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### Find $\Delta S_{\text { rxn }}^{\circ}$ for the c…

07:04
University of Massachusetts - Lowell

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Problem 35

Find $\Delta S_{\mathrm{rxn}}^{\circ}$ for the combustion of ethane $\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$ to carbon dioxide and gaseous water. Is the sign of $\Delta S_{\mathrm{rxn}}^{\circ}$ as expected?

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## Video Transcript

So in problem 35 work has to find the change in tribute for the combustion a thing and to determine whether if the sign of that changing entropy is expected me So for the combustion of engine event, they have nothing. C two h six. We have this reaction here or this equation here on and things Is that what you made for May not know eyes were you have to remember is that when you have combustion reaction, you have the reactive is always gonna be oxygen. Oxygen is always a reactive oxygen and your fuel in this case that a C two h six and in the problem states that it rahm reacts to reduce carbon die outside in gashes water, which is typical of a collection reaction and were asked to determine if the signs expected. So what is the sign that we do expect? Let's determine that now. Well, the thing is, we can't really determine that now because you don't have them about the moles of their respective mole self. Whatever we're working with. So it's first of balance this reaction, OK? And so if you balance this, we have two moles of C two h six or nothing and seven moles. Auction informal is the common at state and six miles off water. So we don't notice is that if you have, you will risk for personal will be. You have just all gases here. It's all the same state, so that's pretty simple. It worked with much easier to work with. So let's just compare where do we have more gases? Because more gases means more interesting, more randomness. So we have nine most gas here and we have 10 mostly gas here. Doesn't really matter that this is a higher Miller Messan and anything is the highest, more massive all of them. But the fact that they're more most gas here is just it are drinks that in terms of how much, um, how much weight that entropy ends. So in this case, we have increasing bench if we go from nine mostly gas to 10 miles against. So this we expect that our calculation when yield a positive delta s reaction. OK, so let's just keep that in mind here, okay? And that right there, or that's circle that I want to confuse you guys. So we have it. We are expected sign is positive. Okay, Now let's actually calculate that. So we have here Delta as a reaction. That's just what I'm, uh, touches what I have. And replace of the symbol is six before I I continue, but it's very important is that if you when you're running this out, you have to specify the states because, as, uh, just mentioned, um, different states have different entry loves, even for the same element or ST compound. So we have to specify the state here. Okay, So if we're in order to determine the don't ask every, actually we simply just take the Summit's products and subtract the adult s values of that of the react. It's so we have here the Delta s values of the, uh, other products. I'm not running the actual values here. It kind of varies from here there. And it's possible that that I'm just on my calculations. Also, just that's just what happens. So we have just labelling the values here or have these as placeholders. There's cause or six waters. There's for coming accents. There's six values off the entropy values here and for our react. Inside, we have seven oxygen's and two moles of ethics. Thus seven values off Ventria, intraday of oxygen and seven in two values of the entropy of Fade where we do our classic products minus reacted when you should end up or release of what I ended up. This is 93.12 Jules per moles, Kelvin. And as we note, this is also positive and so always our our prediction. Our prediction is positive. That is correct. And what I also want to note here is that the units here really matter. I know there's a lot of people hate like I don't like putting units. It's kind of annoying, but here it's very important because here I have jewels and all the other during dynamic values and the are measured in killer jewels. So when you you dio there with Mitt, take a math with, um, you working with them and working with intricate you're working with and will be working with, um whatever and and Gibbs free energy that is always in killed Jules. But here, entreaties is always in jewels, not just something to note here