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### Predict which substance has greater molar entropy…

02:35
University of Massachusetts - Lowell

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Problem 23

Predict which substance has greater molar entropy. Explain.
(a) $\mathrm{NO}_{2}(g)$ or $\mathrm{N}_{2} \mathrm{O}_{4}(g) \qquad$ (b) $\mathrm{CH}_{3} \mathrm{OCH}_{3}(l)$ or $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)$
(c) $\mathrm{HCl}(g)$ or $\mathrm{HBr}(g)$

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## Discussion

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## Video Transcript

All right, So Okay, so we are asked to determine the which of the two molecules has a higher standard molar entropy in various of various pairs of molecules. So start with N 02 and n 2041 thing to notice is that you know, to and to afford their very similar in one aspect is that into a four is basically double and no to. And in that case, there's, uh there's more permutations for n 204 to have that and go to just can't partially because of this, thats funny little single single bond right here and allows, um, this half and this half to rotate at free will. And so basically any permutation that you know to can have and 204 could have, but probably just much more face. So in this case, and into a four probably has much higher standard Moeller Entrekin that of n 02 And now we have dimethyl ether and ethanol to very similar molecules. In fact, they're ah, they're, uh, chemical formula. It's exactly the same scene about a carbon, oxygen and hydrogen. The only difference is that the placement of the fun oxygen in the dimethyl either is in the middle, and the ethanol, as you can see, is at the end, and something to notice is that in these little structures that are dry, I'm not trying to fool one, because that's just gratuitous. There's and enjoy the hydrogen excited need to. The main part of this is just to get the structures, just skip the structures. That's what I want to focus on. And in this case, at the end of every lank bond, I guess that's what the hundreds. I just didn't write them because I, like I said, that's gratuitous. So, uh, how would you go to church? Determine what has I am what? It's hard to be in this case because they're very was very similar. It's just the placement of the Astra. And so what you might be able to guess is that, um, in this case, let's say this one has been imagined last time in the previous one right here. There are two single bonds here, so there's this. This part can rotate every will and so in this one. And this soak in this part is well. But thing is, is that when you flip them, Maybe foot this molecule. If you married, this would look exactly the same. So there's just one permutation that that's just it's an exact copy. And the thing is that when you if you just the same thing over here, he flipped us. If you mirror this, um, you would not have the same molecule or you would have. Samarco just is in a different permutations, like I said earlier. So in this case, you actually have a higher Moeller entropy with the ethanol than you do with a dimethyl either. And it's partially because of that. There's other factors that you don't know off, but in this case, that's the one on one of us. That's the main part here is that they're just slain like they're slightly more permutations that can be here. This pregnancy been there, they open spin around the run thes thes single sima access bonds, bond, axes, axes. But the fact that this can conspiracy and the this can If I like this something like that, um, that this is this looks different than from what it had before like this. The thing is, if I did the same thing over here. Yes, we look exactly the same. So there's no much difference, would it? So nothing in this case that ethanol has is the winner. In this case, we have a slightly higher standard molar entropy off that for that of ethanol. Next, we have hydrogen chloride and hydrogen bromide. And so, um, in this case, I drew dietitians in. And, uh, because and see, structure is not it's not too big a factor in this case. Um, I don't think about it. I'm not sure why it would be if it was so. Anyways, we have hydrogen chlorine and hydrogen brahmi s. Oh, these are pretty much something, and only except for the Romeo and chlorine. And why is that significant? It's because the chlorine here it has a tough number of 17 and Roman has tough number of 35. And what exactly does that meet? So entropy takes into more account, not just the bonds or how they're arranged, or the phases that their interests are my moles. Where, but when it becomes down to the lake. Uh huh. At the very core of it or theme, the absolute measurement sent me. There's no other way to distinguish the two is by the amount for the different species at the different element. It isn't, um is because they are different amount of electrons for the chlorine atom in the broken Adam. Because this 35 in this is 17 Romaine has basically twice as many electrons to play with that. The electron cloud. It's just that much bigger. Therefore, the electrons big the they flow everywhere or around the broom. At least, um, there's just much bigger electron cloud. So that's a fake Drew the electron clever bro. Me, Let's say, like if this not a very good drying, but I hope he gets the idea across for chlorine. You'll be much smaller to be like this. Maybe, And if electrons can flow all all around here, it's just take. They can take a much more space than that of a Korean. They just go here in this case, so we just have more electrons here. In this case, we just have more electrons, and because of that, the hydrogen bromide is the went in the winter. This is is the winner of this case in this case, and so the the come out with hires standard molar entropy would be the hydrogen bromide