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Hydrazine, $\mathrm{N}_{2} \mathrm{H}_{4},$ decomposes according to the following reaction: $$3 \mathrm{N}_{2} \mathrm{H}_{4}(l) \longrightarrow 4 \mathrm{NH}_{3}(g)+\mathrm{N}_{2}(g)$$ (a) Given that the standard enthalpy of formation of hydrazine is $50.42 \mathrm{kJ} / \mathrm{mol},$ calculate $\Delta H^{\circ}$ for its decomposition. (b) Both hydrazine and ammonia burn in oxygen to produce $\mathrm{H}_{2} \mathrm{O}(l)$ and $\mathrm{N}_{2}(g) .$ Write balanced equations for each of these processes and calculate $\Delta H^{\circ}$ for each of them. On a mass basis (per kg), would hydrazine or ammonia be the better fuel?

$-9.01 \times 10^{4} k J / k g$

Chemistry 101

Chapter 6

Thermochemistry

Drexel University

University of Kentucky

Brown University

University of Toronto

Lectures

05:27

In chemistry, a chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Both reactants and products are involved in the chemical reactions.

06:42

In chemistry, energy is what is required to bring about a chemical reaction. The total energy of a system is the sum of the potential energy of its constituent particles and the kinetic energy of these particles. Chemical energy, also called bond energy, is the potential energy stored in the chemical bonds of a substance. Chemical energy is released when a bond is broken during chemical reactions.

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Hydrazine (NzH4) is synthe…

for this problem. We are given the key competent reaction of hydrogen hydrazine, and we want to solve two problems. First part, just put a asked us to find the delta h of the decomposition reaction, and we know that the decomposition reaction is that three moles of hydrogen do you compose into formals of ammonia and one more of next guests. So we know that the strategy and resolving this problem is going to be finding the heats information for the products that's attracting them from the heats. Information of the reactions so strategically you know that the heat information for an extra gas zero because it's an element. So the only thing that we need to do is find the heat information of ammonia and hydrazine and multiply them. But they're specified. Maury Shoes has given to us in the chemical equation, and we confined both the heat's information for ammonia as well as high treason. So we know that for one mole of ammonia produced that the heat of formation is negative. 45.9 Serial killer fools Ramon, and you know that for each more of hydrazine given to us in the problem that the heat. Information. The positive 50.42 killer jewels promote so that expression will give us a dealt age. Did the entire decomposition reaction. When you plug all of the values into your calculator, you will get that the final tell to each value is negative. 334 from eight killing joules per mole to that resolves, part eight Part B is a little longer. It asks us to write the balance equation for the process of both hydrazine and ammonia burning in oxygen, which is a combustion reaction. So the first will first do hydrazine, so we know without writing the balance equation. Yet there's one mole of hydrogen hydrazine plus oxygen, which produces nitrogen gas as well as water to the balance equation. So we know that in this case we have a balanced equation in terms of neck Shin, because there's two moles of naturally Adams up of sites hydrogen. You know that that's not balanced because there's four moles of hydrogen atoms on left side, while there's only two on the right side. So we're gonna ad two of this coefficient reflector that we can also see that this corrects for the oxygen as well, because now we have two and two instead of two, and one does that automatically balances out for us as well. So now we're supposed to find the Delta H for this reaction. And so just like part A, we're going to find the heats information for the products and subtract them from the heats. Information from the reactions. We know that for 02 and friend to those air zero. And they're not important to our calculations because they're elements, which means that we don't need to calculate. So we know we need to find the heat of formation for water and the heat information of hydrazine. You know, both those information the Delta Age. This reaction is two moles of water. The heat information for water is negative. 285 0.8 killed jewels from all the meaning that given that there's only one mole of hydrogen in the reactive side and we know also know that we get information is 50 point for two your jewels more. When you plug those into the calculator, you'll get that the final value is equal to negative 622.0 killer jewels promote. So now we're going to move on to ammonia. So similar combustion reaction. It's hydrazine. We're going to write the unbalanced chemical equation first. Her This is just me one h 20 So that is our unbalanced chemical equation. So the first thing that we want you is one of balance the next regions. So we see. But there's only one mole of nitrogen on the left side. But there's two moles of inaction, Adam, from the right side, such a crook that we're ready to there. Then we're going to have to balance the hydrogen. So given that we added to to hear if the coefficient of ammonia now we're going to have to balance the hydrant now they're six. On the other hand, on the right side, we only have two moles of hydrogen atoms. Such a work for that, we want six. We're gonna multiply this by three. The last element that needs to be balanced is oxygen. So on the left side we have three moles of oxygen. But on the other on the left side BF two moles of oxygen atoms. And on the right side we have three moles of oxygen atoms. So to correct that we're going to write a three opportune. But we also know that in chemical equations we cannot have fractions. That's coefficients truck. For this, we're gonna multiply everything by two. And this will yield us for their balanced equation of four moles of ammonia, three moles of oxygen, gas, two moles of nature gas and six moles of water. So just like last time, we want to find the guilty for this reaction. And it's almost a genital, except we need to change. The coefficients once again heats information for oxygen, nitrogen or zero, so we just do six times. But if 285 minus former's fighters and you will get the Delta H for this entire reaction will be negative. 1000 531 point to kill jewels promote you plug that in to your equation. So then the last part asked on a mass basis per kilogram with hydrazine or ammonia be the better fuel. So in this case, we want to find which fuel on a mass basis pneumonia or hydrazine will release less heat because that will make it safer and more efficient. We already know on a more bases. How much is released for both hydrazine as well as ammonia. But we want to change this in to a grams or mass basis. Let's first do hydrazine. So we know that for one mole of hydrazine that's being decomposed, that 622 killer jewels of heat are released. So we're just gonna 622 killer joules of heat are released, and that is based on one more of hydrazine. So now all we need to do is we're going to divide by the molar mass of hydrazine so that we can find how much heat is released on a per gram basis. We know that the mullahs Graham or the molar Mass of hydrazine is 28 grams. But the problem asks for it to be on the kilogram basis that we're gonna do one more step of dimensional analysis. When you plug all of that into the calculator, you will get that the answer will be negative 2.2 times 10 to the four killer jewels per kilogram of hydro team. So that's what that's our number for hydrazine. Now we're gonna do the same thing for ammonia. You know that for ammonia, there is 1531 points to kill a jewels of heat released when there are four moles of ammonia. So we're to write foremost pneumonia. And then once again, we're gonna multiply or divide by the molar mass of ammonia so that we can get our answer. And the more math ammonium 17 grams convert that two kilograms. And the final answer that we get is negative. Nine point. 01 times 10 to before chilled jewels. Uh, killer jewels per kilogram, l ammonia. So immediately Right off the bat, we can see that ammonia releases more heat almost four times as much heat per kilogram of hydrogen. So from this, we can see that because hydrazine, um, it's less heat per kilogram. That hydrazine is the better fuel. So that is the conclusion to a long part.

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