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Thread: Electrolysis

  1. #1 Electrolysis 
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    Hi guys, I have some question about electrolysis.

    1). If I were to split water via electrolysis inside a closed vessel and allowed the two products to mix would the Hydrogen and Oxygen reform into water and condense? Would this happen quickly?


    2). I know electrolysis is commonly demonstrated with water, but I'm interested in what other substances I can investigate for use?

    The criteria is;

    liquid at 0 - 25C
    generates an inert or non-corrosive gas.
    generates products that are not likely to reform and condense.


    Basically I'm interested in liquids that could electrolyse into a large volume of non-explosive, non-corrosive gas, that would remain stable and not recombine and recondense, if I have to keep the two products separate then I guess that's not a huge problem either.

    Thanks for your time,

    Stuart


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    Mixtures of hydrogen and oxygen gas will not spontaneously reform water unless you apply a spark or a flame (there is an activation energy barrier as the H-H bond in hydrogen needs to be broken to initiate the reaction). Be careful though as if a spark or flame is applied the recombination is explosive. As for your second question, in that temperature range I think you are restricting yourself to aqueous solutions of various salts.


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  4. #3  
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    Thanks,

    I'm just trying to read a little around this, but I believe (rightly or wrongly), that the electrolysis of water produces Oxyhydrogen?


    My understanding is that oxyhydrogen is unstable and has an autoignition temperature of about 540C, but that it has no autoignition pressure, is this correct?


    I also believe (rightly or wrongly), that 1 liter of water would electrolyse into approx 669 liters of O2 and 1337 liters of H2 (read it elsewhere on the net), so in a very slow isothermal process in a closed volume that would be a pressure gain of around 30,000 psi which is an enormous pressure, that I would never want to see.

    But would HHO theoretically be stable at pressures of circa 10,000psi if it is compressed in an isothermal process? Might HHO become corrosive at an elevated pressure?


    Are there any safer/more stable options for this scenario? I guess if there was any liquids that would not produce oxygen as a product they would be much safer?

    Cost and availability are not a consideration as this is a theoretical exercise. I'm just wondering if this is possible in theory.
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  5. #4  
    Bullshit Intolerant PhDemon's Avatar
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    Electrolysis of water produces oxygen at the anode and hydrogen at the cathode in a 1:2 ratio.

    This mixture is stable at room temperature, as I said, unless you provide energy to overcome the activation energy by lighting it. The temperature of 540 C is the temperature at which the molecules themselves are energetic enough to overcome the barrier and the reaction is spontaneous.

    If you want to check your numbers, for each mole (18 g) of water you electrolyse (not the volume of water in your system -- the amount actually broken down) you will get 1 mole of H2 and 0.5 moles of O2, at 25 C 1 mole of gas has a volume of ~24 Litres.
    Again, mixtures of hydrogen and oxygen are stable unless the activation energy barrier is "broken".

    Most aqueous solutions when electrolysed produce O2 (exceptions are those that contain anions such as chloride which can liberate chlorine instead of oxygen -- you don't want that, I've gassed myself with Cl2 in the lab before, it's not nice)
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  6. #5  
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    Hydrogen oxygen mixed gases are still used industrially
    It was actually the first welding gas and was made by electrolysing water.
    It is the H2 molecule and they keep it and the O2 separate for safety. Trying to store them mixed is quite dangerous.
    Theory says it can be burned at a 2:1 mix but that gives an oxidizing flame and for welding it should be burned at about a 4:1 ratio.
    You can buy the hydrogen and oxygen in tanks now, so running your own generator is not really required.
    Hydrogen is not the best welding gas for general use. The hydrogen tends to make welds on iron brittle but it is OK with aluminum.
    However it has uses where very high and concentrated temperatures are needed. Jewelers sometimes use it for just those reasons.
    A hydrogen flame also burns very cleanly, and almost invisibly.

    Be aware that there are a lot of crank sites calling it Brown's gas and making outrageous claims about it.
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  7. #6  
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    It seems that at higher pressures ammonia is a stable liquid?


    Therefore if we take the closed vessel and fill it with liquid ammonia at say 200 psi, the ammonia should be a stable liquid as the boiling point is ~100F?

    If we then apply electrolysis to the ammonia we would produce H2 and N2, which should be much more stable/safe than Oxyhydrogen?


    A gas cap would form at the top of the closed vessel and the pressure would rise accordingly. The electrolysis would be slow enough as to be an isothermal process.


    Is that correct? What considerations are there regarding the reformation of Ammonia from the H2 and N2? are the H2 molecules locked with the activation barrier mentioned earlier?
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  8. #7  
    Bullshit Intolerant PhDemon's Avatar
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    Yes, but from the temperature range given in your OP, I was under the impression you were interested in ambient conditions and gave an answer corresponding to this.

    You might find this interesting: Liquid ammonia electrolysis by platinum electrodes


    Nitrogen/hydrogen mixtures are stable, you need high temperature and a catalyst to reform ammonia (look up the Haber process).
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  9. #8  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by Stuart255 View Post
    The criteria is;

    liquid at 0 - 25C
    generates an inert or non-corrosive gas.
    generates products that are not likely to reform and condense.
    Two completely away from the main thrust of the thread, which I think has been addressed:

    1. Criteria is plural, not singular.
    2. Oxygen is a decidedly corrosive gas.
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