Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: czmtzyddl on September 18, 2024, 12:19:27 AM
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There is an equilibrium in the reaction 2NO2 ::equil:: N2O4.
Consider a constant-pressure system where such reaction has reached its equilibrium. How will the color change when adding extra NO2 to the system? And why?
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Well, from my perspective, when adding extra NO2 to the system, the concentration of NO2 increases. Therefore the color goes darker. However, according to Le Chatelier's principle, part of the NO2 should turn into N2O4 and the color should turn lighter correspondingly. And that’s the problem, the extent to which the color changes.
My friends and I both agree that the color should be darker than the origin, while our teacher believes the color should stay the same. Since I don’t have the equipment to conduct the experiment, I tried to calculate the Δc during the process. It seems to be so complicated that it confuses me even more.
So what’s really happening in the system? How should I think about a problem like this the next time? I hope the answer could include some fundamental logics of chemistry rather than some conclusions I don’t even know where they are derived from. And this is also why I posted this high school chemistry problem here. Thanks! :)
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There is a very important hint in the problem: system is kept at the constant pressure.
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Hmm. So does it mean that although the reaction will consume more NO2, the volume shall shrink at the same time, causing the reaction to keep on going till the concentration turns back to the origin?
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Or is there an appropriate mathematical formula to explain this problem?
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Hmm. So does it mean that although the reaction will consume more NO2, the volume shall shrink at the same time, causing the reaction to keep on going till the concentration turns back to the origin?
Exactly.
And yes, there is a formula - just for the equilibrium constant. In this particular case (one reagent, one product, it doesn't have to be identical for other cases) no matter what the initial composition of the mixture is (you can start with just NO2, just N2O4, or anything in between*) once the system gets to equilibrium partial pressures of both gases will have the same value - which also means identical concentrations and identical absorption of light (or the same color).
*technically preparing things like "pure NO2" or "pure N2O4" is impossible, as they immediately start to interconvert.