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Thread: will hydrogen gas be on top in atmosphere?

  1. #1 will hydrogen gas be on top in atmosphere? 
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    hi, just now i read the concept of diffusion of gases, according to this concept hydrogen gas should be on top of earth's atmosphere(low molar mass)is it true in real or all the gases are in same level, please explain it...


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    Quote Originally Posted by chinna View Post
    hi, just now i read the concept of diffusion of gases, according to this concept hydrogen gas should be on top of earth's atmosphere(low molar mass)is it true in real or all the gases are in same level, please explain it...
    Well diffusion in the atmosphere will tend to stop it separating by density, I think.

    But the reason why there is so little hydrogen in the atmosphere is that the velocity of a significant fraction of the molecules exceeds the gravitational escape velocity. So most of the hydrogen has been lost into space. This happens far less with heavier gases, as for them the molecular speed for a given temperature is not so high.

    The relationship between molecular mass, velocity and temperature is explained here: Kinetic Temperature, Thermal Energy


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    Bullshit Intolerant PhDemon's Avatar
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    It is true that lighter gases tend to rise up in the atmosphere but there are other factors which mean that gases in the lower/middle atmosphere are well mixed such as convection due to solar heating which causes turbulence and rapid vertical mixing in the troposphere. In the stratosphere there is less turbulence and the atmosphere is in layers with warmer air above colder air (hence the name) and vertical structure is due to temperature not composition, but there is still a lot of horizontal mixing. Above this you have the mesophere. In all three of these regions chemical species are well mixed and molecular weight has no effect on the height distribution. It is only above this when you enter the mesopause/thermosphere (at an altitude of about ~80-100 km) that the atmosphere becomes "thin" enough (i.e. low enough pressure) for molecular diffusion to be dominant over turbulent mixing and the molecular weight of the gas has a significant effect.
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    Quote Originally Posted by PhDemon View Post
    It is true that lighter gases tend to rise up in the atmosphere but there are other factors which mean that gases in the lower/middle atmosphere are well mixed such as convection due to solar heating which causes turbulence and rapid vertical mixing in the troposphere. In the stratosphere there is less turbulence and the atmosphere is in layers with warmer air above colder air (hence the name) and vertical structure is due to temperature not composition, but there is still a lot of horizontal mixing. Above this you have the mesophere. In all three of these regions chemical species are well mixed and molecular weight has no effect on the height distribution. It is only above this when you enter the thermosphere (at an altitude of about ~100 km) that the atmosphere becomes "thin" enough (i.e. low enough pressure) for molecular diffusion to be dominant over turbulent mixing and the molecular weight of the gas has a significant effect.
    That's interesting. Does that mean that diffusion on its own would lead to significant stratification of gases by density? I had sort of thought intuitively that the diffusion process would mix them fairly completely and the density difference would not be enough to separate them.
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    It's a while since I've considered the atmosphere that high up (I normally only think about the troposphere at work) but IIRC there is a region called the "turbopause" which is basically the altitude below which the mean molecular mass is constant and above which the mean molecular mass is altitude dependent due to molecular diffusion.

    I had a little look to refresh my memory and found this:

    http://www.ann-geophys.net/32/431/20...2-431-2014.pdf
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    ok thankyou very much...
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    Very few gases exist as pure elements and it is the molecular mass of the gaseous compound that matters.
    Hydrogen reacts with carbon and oxygen easily and forms heavier molecules like CH4 or H2O.
    Helium is a less reactive gas and since it does not easily combine with heavier elements it is light enough to escape from the atmoshere into space.
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    Seeing as most of the atmosphere is nitrogen and oxygen (ok in diatomic molecules) which are elements I'd disagree with your first statement.

    Also some H2 is formed in the atmosphere from methane (much of the rest is from from biomass burning) and in the lower atmosphere the major sink is soil uptake not loss to higher layers of the atmosphere and ultimately space...

    http://onlinelibrary.wiley.com/doi/1...00788/abstract

    For more info on H2 in the atmosphere here are some open access papers:

    http://www.atmos-chem-phys-discuss.n...2012-print.pdf
    http://www.atmos-chem-phys.net/11/33...-3375-2011.pdf
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    >>>Hydrogen reacts with carbon and oxygen easily and forms heavier molecules like CH4 or H2O.

    I don't think hydrogen reacts with carbon easily to form methane. In fact it is quite difficult to get this reaction to work. Besides which, where is the elemental carbon in the atmosphere going to come from? - it should have already reacted with oxygen, by your argument.
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