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Kinetic vs thermodynamics control

  1. Feb 10, 2016 #1
    So I understand that thermodynamics of a chemical reaction is the change in Gibbs free energy and that kinetics mainly deal with the activation energy (or production of a high energy intermediate). My question comes from something that my organic chemistry professor stated in class. He said the thermodynamically controlled reactions were reversible and kinetic controlled reactions are irreversible. Would anyone have any insight on this statement or where I can look to find any info about it.
     
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  3. Feb 10, 2016 #2

    Bystander

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    He's "zero for two," given that you've not misquoted him.
     
  4. Feb 13, 2016 #3
    He did say that he doesn't use microwave ovens because it's a type of radiation (not a misquote). I don't believe that I misquoted him because I talked to my study group about this (they were in the lecture as well) and they had no idea either.
    One thing that I can think of is that when a reaction is under thermodynamic control it can establish an equilibrium because there is enough energy in the system to overcome the activation energy going both ways in the reaction(product to reactant and reactant to product).
    Please correct me if I am not correct on that statement!!
    Also under kinetic control there isn't sufficient energy to overcome the activation energy going form product to reactants and therefore an equilibrium isn't established?
    Maybe this is what he was meaning by reversible/ irreversible.
    So have I interpreted these types of controls (thermodynamic/ kinetic) correctly?
     
  5. Feb 13, 2016 #4

    Bystander

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    "Microwave ovens?" Photolysis, yes; there're some reactions that can be induced to proceed in higher yields with specific wavelengths of higher energy radiation (well into/beyond visible) than just general thermal excitation (the microwave). But the "kinetic/thermodynamic" arguments are nonsense.
     
  6. Feb 13, 2016 #5

    Ygggdrasil

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    The explanation sounds okay to me. Here's another nice explanation of thermodynamic vs kinetic control of reactions: http://www.masterorganicchemistry.com/2012/02/09/can-opener-economics/

    Is it possible for a reaction to be under kinetic control if the reaction is occurring under reversible conditions? How can you reach the thermodynamic product if the reaction is occurring under irreversible conditions?
     
  7. Feb 13, 2016 #6

    Bystander

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    H-O fuel cell vs. H-O explosion.
    See last: first is reversible; second is not.
     
  8. Feb 13, 2016 #7
    What does it mean for a reaction to be under irreversible conditions?


    When you change the temperature, you can change the direction of the reaction. Like ice and liquid water, though that's not a chemical reaction, the idea is the same. Think heat packs

    With kinetic control, you only control the rate of a reaction, not the direction. Like adding an impurity to pure water just below 0 C to initiate nucleation.
    A reaction that goes extremely slow to the point that it is approximately an equilibrium, though there is a thermodynamic driving force, you can speed up and make viable/happen at working day time scales.

    Important to remember is that kinetics are about rate of reactions i.e. speed. Thermodynamics is about direction in time. Either A goes to B or B goes to A. Where is the equilibrium, where is the Gibbs energy of the reaction zero? At what ratio/concentrations?

    You can have an extremely fast reaction that does not happen because it already went from B all the way to A, and the other direction is unfavoured/doesn't produce net entropy. It already found the concentrations of A and B where the Gibbs energy is minimized. Think acid-base reactions. Those proton shifts are fast, near diffusion speed. But they go one direction, not the other, even though the reaction going in either way would happen at diffusion speed rates. Change concentrations and it will be fast, in the reverse direction. Think acid-base solutions with ph indicators where you add acid or base, go through a pH gradient and see all colour changes instantly.

    You can have an extremely slow reaction that produces tons of energy. Like a bunch of TNT just sitting there, not decomposing and delivering that sweet entropy. Or diamond turning into graphite.

    So kinetics and thermodynamics are completely orthogonal.


    But both are the same reaction. In one you do work on the system, in the other you waste all the work you could be doing by making it all go into heat.


    Every reaction is reversible. Some just mean you need to go to very extreme concentrations or temperatures/pressures.
     
    Last edited: Feb 13, 2016
  9. Feb 13, 2016 #8
    I have a sneaking suspicion that my prof sometimes spews b.s.. This is why I mentioned the microwave thing. However, I confused the statement. He was saying to not use microwave ovens on your food because it was unsafe to the consumer of that food. From what I understand and what the evidence backs up is that microwave ovens are safe to use on food and if made properly are safe to use.
     
  10. Feb 13, 2016 #9

    Ygggdrasil

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    Both give the same product, so the kinetic product and thermodynamic product are the same.

    Anyway, talking about reactions being under kinetic/thermodynamic control and reactions being reversible/irreversible probably oversimplifies things, but this is the language organic chemists use to talk about competing reaction pathways such as the one discussed in the link I posted earlier.
     
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