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Calculate the standard molar enthalpy of formation of $\mathrm{NO}(g)$ from the following data:$$\begin{array}{ll}{\mathrm{N}_{2}(g)+2 \mathrm{O}_{2} \longrightarrow 2 \mathrm{NO}_{2}(g)} & {\Delta H_{298}^{\mathrm{o}}=66.4 \mathrm{kJ}} \\ {2 \mathrm{NO}(g)+\mathrm{O}_{2} \longrightarrow 2 \mathrm{NO}_{2}(g)} & {\Delta H_{298}^{\circ}=-114.1 \mathrm{kJ}}\end{array}$$

$\Delta_{\mathrm{r}} H^{\mathrm{o}}=90.25 \frac{\mathrm{k} \mathrm{J}}{\mathrm{mol}}$

Chemistry 101

Chapter 5

Thermochemistry

University of Central Florida

Drexel University

University of Toronto

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So today we're trying to calculate the entropy of formation of end of gas, so it's important to know what the definition of entropy information is. But we are going to be creating one mole of products with N O, and it's gonna be comprised of its individual elements. So it's comprised of nitrogen, which is naturally found in the world, and to and oh to which is how oxygen is normally found. Now the interesting thing is with Hess's law and will be a formation were only producing one mole. And that means we are gonna actually end up with a fractional coefficient in front of each of our reacted. It's This might seem strange, but it is kind of just the convention that we use so that we can scale things. So if we're trying to create three moles of Eno gas, we can multiply this by three or any number etcetera. So we're gonna use these two equations that we've been doing in the problem to find the entropy. For this reactions of Delta H is what we don't know. My first step is to see that we need to have Eno end up in our products and we can see that N o really only shows up in the second equation. And because it's in the reactions of this first equation, we're gonna have to flip it. We can also see that the ah, the coefficient in front of N O is to in order to get to our one and we're gonna have to divide everything by two. So first I'm gonna flip it So it would become too ano too. Becomes too, eh? No. Let's go to when I flip it. We know that Delta H should chain signs it becomes positive 114.1 Killer Jules. And then I need to divide everything by two. So if I divide everything by two, this becomes one and 02 goes it N o plus 1/2 +02 If I'm all divide everything by two. I didn't do the same thing for my ah, for my entropy. And so that's gonna become 57.5 Killer Jules. So we've modified our first reaction bread and ran. Now let's think about this green reaction on top. How can we use this to help us get to our final reaction here and so we can see that in this reaction, we're gonna have to There is no n 02 some things we have to get rid of n 02 and the only way to get rid of him too. For this red reaction here is have one and no two in our re our product side. So if I look at my green reaction to get one No. Two in the product sign, we just have divide everything by two because no two already ends up in the right spot. So if I divide everything by two that's living 1/2 and two Quest 102 becomes one and two Again, Thio, get our new Delta h of this reaction. We just divide everything by 1/2. So that becomes 33 0.2 Killer Jules. So this is our new equation. And so we can see if we now add these equations together, are, you know, to they're gonna cancel out. So attitudes and our product attitudes are reacting. We're gonna end up having 1/2 02 in our product signed and 102 r reactant. So this is gonna end up canceling out, and this is gonna become 1/2. And so therefore, final equation to be 1/2 and two, That's 1/2 02 he comes. And oh, which, as you can see, is our reaction that we're looking for us. So now we're just gonna look for adults. H because we added these two equations to get to this blue equation. We just add their end will be together. And so, in order to get that, we're gonna be we're gonna get 90 point to buy. Killer Jules, what about house?

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