[apteryx]

Hey everyone, just found this site, seems very informative! I've got a practice question from P. Chem (although it does seem like more of an analytical chemistry question.) Here it goes:

Consider a chemical reaction where A and B react to form a product C in the form
3A+B�reaction equilibrium double arrows go here)2C
where the double arrow indicates a chemical equilibrium between the reactants and the product.
Write down the reaction quotient for this reaction. How do you calculate the equilibrium
constant for this reaction from tabulated thermodynamic data? Could absolutely pure C be
stored in some container? Give a reason for your answer.

Figuring out the reaction quotient and equilibrium constant is simple enough, but pure C being able to be stored in some container? I'm just not 100% on this. Is this saying that since in equilibrium, there will be some amount of A and B in the container? Maybe entropy decreasing on the product side? I obviously have no clue.

[Borek]

Is this saying that since in equilibrium, there will be some amount of A and B in the container?

Yes.

[apteryx]

Cool, thanks!

[Yggdrasil]

It would depend on the kinetics of the reaction.  If the reaction proceeds extremely slowly at the storage temperature, you would not be wrong to assume negligible amounts of A and B are present.  For example, you could say that there is an equilibrium between diamond and graphite (favoring graphite) because these two allotropes of carbon can interconvert.  However, if you store a piece of diamond for a long time, you won't see any appreciable contamination with graphite.

[Borek]

It would depend on the kinetics of the reaction.

Good point, at the same time it is pure semantics - negligible amounts vs absolutely pure

[apteryx]

Well, I went and asked my professor for some clarity on the question, and he said for the "Reasoning" it asks for, to show with a thermodynamic property what would happen (ie with delta G). Somehow it made sense when he told me, but now I'm even more confused. He said something to the effect of 'if delta G was infinitely positive, the reaction would move in the left direction.' So for reasoning, could I just give a value of delta G that would result in the reaction going all the way to the right (inifintely negative G?) Totally confused now...I'll be reading my textbook to try to come to an epiphany, but if anyone would care to enlighten me along the way, I'm all for it. I may throw another qualitative question up here if I can't solve it in the next hour or so, so stay tuned.

EDIT: I'm just going to think out loud (and by out loud I mean on this thread). If Q = reaction quotient, then when Q is equal to 0 (zero meaning there is no product and only reactant), delta G should be equal to negative infinity, correct? So then, if there was all product and no reactant ( or at least as the reactant goes to zero), Q should go to infinity, and thusly delta G would go to infinity, right?

[Yggdrasil]

Yes.  If you wanted a reaction that yielded 100% C and 0%A and B at equilibrium, the ΔG would have to be infinitely negative.  Because you can't have an infinitely negative ΔG, it is not possible for a system at equilibrium to be absolutely pure.  There will always be some reactant and some product.

[renge ishyo]

So for reasoning, could I just give a value of delta G that would result in the reaction going all the way to the right (inifintely negative G?)

If we follow your professors reasoning then an infinitely negative delta G would be a situation that one might think would produce "absolutely pure" product C and leave nothing left over of A and B. However, I am unsatisfied with that answer for the following reasons. Yggs beat me to it, but I will elaborate with a few more details for you:

1. Division by zero is mathematically undefined. Therefore, for any defined value of delta G it is impossible to make A and/or B disappear completely because the reaction quotient for such a situation is not defined. One can also argue that no matter how negative the delta G becomes there will always be some small (non-zero amount) of A and B present. After all you cannot prove that all of A and B will be gone when infinity is reached because the function is not defined for infinity (so in other words, who is to say what will happen when that is reached?).

2. Experimentally, there is no known process in chemistry that produces "absolutely pure" anything. It doesn't mean that it doesn't exist, but that we would have no way of detecting it because there would be no change for us to observe. With no known examples of absolutely pure chemical substances it is hard to argue that such a substance can exist.

My take anyways  

[apteryx]

Yeah, I was just trying to apply a limit to the denominator for theoretical purposes. So quantitatively, there's no way you can actually have pure C. I would probably say the first reason renge ishyo posted if this question ends up being on the test tomorrow, but if I'm feeling particularly like a know-it-all that day, maybe I'll throw in the second reason too. I can't tell if that's a thing a professor would be impressed with, or if it would be "this kid has no idea what he's talking about; this isn't relevant. Fail."

Thanks a lot guys, it's been a really big help. I wish I found this forum when I was doing quantum at the beginning of the semester, lol. I'll be sure to stick around from now on though.

[apteryx]

Of course, a form of this question didn't show up on the exam after all the thought and time I put into it. Oh well, hopefully it'll be useful some other time in my life.

[renge ishyo]

know-it-all that day, maybe I'll throw in the second reason too. I can't tell if that's a thing a professor would be impressed with, or if it would be "this kid has no idea what he's talking about; this isn't relevant. Fail."

It might seem strange to mention the lack of experimental evidence of a "pure C" as an answer to the question, but this is hardly "know it all" thinking and it probably is the better answer (the math doesn't describe the situation directly after all). The reason why experimental evidence is so important is because nobody can "see" what happens at the molecular level directly, so you need experimental evidence to distinguish among the various theories. Without experimental evidence, there is no way to separate one theory from another, and the "truth" becomes a matter of authority only.

I am happy you weren't asked this question on your test though