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  • Chromium is a really surprisingly exciting element. We've had real fun in the lab and

  • we've had volcanoes,

  • fires,

  • Neil nearly caught fire. You can see some quite exciting things

  • And also, I hope to learn you something about the chemistry of this really interesting element. It has played a big role in my life.

  • Particularly in my research ever since I've been the schoolboy.

  • Chromium is element number 24 in the middle of the so-called

  • transition metals in the first row. It was first discovered or rather its

  • first compounds were discovered in the middle of the 18th century in mines in the Ural Mountains

  • in eastern Russia. One of the

  • minerals that was discovered was a very bright-yellow pigment lead chromate.

  • You can make it in the lab

  • by dropping ammonium dichromate. We'll show you another interesting reaction of that later.

  • Dropping a solution of ammonium dichromate which is orange

  • into a beaker containing

  • lead acetate which is colorless - well slightly cloudy - and you get this

  • fantastically bright yellow precipitate formed. It used to be called chrome yellow and be used as a paint.

  • Now people don't really want to use lead salts in paints but you can still see this terrific color.

  • It's important to say that even for chemists like me, to see this reaction yet again

  • is really exciting and particularly because the formation in the beaker is

  • governed by the swirling of the liquid and the mixing and you see really beautiful patterns

  • and it always gives me a thrill. Then in the later

  • 18th century a French chemist Louis Vauquelin

  • discovered that you could convert chrome yellow

  • into an oxide of chromium (chromium trioxide) by treating it with acid. Later he

  • reacted this oxide with hydrogen to make chromium metal and discovered the element.

  • I think it's just worth mentioning that the reason that so many elements were discovered at the end of the 18th century

  • beginning of the 19th century is that's when the idea or the modern idea of chemical elements

  • was first formulated so before then people were not

  • looking for chemical elements. And once the idea was formulated, a whole mass of elements were found

  • relatively quickly. While we're talking about chromium trioxide,

  • it's one of my favorite compounds. You may have seen an earlier video where I set off all the fire alarms in the chemistry department

  • by mistake with chromium trioxide and ethanol.

  • Brady: "You put that smoke alarm off?"

  • Professor: " Yeah...oh"

  • [fire alarm siren beeps]

  • Professor: "They were meant to put it off".

  • Professor: "You forgot to switch it off".

  • Cameras have got better and I persuaded Neil to try this reaction himself.

  • You pour ethanol (which is a compound of carbon, hydrogen and oxygen) on to chromium trioxide

  • (which is a compound of chromium where the chromium has given all its electrons to the oxygen).

  • It is a very powerful oxidizing agent;

  • it makes things burn and it really made the ethanol burn.

  • Poor Neil's beaker caught fire.

  • He should have of known perhaps that it would, if he'd watched my video he would have got the warning.

  • The reaction not only produces flames, but it converts the chromium trioxide

  • into a different oxide of chromium where there are some electrons on the chromium and it is bright green. It is

  • so-called chromium(II)

  • oxygen 3. Some people call it chromium sesqui oxide

  • Brady: "you are joking"

  • And the flames all went on a GoPro camera

  • but I'm pleased to say the GoPro camera was not injured so it'll survive for our next video.

  • -Neil is laughing in the background-

  • This brings us back to the name chromium which is derived from the Greek word meaning a color.

  • And you've heard of monochromatic and words like that.

  • And chromium forms the most

  • amazing range of different colored compounds. You've already seen it going from red to green and you can get

  • almost any color you want. And the reason that it has all these different colors is

  • because as a transition metal it has

  • 4 D electrons and 2 s electrons, so that's

  • six electrons. In different compounds, it gives different number of electrons to the neighboring atom and

  • The colors are associated

  • with the energy levels of the electrons that are left on the chromium atoms so, you have different numbers of electrons and the

  • energy levels can also differ depending on the atoms around it. If the energy levels are far apart

  • they will absorb blue light and so the compound will look red. If the energy levels are close together

  • they will absorb red light and look blue.

  • You can see a really nice color change when you drop sodium chromate into acidified hydrogen peroxide.

  • The chromate liberates the oxygen from the hydrogen peroxide and

  • part of the chromate is reduced, that is, it receives electrons

  • and so the chromium will go to a greenish color.

  • Now when you actually look at the experiment what Neil did was that he took a dropper full of the

  • chromate solution and stuck it at the bottom of the beaker and then

  • squeezed it so the chromate solution went at the bottom. It reacted immediately,

  • going a sort of greenish brown color

  • which, if you'd looked very carefully, you will see is actually a mixture of a blue compound and a green one.

  • But the oxygen that is liberated

  • so the bubbles go up to the surface and as it goes up,

  • it takes up streams of this colored liquid, so you have quite a strange

  • almost like a garden growing. And again for me as a chemist,

  • it's really exciting to see how these things behave

  • You can't predict exactly what's going to happen so every experiment is exciting. In the past

  • We've done this reaction before where we did it the other way around, pouring hydrogen peroxide into

  • chromate solution and there you do get quite dramatic color changes and as I mentioned a

  • professor that I knew blew off two of his fingers with this reaction, so you've got to be careful.

  • Neal did one other really spectacular reaction with the

  • ammonium dichromate.

  • Now, ammonium which is NH4 plus.

  • If you think about it contains hydrogen which can burn and on the other hand it has chromate which can give up oxygen.

  • So if you like, it's got both the oxidant and the fuel all in one compound. The bottles say is careful

  • explosive and Neil

  • heated this up in a nice round bottom flask,

  • quite a sizable glass and when the reaction takes off it takes quite a bit of heating to get it going,

  • then it

  • looks just like a volcano

  • because you have the green

  • chromium oxide being formed which looks a bit like lava and the flame is shooting up.

  • All three of us: Brady, me and Neil were really quite impressed by that reaction.

  • Unfortunately the filter that Neil used at the top wasn't strong enough

  • thermally to resist the heat and it caught fire and then it melted. But apart from that the

  • reaction was quite a success.

  • Chromium plating was used very widely in the United States in the car industry,

  • the automobile industry and with expensive cars were covered in chromed

  • components, so they looked really shiny and silver.

  • Nowadays people are trying to save weight on cars

  • and you don't use metal for the bumpers and things like that, so there's almost no chrome plating on cars.

  • Let me tell you about

  • chromium in my research, I've worked for many years with this compound chromium hexacarbonyl.

  • Chromium with six CO-groups around it. if you

  • isolated it in a solid at a very low temperature,

  • -250 degrees, you can irradiate UV light on it and generate chromium pentacarbonyl.

  • A chromium with 5 CO-groups around it

  • which is enormously reactive and would react immediately and disappear if it was not frozen at such low temperature.

  • At that sort of low temperature it will even react with argon.

  • We all think that argon is this really innert gas. My fellow student Robin Perutz

  • discovered it would react with argon and form a really nice purple color.

  • I reproduced his experiment for my first lecture that I gave outside my own University.

  • I still show the slide 43 years on the lecture was a bit of a disappointment.

  • There was a Nobel Prize winner, so George Porter was meant to be there

  • but it turned out he had another engagement so I never saw him at the lecture. It then turned out that

  • Cr(CO)5 will react with xenon. Me and my colleague Jim Turner (my professor)

  • published the first paper in which Cr(CO)5Xe was discovered in solution.

  • And now this is still a major research

  • area of Nottingham. My colleague Mike George is using very fast spectroscopy to look at

  • metal xenon compounds. Most school teachers don't know that xenon will react with transition metals

  • but it's now quite a widespread phenomenon. When I was a schoolboy , aged 16,

  • I passed my exams a year earlier than my classmates, so I was allowed to do a research project all by myself.

  • Wouldn't be allowed nowadays. For reasons I can't remember now,

  • I decided to look at the reaction of copper chromate and ammonia. For nostalgia state

  • I asked Neil to repeat this experiment

  • taking copper sulfate, which is a blue solution,

  • adding to it a solution of sodium chromate and you get a rather sort of muddy precipitate.

  • It doesn't look at all appetizing, and then if you add ammonia

  • It goes dark blue if you haven't mixed it up properly like Neil hadn't and then it goes green.

  • Now what I want you to see from this experiment

  • is it a pretty messy experiment and it was completely mad for me as a

  • schoolboy to try and study this reaction. But I did it with great determination I

  • never really discovered anything important

  • but it did really hooked me on doing chemical research

  • I've been doing it nearly ever since. I was so devoted to this reaction

  • I had a rack of test tubes with these different samples above me on the shelf above my bed in my bedroom

  • I'm not sure what my parents thought of this

  • Hi, there everyone! This is the first video we've posted since passing 1 million subscribers

  • I thought I'd take this opportunity to thank everyone who's watching periodic videos.

  • A special thanks to everyone who subscribes.

  • A special special thanks to everyone who's used a little notification bell with their subscription.

  • A special, special, special thanks to everyone who supports us on patreon, there's an address on the screen.

  • Also, if you're someone who's gotten into watching podcasts, I thought I might show you how I actually have two podcasts.

  • I do one with a fellow youtuber called CGP Grey, you may have heard of him, and our podcast is called Hello Internet

  • And there's a second podcast I do a newer one called The Unmade Podcast that I do with an old friend of mine called Tim

  • The names of them are on the screen, you can find links in the video description

  • Or just search Hello Internet or The Unmade Podcast on your podcast player of choice. Thanks for watching

  • We'll see you again here soon!

  • Subtitles by Luna Versmissen

Chromium is a really surprisingly exciting element. We've had real fun in the lab and

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    林宜悉 に公開 2021 年 01 月 14 日
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