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Hydrogen, in the presence of oxygen, will combust when exposed to a

  1. Sep 2, 2012 #1
    Hydrogen, in the presence of oxygen, will combust when exposed to a heat source and form water. At least this is what I have gathered from some googling. But I haven't been given a satisfactory, detailed explanation as to why this occurs. Would anyone here be willing to describe this process to me? Also, would it be possible to combine H and O to form water without an explosion? Thank you.
  2. jcsd
  3. Sep 2, 2012 #2

    Simon Bridge

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    Re: Hydrogen/Combustion

    Define "satisfactory"?

    Can you provide an example, and what it is about the explanation that you found "unsatisfactory". That way we can avoid reinventing the wheel.

    You can use an intermediate process.
    eg you can combine the H with C first and then burn the result in oxygen gas.
    You can combine the H with Cl to make HCl and the O with Ca and C to make CaCO3 then react them in solution to get more water, and some other stuff.
    You can react very small quantities.
    The energy output from the reaction is a characteristic of the reaction.
    Last edited: Sep 2, 2012
  4. Sep 2, 2012 #3
    Re: Hydrogen/Combustion


    I would like some more detail about the following, from the above link:

    "Hydrogen molecules and Oxygen molecules have a much lower energy state when combined to make the molecule H2O (water) than the total energy when they are separate molecules of hydrogen (H2) and Oxygen (O2). There is plenty of Oxygen in the atmosphere here on Earth, so if you have a source of Hydrogen and you add some energy, like with a flame, they will react (violently) to make water and give off heat.

    Other molecules might not do this --- If you had nitrogen and a flame, you wouldn't get an exothermic reaction to make NO2 or any of the other oxides of Nitrogen. If you had helium or argon or any of the other 'noble' gases, you would never get a reaction at all.

    Hydrogen is very reactive because it gives up its electron so easily, like all the elements on the left column of the periodic table."
  5. Sep 2, 2012 #4

    Simon Bridge

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    Re: Hydrogen/Combustion

    Where are there missing details - it looks all there to me?
    Are you after the quantum mechanics?
  6. Sep 2, 2012 #5
    Re: Hydrogen/Combustion

    I am hoping for a detailed elaboration of the 'energy state' concept as it applies here. It says here that water exists at a lower energy state than separate H and O atoms. I understand that concept - but how does this process take place? What prevents H and O from spontaneously forming water, since a lower energy state is where they would 'like' to be? And why does adding energy in the form of a spark or flame make this reaction happen? In short, yes, the quantum mechanics.
  7. Sep 2, 2012 #6

    Simon Bridge

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    Re: Hydrogen/Combustion

    OK - I'll try to come short of the quantum for now and just explore the concept a bit.

    The H2 and O2 nuclei form potentials that their electrons roll about in like balls rolling back and forth in a valley or a bowl. The walls are too high for the balls to get out. The electrons being shared between the common nuclear well is what holds the thing together.

    So you have these two valleys, with a high ridge between them.

    In order to get the bonds to switch - you have to lower that ridge or give the electrons more energy to get over it.

    You can lower the ridge by pushing the valleys closer together ... but H2 and O2 are neutral - there's no special reason for them to do that. I imagine if you put them under a LOT of pressure you'd get the reaction without the initial flame.

    So, instead, we heat them up - electrons leave the valleys. The heat also makes the molecules vibrate a lot - earthquake! The molecules break apart. Now you have a bunch of ions and free electrons as well as O2 and H2 and just normal neutral O and H.

    Start to get the idea?

    The combination with the deepest well has the lowest energy, and attracts the most electrons, and holds them the tightest.
    Interactions go in the direction of the steepest slope.
  8. Sep 2, 2012 #7


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    Re: Hydrogen/Combustion

    No problem.

    First, there is a hydrogen burner, where hydrogen burns mixed with oxygen.

    Then, even if you have a mixture of hydrogen and oxygen, you can combine them using a catalyst (I believe platinum black works) - they combine on the surface without explosion and water drops from the catalyst. Trick is, catalyst is getting hot fast, so you need to find a way to control the temperature to avoid explosion, but it should be perfectly doable.

    Note lead batteries used in cars these days often contain some kind of catalyst that turns hydrogen evolving during charging into water - but there is not much hydrogen, so there is no problem with heat. Plus, some kinds of devices that combine hydrogen with oxygen (without explosion) are used in nuclear reactor buildings.
  9. Sep 3, 2012 #8
    Re: Hydrogen/Combustion

    Thanks a lot guys. Great explanations here, I really appreciate it.
  10. Sep 4, 2012 #9
    Re: Hydrogen/Combustion

    The reaction between hydrogen and oxygen is a very complicated reaction system known as a branched radical chain system.

    It starts with a free radical from the environment -- usually •OH

    We then have

    •OH + H2 --> H2O + H•
    H• + O2 --> •OH + O

    the free radicals, marked with •, are very reactive chemical species that will undergo reaction on nearly every collision. Usually if a free radical is a reactant in a reaction step like the two immediately above, then another free radical is formed as a product, and the reaction carries on as a normal chain reaction.

    But oxygen reactions are different.

    H• + O2 + M --> HO2• + M

    and the peroxy radical HO2• , formed in a 3-body collision, is much less reactive than the other free radicals in this particular system. BUT

    O + H2 --> •OH + •H

    is another reaction that also happens readily, and this step creates new free radicals. This reaction step makes the possibility of the number of very reactive free radicals increasing very rapidly, and the reaction can rapidly build up speed and intensity (a very similar process to the multiplication of neutrons in a nuclear explosion), leading to a radical explosion.

    Whether or not an explosion occurs depends on the relative weights of these last two steps in the mechanism, and on the possibilities for removal of free radicals by vessel walls or other surfaces, or slower deactivation steps with the peroxy radicals.

    The very high temperature of a hydrogen flame brings nitrogen from the air into the action:

    N2 + O2 <==> 2 NO

    Nitric oxide is actually a very unreactive free radical, but it will react with the peroxy radical to bring it back into the action:

    NO + HO2• --> NO2 + •OH

    Reactions with oxygen always suffer these sorts of complications, and the possibility of explosion of any fuel in an oxygen/air fire is always a very real one. The detail of the hydrogen combustion is better understood than most others.
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