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I'm wondering what happens when $\ce{H2}$ leaks from say a transmission pipe in an unenclosed area. No immediate source of ignition, I know it rises more quickly than helium (x2 I believe) and dissipates in concentration quickly but does it react with gases in atmosphere to form methane or other compounds?

Short of ignition and burning causing more $\ce{H2}$ to dissociate what can happen, especially in upper atmosphere with more UV and so on. Methane has reaction in the upper atmosphere giving it a half life of ~7 years for example.

The covalent radius of a neutral hydrogen atom is 0.0371 nm, smaller than that of any other element. Because small atoms can come very close to each other, they tend to form strong covalent bonds. As a result, the bond dissociation enthalpy for the $\ce{H-H}$ bond is relatively large (435 kJ/mol). $\ce{H2}$ therefore tends to be unreactive at room temperature. In the presence of a spark, however, a fraction of the $\ce{H2}$ molecules dissociate to form hydrogen atoms that are highly reactive. Source: chemed.chem.purdue.edu

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  • $\begingroup$ Hmm, UV can readily dissociate $\ce{Cl2}$, but I'm not really sure about hydrogen gas. It's known to be reacting vigorously, but only if some good amount of activation energy is provided. $\endgroup$ – M.A.R. ಠ_ಠ Aug 16 '15 at 15:27
  • $\begingroup$ interesting, activation energy in the form of radiation or kinetic agitation? $\endgroup$ – wide_eyed_pupil Aug 16 '15 at 15:59
  • $\begingroup$ Activation energy can be kinetic or potential. Basically, you need the reacting species with correct orientation and enough energy to collide. See collision theory and The Arrhenius Law. $\endgroup$ – M.A.R. ಠ_ಠ Aug 16 '15 at 16:23
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The most common, reactive stuff in the atmosphere are $\ce{-OH}$ radicals (Wikipedia). So I guess they're the natural antagonist for $\ce{H2}$ molecules.

Btw., as soon as it's diluted in the surrounding air, it will no longer rise. Only bulk masses have buoyancy. The g gradient in the atmosphere is much too small to separate gases.

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  • $\begingroup$ Please visit this page, this page and this ‎one on how to make your future posts better.‎ || I like the "prey" analogy, but I believe the $\ce{H2}$ will be the prey and hydroxide anions the predator. $\endgroup$ – M.A.R. ಠ_ಠ Aug 16 '15 at 16:24
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    $\begingroup$ Your point about buoancy is not really true on geologic time scales. Early in Earth's history, large amounts of hydrogen (H2) gas were lost to space. The process continues today albeit at far lower rates. Even so, the rate of H2 loss to space far larger than say N2 or O2 loss to space. $\endgroup$ – Curt F. Aug 16 '15 at 19:04
  • $\begingroup$ en.wikipedia.org/wiki/Atmospheric_escape - Yeah hydrogen and helium do escape albeit in little amounts $\endgroup$ – Mithoron Aug 16 '15 at 20:26
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    $\begingroup$ Has nothing to do with buoancy. The speed of H2 molecules( or He) in the upper atmosphere is above the escape velocity. He and H2 have a net flow upwards because of diffusion in the ensuing concentration gradient. $\endgroup$ – Karl Aug 17 '15 at 0:31
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    $\begingroup$ No, it just diffuses up there. As it can't diffuse back (it's destroyed up there), there is a net flow upwards. An entropy gradient drives it up, not one in potential energy. $\endgroup$ – Karl Aug 17 '15 at 9:25

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