Potassium explodes without Hydrogens help


Thus far almost all the stuff I’ve done on the alkali metals has been on ~250 mg quantities.

Self preservation says, “Start Small!”, and with good reason, the reactions of alkali metals with water are known to scale in treacherous fashions.

In the first instance I went for 1.6 g of potassium and 3.5g of sodium.

Note here there is not the slightest hint of green gas with the potassium!

Now the potassiums the one I’m going to focus on for the moment, simply as when this is played back at about 1/100ths speed it becomes clear that the hydrogen burning plays essentially no part in the violence of this reaction.

Fascinatingly the explosion appears to go in two stages here!

So potassium, molecular weight ~40, so 1.6 g is 0.04moles. 2moles of potassium gives a mole of H2 , so this reaction will produce about half a liter of hydrogen.  Just for scale the jug is about 3 liters.  It’s also fascinating to see the ‘springiness’ around this generated gas.

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12 Responses to “Potassium explodes without Hydrogens help”

  1. Kittrana Says:

    if you note though there a flash of something igniting just before the first explosion so there had to be some gas there no? you see the same flash with the second explosion too

  2. BlitzNeko Says:

    Well you could always go into the pyrotechnics field… Fireworks is a growing industry…

  3. Gooda Says:

    What I thought was amazing is that after the first explosion there was a perfect sphere, would the second explosion also have been a sphere given enough room?

  4. DMT Says:

    So you’re positive that the first “sphere” is hydrogen? Maybe there’s water vapour?

    • Kittrana Says:

      if you look theres a burst of flame just before the first explosion ergo theres gotta be some gas.

  5. Prelude610 Says:

    Underwater fireworks. Very cool.

  6. dreadnaught Says:

    Youtube will be the poorer if you’re “out,” Tf00t. Just so you know, your vids helped to undo the confusion resulting from K-12 homeschool by creationists and am grateful to you for taking the time to make them.

    If your future is in blogs, so be it. Am looking forward to an explanation of potassium, hydrogen, green gas and lack thereof.

  7. Vic Says:

    Have you thought about having the metal pass through a layer of inert oil or something before it hits the water so that it doesn’t encounter both the atmospheric oxygen and the water at the same time?

  8. poleschs Says:

    Nerdgasm, that’s awesome.

  9. poleschs Says:

    I’m not sure where you are going with this, but I would recommend a UV/IR hydrogen flame detector. I’m not completely sure how they work, but the brochure say it will only detect flame from hydrogen. With that tool, you can see if there’s any hydrogen gas burning, and how fast the hydrogen is produced in the reaction. I’m not sure about the cost either, just something you could look into. Here an example: http://www.generalmonitors.com/products/flame_fl3100h_hydrogen.html

  10. Patricia Bianconi Says:

    The melting point of K (65 degrees C) is much lower than that of Na (100 degrees). The first reaction is evolution of H2 from the surface of the K in contact with the water, and that ignites because there is also a lot of heat evolved when the surface layer of KOH dissolves.

    The heat of the fire melts the rest of the K, and there is a sudden evolution of H2 from all the rest of the 1.6 grams. So it’s the instantaneous evolution of so much gas that causes the violence of the reaction. The interior of the K didn’t have any KOH, so the heat evolved from that dissolving isn’t added, and apparently the absorption of the heat of H2 evolution by the cooler water is enough to not ignite the H2 until it hits the surface.

    Atmospheric oxygen causes the formation of KOH, not hydrogen, so that’s not involved.

  11. Patricia Bianconi Says:

    Whoops! Made some mistakes there.

    H2 needs to react with O2 to ignite. Water being very polar, while O2 is not, there is not much O2 dissolved in water. So H2 hits the surface before it inflames.

    K forms KO2, potassium superoxide, on exposure to O2. KOH is formed on reaction with water vapor. Those two salts are the white layer that form on the surface.

    Sorry. I usually drink my coffee before I think about chemistry.

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