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Manhattan College

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Problem 44

Consider the following reaction:
$$2 \mathrm{CH}_{3} \mathrm{OH}(g) \longrightarrow 2 \mathrm{CH}_{4}(g)+\mathrm{O}_{2}(g) \quad \Delta H=+252.8 \mathrm{kJ}$$
(a) Is this reaction exothermic or endothermic? (b) Calculate the amount of heat transferred when 24.0 of $\mathrm{CH}_{3} \mathrm{OH}(g)$ is decomposed by this reaction at constant pressure. (c) For a given sample of $\mathrm{CH}_{3} \mathrm{OH},$ the enthalpy change during the reaction is 82.1 kJ. How many grams of methane gas are produced? (\mathbf{d} ) How many kilojoules of heatare released when 38.5 $\mathrm{g}$ of $\mathrm{CH}_{4}(g)$ reacts completely with $\mathrm{O}_{2}(g)$ to form $\mathrm{CH}_{3} \mathrm{OH}(g)$ at constant pressure?

a) Endothermic
b) 94.8 $\mathrm{kJ} \mathrm{CH} 3 \mathrm{OH}$
c)10.43 $\mathrm{g}$
d) $-303.36 \mathrm{k} \mathrm{J}$

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## Video Transcript

All right, guys, we're going to be doing problem forty four. In fact, they're five in chemistry, the central science. So we're gonna look at the following equation. We have it right here where methanol is being converted into methane and oxygen. Now, part a were asking if this reaction is except America ended. Since we have dealt ages, great zero if by definition and other, if there's a lesson here than accept. So we're going to calculate the amount of heat transferred when twenty four grams of methanol is composed by this reaction action. So we have to change this equation. We have tio act like we're using one one gram. Sorry. One mole of methanol produces one mole of methane Jane and half a mole of oxygen. Question This is going to be so that's going to change this, for it is from one point six point four killer jewels that one point six point four killed. Jules, her mole. So permal of methane we used This is how much energy we're producing. So if we do this, we have to convert this into malls twenty four or grams over us thirty two grams per mall. Oh, so that's going to give us your point seven five malls calls. Now we're going to multiply zero point seven five one twenty six point four Earth. And that's going to give though the most can flat, and that's going to give us ninety four point eight eight. Kill it, Jules. And in this case, hey, C'Mon aki transferred is equal to the to the the entropy cues. Q is equal to Delta h h. So that means that this is the value of our cue, our humanity that transferred now for section C, we're going to as they're looking, seeing the age for this transition. Forgive example of methanol is eighty two point one killer Jules. So how many grams of methane gas are produced? So first? So the way we figure this out is we know how many killers use aircrews for mole. So by using this, we can find out many balls we have so do eighty two point one one killer Jules over one twenty six point four villages for more. Well, so a key point. One divide by one twenty six point four. There's going to be zero point six five moles right now. We're going to convert that zero point six five moles in two grams. So we're going to multiply this by the molecular weight. Eight of our methanol. Sorry, our methane. So thirty, so sixteen grams per mall and that's going to give us ten points. Ten point four grams Ems of methane are produced. Give it They've given a sample that produces this much energy. Now for a party. The harmony killed jewels of heat are released when thirty point five grams of methane react simply with oh, two different form form seat form C, C for methanol. So we're actually going tohave to flip this with this equation because we're not using methanol to produce. There, you see age we're not using methadone procedure for and oxygen were used reacting methane oxygen to produce. You see, it's three now. Now to start off, we're going to have to flip that. So we have have negative one hundred twenty six point four killer just formal. So we're going to convert. No, now we're going to convert Earth our mass into moles confirmed. I think so. That's going to produce two point four for most. So then we're going to multiply and negative one twenty six explained four two point four. And we get atnegative. That's it. Negative. Three. This negative three hundred, you point four Killer. Jules LS. And since the remember Delta H sequel to Q. Yes, thank you. So we're going to. So we're releasing a magnet as so when we're talking about the key thing about amount of heat released. So we're dealing with it in terms of magnitude. So in this case that the age is going to be equal to negative. Q. The negative delta is going to be equal to Q. Because if if negative amount of heat is released, then that means that you're absorbing heat. So the magnitude of Q is going to be a three hundred and three point four our killer Jules. So you should think about the wording off the question when you're answering it, because that can affect the sign of a sign of your answer.