Because I'm ALSO doing the math for Hydrazine + N2O4 combustion... and getting a different-than-published-ratings result (15% too low).
The only "published figure" you've cited is the 19.5 MJ/kg figure that is the
standard heat of combustion. That's for the reaction between hydrazine and
atmospheric oxygen under
standard conditions (298 K and one atmosphere of pressure). The reaction between hydrazine and nitrogen tetroxide is expected to have a different change in enthalpy because the oxidizer is not free oxygen—it's an oxygen-bound compound.
I was puzzling over some of the questions you asked a few days ago—not about the answers, but why you would be asking them. As we previously explored, this is often the hardest part of teaching. You asked a question to the effect of whether it was possible to compute the heat outputs of reactions rather than just looking them up. Well, yes, that's what thermodynamics is largely concerned with, and most technical people know this. And I was initially baffled by the question I answered briefly last night. Now it has finally fallen into place. You're just now starting to grapple with the extremely basic principle that different reactions among different reactants produce different changes in enthalpy!
I'm guessing you've never studied thermodynamics.It's hard to express just what a fundamentally wrong misconception you seem to be laboring under. This is worse than looking into the big end of the telescope or pouring oil in the car radiator. You're not just dusting off 30-year-old knowledge. This is knowledge you evidently never had and certainly aren't learning very well now. I was expecting it to be hard slogging to get through a discussion of non-standard conditions and how to adjust for them as we'll have to do in our problem, but this is a true facepalm moment.
It doesn't matter how long it's been since you took the class. There is literally
no way a student who sat through even just the first week of introductory thermodynamics would think that there is some pat number you can just look up in a book and that this will be the change in enthalpy that works in all cases or after you change one of the reactants. That extremely broken expectation is the piece that finally fell into place.
No wonder you want to speed past this part of the examination—it's way over your head. No wonder you didn't feel like exploring the questions I asked you regarding how we adjust our values for non-standard conditions and what our next steps of analysis ought to be—you don't appear to know enough about the field even to realize what the ramifications of any of those questions would be.
No, you can't discuss the performance of a thermodynamic engine without a thermodynamics analysis. No, you can't just look up numbers in some "industry standard" for every question and avoid having to do hard work. No, we're not going to skip the details and just fly by the problem at some high level that you can easily sidestep (as you've done every other time).
Speeding up is definitely not the order of the day. In fact, we're going to have to slow
way down and bring you up to speed from about 2nd-year high school chemistry. And no, I have no plans to spend the holiday season as a remedial tutor, so this might be my post on the subject until after the holidays. I just can't even...
