JefferyH
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Reactions in liquid ammonia? Thermal/Kinetic issues?
I was reading about various reactions in liquid ammonia and their benefits. Some studies refer to "reactions being conducted in liquid ammonia at room
temperature", others "liquid ammonia at reflux". All of these are notoriously low under -33 degrees. I don't see the purpose of saying, in liquid
ammonia at room temperature, because the liquid ammonia itself could not be at room temperature, at least, not without being under pressure.
Some of these reactions, can go to completion in this refluxing liquid ammonia in just (3-5 hours). When temperature is increased in reaction mediums,
the rate of reaction generally increases in an exponential fashion, not linearly.
Due to ammonia boiling above -33 degrees, will the substrates in solution behave as though they are actually that temperature in liquid ammonia, or
does the 'reflux' at -33 degrees modify the reaction somehow and make the substrates behave as though they are at a higher temperature?
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BromicAcid
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Interesting note, the boiling point of a solvent can be affected by the solutes dissolved in it. You mention liquid ammonia at room temperature, a
saturated solution of ammonium nitrate in ammonia can actually be brought to room temperature without boiling, but the amount of ammonium nitrate that
will dissolve in liquid ammonia... at that point it is more ammonium nitrate than ammonia and it might be a bit of a blaspheme to call it a solution.
None the less, it highlights my point that -33C might not be the hard and fast number you think it is.
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JefferyH
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Yes, but would these reactions 'exponentially faster' if brought to a higher temperature, such as under pressure room temperature? For instance, some
of these reactions I read about are inhibited by steric hindrance at -33 in refluxing ammonia. That steric hindrance could likely be overcome if the
temperature is raised, correct? I know it depends on a case-to-case basis, but in general?
Are these molecules behaving as if they are at -33 degrees, when in refluxing ammonia, or do they behave as if they are at much higher temperatures?
Usually reactions can't move forward at such a low temperature, so it would reason that if the temperature could be raised, the rates would be much
faster?
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DraconicAcid
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If it's refluxing in liquid ammonia, it will act as if it was at the temperature of -33 C. The fact that it's refluxing just means that it will be at
that temperature consistently. If you pressurized it so that it was at a higher temperature, the reaction would happen faster (often, the reaction
rate will double for every ten degrees increase).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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JefferyH
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Okay, thank you. Such a simple concept finally made clear . Just had to clear
that up.
By the way, when you say the reaction rate doubles every 10 degrees increase, is this compounded? Meaning, 60 degrees increase would be -> 2^6
increase?
[Edited on 16-5-2014 by JefferyH]
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DraconicAcid
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Quote: Originally posted by JefferyH  | Okay, thank you. Such a simple concept finally made clear. Just had to clear
that up.
By the way, when you say the reaction rate doubles every 10 degrees increase, is this compounded? Meaning, 60 degrees increase would be -> 2^6
increase? |
Yes, but that's just a rule of thumb. The exact rate increase will depend on the activation energy of the reaction, and some reactions will switch
from being favourable to unfavourable (or vice versa) at particular temperatures (this can apply to the desired reaction as well as to undesired,
competing reactions).
An example I give my students is the cooking of a turkey. Based on how long it takes a turkey to cook in the oven at one temperature and to deep-fry
at another, one can calculate the activation energy of the process. One could then extrapolate that a turkey will take 32 days to cook at 30
C.....but nobody wants to try cooking a turkey that way.
[Edited on 16-5-2014 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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