This is a good question. I believe there are a
few reasons why we don't see this reaction occur
in nature. First of all, most of the hydrogen in
nature is already bound to other chemicals (either
in water or something else). We don't see much
free H2 because it is light enough to
escape the Earth's gravitational pull.
Furthermore, to get free H2 from the
compounds where H's are "embedded" would require
some kind of activation energy, which, depending
on which compound it is coming from, could be high
enough to make the overall set of reactions
Now, say we did have free H2 and
O2 that we could combine in a chamber.
It still would not react without the necessary
activation energy (e.g. from lightning). I think a
point of confusion for many of us as we learn
chemistry is that just because a reaction is
spontaneous doesn't mean that it will happen.
Spontaneity corresponds to a -∆G(change in Gibbs
Free Energy) for the reaction.
However, there are reactions with negative ∆G that
would take the age of the universe to occur (not
our case, but others), because the activation
energy required for the reaction to occur is
simply too high. I hope this helps.
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