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  • After this, whenever he was lecturing the reaction,

  • he would hold up his hand and say, "I had a hand in that reaction."

  • So this time, I've got potassium dichromate,

  • some sulfuric acid,

  • and then finally on top of this mixture, which is in- dissolved in water, I'm going to add some diethyl ether.

  • Diethyl ether is an organic compound which does not mix with water.

  • So if I add it carefully, it should float on the surface.

  • So if you look now, you can see,

  • there's a colorless layer here of ether, with the dichromate underneath.

  • So now, I put some hydrogen peroxide into this small beaker,

  • and we'll add a little and see what happens.

  • So watch carefully

  • and with luck something will change.

  • So...

  • What you can see

  • is that there's quite a violent reaction,

  • and the ether layer has gone bright blue.

  • And the reason it's gone bright blue is because

  • we have formed a chromium compound with the formula chromium-O₅.

  • Which, unusually for an inorganic compound, will dissolve in an organic solvent.

  • And so the hydrogen peroxide has oxidized the chromium.

  • It was chromium-O₄⁻², it's now chromium-O₅.

  • A quite dangerous compound.

  • And it's important with this blue compound,

  • it's very intensely colored, so there isn't very much here...

  • but one must be very careful not to isolate this material.

  • There is

  • a famous story of a chem- a chemistry professor in Cambridge,

  • whose student made a whole test tube full of this solid compound.

  • And the professor was shaking the test tube,

  • when it went off with an enormous bang,

  • and removed 2 of his fingers.

  • And after this, whenever he was lecturing the reaction he would hold up his hand and say,

  • "I had a hand in that reaction."

  • And all the students got very frightened.

  • Slowly, what is happening,

  • is that the hydrogen peroxide is mixing

  • and reacting with it.

  • But in a long tube like this, you don't get very good mixing.

  • If I was to stir it with a rod or something like that,

  • I don't have one here, it would all react at once.

  • But this is quite nice to do it like this, because then you see the before and after color at the same time.

  • And you can see the reaction is taking place here,

  • and the oxygen, O₂,

  • which is released by the reaction, is coming up.

  • And you're left with some the, um, chromium in solution here, and it's a, um, greenish color for chromium 2.

  • So kind of like the reaction point is like a wave that's moving down the...

  • Yes.

  • Huh.

  • But, that's nothing to do with the chemistry, that's because the mixing is so bad in this sort of vessel.

  • That's why chemical engineers have to design proper-shaped vessels

  • so that reactions go in a controlled manner.

  • You saw in this case it wasn't very controlled to begin with, it all splurted out of the top.

  • Which looks quite spectacular, but is not very good practice.

  • We're trying again, this time I thought to put water.

  • I put warm water in here so the reaction may be faster,

  • and I'll put in most of the acid, as well.

  • [Nerf gun firing] [clinking]

  • And...

  • then a bit of permanganate...

  • and now we've got enough peroxide still left.

  • You can see it's decolorising straight away.

After this, whenever he was lecturing the reaction,

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化学に指を失う - 動画の周期律表 (Losing fingers to chemistry - Periodic Table of Videos)

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    林宜悉 に公開 2021 年 01 月 14 日
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