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Activation energy for chemical reaction

  1. Aug 28, 2016 #1
    Only particles that possess enough energy to overcome the activation energy and orientated correctly will collide and form products. How about those particles which do not possess enough kinetic energy to overcome the activation barrier? Does it mean that for a particular chemical reaction where 1 mol of a reactant is needed, the actual amount of reactant that reacts is less than 1 mol?

    I
     
  2. jcsd
  3. Aug 28, 2016 #2

    Bystander

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    You've compared "apples to oranges:" can you rephrase the question in a way that it answers itself?
     
  4. Aug 28, 2016 #3

    Borek

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    You are aware of the fact molecules are in constant motion, they rotate and collide all the time, exchanging energy?
     
  5. Aug 28, 2016 #4
    What do you mean with " [...] where 1 mol of a reactant is needed"? Needed for what?


    Obviously, one mole of reactant doesn't react instantaneously. But that's not your question, right?
     
  6. Aug 29, 2016 #5
    I just started reading about activation energy today, but from what I gather, once the initial energy is supplied to kick-start the reaction, then the energy released (let us assume that the reaction is exothermic) is enough to activate the other molecules and finish the reaction.


    EDIT: On second thought, it seems to me that some energy must be supplied externally in order to sustain the reaction?

    Example: Suppose we wish to combine hydrogen molecules with oxygen molecules to form water. We raise the temperature enough so that the activation energy is reached and at least one molecule of hydrogen has combined with a molecule of oxygen. This is an exothermic reaction, so let us now remove our external heat source. Surely the heat released from that first combination of hydrogen and oxygen isn't enough to sustain the reaction until all of the hydrogen and oxygen combines? Am I right when I say this? Intuitively, it seems to me that we must continue to supply heat in order to force the reaction to continue.
     
  7. Aug 30, 2016 #6

    Borek

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    IMHO you are missing the fact molecules exchange energy, and their energies are not constant. That in turn means in a sample some molecules have higher energy, and some molecules have lower energy. As long as the temperature is kept constant fraction of the molecules that have higher energy is constant as well, so you can't just consume in the reaction all these molecules with higher energies and be left with those of lower energies.

    https://en.wikipedia.org/wiki/Maxwell–Boltzmann_distribution
     
  8. Aug 30, 2016 #7
    At a certain temperature, a certain ratio of molecules will have sufficient energy. If they are 'removed' somehow (let's say they reacted) but the temperature is magically held constant, the system must still have the same ratio of molecules with sufficient energy. So it will rearrange to still have the same distribution (Boltzmann).

    Changing this distribution means changing the temperature.
     
  9. Sep 7, 2016 #8

    Tom.G

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    It's called an Oxy-Hydrogen torch; it has a specialized use in the Semiconductor manufacturing industry. Just like the burner of a gas stove, a blow torch, or a welding torch, once the reaction is started you can remove the ignition source and the reaction (flame/burning) continues. (Works with wooden houses too.)
     
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