I am wondering about chemical reactions when the reactants are in a plasma state. Consider hydrogen and oxygen. Heated, these would react to form water.

If hydrogen and oxygen plasma were combined, would the separation of electrons from nuclei prevent normal chemical reactions? Or would the reaction take place in the plasma state?

  • $\begingroup$ Of course it prevents normal reactions. In temperatures like 10^5 K there are no molecules. $\endgroup$
    – Mithoron
    May 3 '17 at 18:57
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    $\begingroup$ Most plasmas are not fully ionized. In fact, many have very low levels of ionization (maybe 1% or so). However, ionizing atoms or molecules, and splitting molecules, certainly makes it easier to achieve a number of chemical reactions (and break up things too!). What, really, is your question? $\endgroup$
    – Jon Custer
    May 3 '17 at 19:15
  • $\begingroup$ The broader question is about chemical reactions with reactants in plasma state. Combining hot oxygen and hot hydrogen would produce water. Would combining these two in plasma form also produce water, or would the plasmas mix and not react. Assume 100% ionization of these plasmas. That is really my question. $\endgroup$
    – Willk
    May 3 '17 at 19:57

Yes, chemical reactions do take place in high-temperature plasmas. In fact, these reactions present some problems in the analytical technique known as Inductively Coupled Plasma - Mass Spectroscopy (ICP-MS), which typically uses an argon plasma.

As noted in the comments, the conditions within a plasma are very different from those in a laboratory at STP, for example. But that just means that the chemical reactions that are observed are different, not that they don't exist. In ICP-MS for example, one is trying to detect the masses of very low concentrations of various elements in the presence of relatively huge quantities of argon, oxygen and hydrogen (the latter assuming that an aqueous sample is being analyzed). Species that are formed within the plasma include $\ce{Ar2+}$, $\ce{ArO+}$ and more. When one of these species has the same mass as the trace isotope of interest, it only takes a very small percentage of these products to interfere with the analysis.

This example of argon plasma can largely be extended to your specific question "If hydrogen and oxygen plasma were combined, would the separation of electrons from nuclei prevent normal chemical reactions?". Depending on the conditions of the plasma (temperature, concentrations, degree of ionization, etc.) the answer is that the reaction would only form some small degree of transient water molecules, as compared to say molecular hydrogen and oxygen gas at high temperatures or in the presence of a catalyst, which would overwhelmingly form water. In other words, the former (plasma state oxygen and hydrogen) would form trace amounts of water, and probably hydrogen peroxide and other oxygen-hydrogen species, where under "normal" conditions the formation of water would be strongly favored (thermodynamically anyway). It's just the nature and definition of a plasma that whatever species you have in your plasma will want to be broken into it's elemental constituents and the formation of molecules will be strongly unfavored.

The conditions within a plasma are very different from conditions typically encountered in a non-plasma environment, and thus the chemical reactions are different. The formation of otherwise exotic species like $\ce{Ar2+}$ are relatively commonplace within an argon plasma for example ("common" being a relative term of course, as concentrations of $\ce{Ar2+}$ may still be relatively low, just much higher than would be seen outside of the conditions present in the plasma). Plasma conditions themselves can also vary widely, from conditions where only a small portion of the gas molecules are charged to conditions where the majority are charged. Regardless, the high energy state of a plasma tends to result in the destruction of molecules to their atomic precursors, but occasionally there are reactions to form products that would not be seen outside of a plasma.

  • $\begingroup$ That's nice answer, not what OP was expecting though. $\endgroup$
    – Mithoron
    May 3 '17 at 21:02
  • $\begingroup$ Yea, I left out a specif part of the OP question. I made an edit to address this, though it looks kind of awkward, but I think it more directly addresses the OP anyway. As usual, thx for the input ;) $\endgroup$
    – airhuff
    May 3 '17 at 21:45
  • $\begingroup$ An excellent answer, with the general and the specific. Thank you! $\endgroup$
    – Willk
    May 4 '17 at 1:01
  • $\begingroup$ Glad it was helpful :) $\endgroup$
    – airhuff
    May 4 '17 at 1:04
  • $\begingroup$ How do you form species like Ar2 and ArO if you're in an plasma? Doesn't being in a plasma imply that everything is ionized--in which case how would you form the covalent or ionic bonds necessary for molecules? Or is your example with the ICP-MS just a case where you have a distribution of atoms, some of which happen to be below the ionization point which allows these species to form in small amounts? $\endgroup$
    – spacetyper
    Dec 6 '19 at 1:05

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