The Lewis electron dot diagram for C2H4 is shown below. In the box on the left, in the box on the right, complete the Lewis electron dot diagram for C2H5O, or ethanol, by drawing in all of the electron pairs.

As they said, this right over here, this is the Lewis electron dot diagram for ethine. They want us to fill in all of the electron pairs for ethanol and what we could do... 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 let, 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. If you were actually taking the AP test, you probably wouldn't even have markers around, but I'm going to do it in different colors so you can see it.

So each carbon has four valence electrons that it can contribute for the covalent bonding. So this carbon right 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 is interesting; it's going to have two lone pairs. So it's going to have one lone pair I could do there, one lone pair like that, and then 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 represents 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 with all of the electron pairs.

Now let's think about part E. What is the approximate value of the carbon-oxygen-hydrogen bond angle in the ethanol molecule? So 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 C2H5, so I'll just write that as C2H5 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, I guess you could say, points of the tetrahedral shape.

And so if we were talking about just water... so water, if we’re 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, let me draw it like this. Let me draw... I could draw it like this. I could, 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 try to eyeball and say, “Oh, that looks a little bit more than 100°,” and you'd be right; this is approximately 104.5°.

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 a little bit closer together with each other.

If you have a pure tetrahedral shape... so you have something in the 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 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 tetrahedral shape, is 109.5 degrees. And this is a reasonably useful number to know, obviously for this question as well.

I don't know if you can... if this is obvious. Let me actually draw the tetrahedron; 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 can see the one in the center. Then the angle here is approximately 109.5°. So this is going to be tetrahedral, but you 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 a more pure tetrahedral shape. So I would say your bond angle is going to be, I don't know, between 104 and 110°. 104 to 110 degrees; in fact, I would estimate that it's going to be more than 104.5° because these... well, I'll just... I won’t try to dig too much into it.

They really just want us to approximate the value, so you could give a... you know, anything in this range would be a suitable answer.