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  • Cobalt is element number 27,

  • between iron 26 and nickel 28,

  • and like most elements, it's really quite interesting

  • It is a catalyst

  • It can be used in magnets

  • It is important in the batteries that we have in all our modern electronic equipments and

  • Above all it is associated with the color blue.

  • The element or its compounds has been known for a long time because it's been used for coloring glass.

  • I found in my cupboard at home quite a nice glass jar which

  • everybody has told me is a 19th century

  • Christmas tree light with a candle in it though i'm sure some viewers will tell me it's something more boring

  • But i still think it's a Christmas tree light and you can see it has got this glorious, blue color.

  • Brady - "Did you buy it in the 19th century?"

  • I can't remember when I bought it but it was a long time ago. They add a small amount of cobalt

  • Into the melt when they're making the glass and so you get isolated

  • Ions of cobalt, which have this nice blue color. Cobalt itself is a

  • shiny metal if it's polished and

  • It's relatively, dense. It's quite heavy to hold. It's used principally in alloys with other

  • Metals it used to be used a lot in making magnets though now

  • You can get more powerful magnets using rare earths

  • Because cobalt is a so-called transition metal

  • The ions of cobalt say cobalt 2+, that's cobalt with two electrons removed

  • it's usually not naked, it will be

  • coordinated by other compounds

  • For example if you, make it using water

  • You get six water molecules round the cobalt,

  • so here you can see cobalt in the middle with six water molecules around it.

  • Now you might ask, how can you form so many bonds,

  • and the answer is that the interaction is partly electrostatic.

  • The negative oxygen interacting with the positive cobalt.

  • If you heat cobalt chloride, which is beautiful red colored crystals to high temperature,

  • you can drive off the water and make a sort of purplish powder.

  • We found a lovely really old sample of anhydrous (without water), cobalt chloride

  • in a bottle that still sealed up with its original wrapping.

  • We felt we really didn't have the heart to destroy this historic sample to destroy the wrapping, so you'll have to look at it through the glass.

  • We had another sample of anhydrous cobalt chloride,

  • where the bottle had split so air and water could get in and you can see it goes back to the pinkish color

  • So we've got two demonstrations for you the first one is to demonstrate

  • how in solution you can remove the water and substitute it with chloride ions,

  • by using concentrated hydrochloric acid.

  • We were so excited to do this that the first time, we didn't do it very, well

  • We had a test tube of cobalt chloride solution

  • Nice the pinkish color, and try dropping in concentrated hydrochloric acid

  • but although you could see some changes of color,

  • the concentration of the acid was not enough.

  • So, we reversed it

  • and dropped the pink solution into the colorless hydrochloric acid

  • so now we had a whole test tube of hydrochloric acid

  • And what we saw and I hope you can,

  • is that as the pink solution goes in it goes bright blue

  • and the bright blue is color is due to

  • Cobalt ions with four chloride ions so it goes to (CoCl4)2-

  • going from cobalt 2+ to 2- ions.

  • And the striking thing is of course that it's a much darker color, even though we're diluting it

  • If you drop the cobalt chloride into water, you see almost no color, because it's so dilute.

  • So the reason why you get this very intense color is because of

  • the change of the shape of the atoms immediately around the cobalt,

  • going from six round the cobalt to only four.

  • And this change of shape changes the quantum mechanical rules,

  • in which the electrons are interacting with light

  • So in the second experiment i want you to see what happens if you drop

  • cobalt chloride solution into sodium hydroxide, quite concentrated

  • and you would expect to see a precipitate of cobalt hydroxide, which is pink.

  • Now watch carefully.

  • As it goes in you get a precipitate a, solid but it's bright blue!

  • And then as you watch,

  • really quite slowly and satisfyingly,

  • it gradually goes pink to make the product that you expect.

  • So this is an example of a reaction,

  • where the starting material goes through some sort of intermediate compound and then makes the product.

  • And it makes the intermediate very quickly and then slowly it goes to the product

  • And what's happening is that

  • first of all, you get a mixed salt where you have both hydroxide and chloride

  • which is still insoluble so it precipitates.

  • And then, slowly, and it's slow

  • because it's in the solid state and has to redissolve,

  • the chloride is replaced by hydroxide to give you the pink color.

  • and it's really quite mesmerizing, how these pink crystals rain down

  • Cobalt is also very important as a catalyst

  • It is used in a variety of industrial processes,

  • particularly a process for converting natural gas,

  • or rather a mixture of carbon monoxide and hydrogen,

  • which can be made from natural gas,

  • and has the rather amusing name of syngas for synthesis gas,

  • and using a cobalt catalyst you can then convert it to liquid fuels for cars or lorries

  • And the advantage is that you can take isolated gas fields,

  • where there's natural gas (methane) and convert it into liquid fuel that can then be transported

  • Cobalt is also very toxic

  • We had some powdered cobalt, so-called cobalt sponge

  • And Neil, our technician, was really quite nervous of this poisonous powder,

  • but cobalt is also vital for life.

  • It occurs in a molecule called vitamin B12

  • which is present in living organisms in rather a low concentration,

  • but catalyzes a whole series of important biological reactions

  • And the structure vitamin B12, was determined, by the british chemist Dorothy Hodgkin,

  • who was the only woman who's won the Nobel Prize for Chemistry from the UK

  • So it is really a very historic structure.

  • So this is Neil's new device. It's a very very fancy erection.

  • Potentially we've used a laboratory stand, and we have an eyelet.

  • So this is just to act as a point, which we can raise up our cesium on a piece of string.

Cobalt is element number 27,

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    林宜悉 發佈於 2021 年 01 月 14 日
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