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