Chemistry Demonstrations

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V4mp
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Chemistry Demonstrations

Postby V4mp » Tue Mar 17, 2009 4:26 pm UTC

Well, I'm coming up to the end of my time in 6th form, and will be going off to uni next year, so our Chemistry teacher said that for the last Chemistry lesson we have, he will do any demonstrations we want (within the bounds of reason). So I'm looking for suggestions for demonstrations that will be exciting to watch :)

Not particularly bothered about the chemistry behind them (alright, actually I would be interested in it, but since i need to persuade the rest of the set, purely chemically, as opposed to visually, interesting demonstrations probably wont cut it) so go for pretty colours, lights and bangs... Bear in mind that there aren't unlimited resources, and these do need to be performed in a typical high-end college lab - so nothing too explosive or expensive - although I think that the budget could stretch to substances such as liquid nitrogen/oxygen.

So far the only suggestions are the usual caesium/other cheaper group 1 metal in water, and conc. sulphuric acid + conc. nitric acid on a cloth leading to nitrocellulose...
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Re: Chemistry Demonstrations

Postby alexh123456789 » Tue Mar 17, 2009 4:36 pm UTC

You could try thermite, it seems pretty cool. Maybe burn through a steel plate or something.

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Re: Chemistry Demonstrations

Postby meat.paste » Tue Mar 17, 2009 5:52 pm UTC

There are many cool color oscillations (look up chemical clocks). They are not too pricey and will oscillate between different colors for a while. You can even experiment with the electron donor compound.

These are iodine clock reactions:
http://en.wikipedia.org/wiki/Iodine_clock_reaction
http://en.wikipedia.org/wiki/Briggs%E2%80%93Rauscher_reaction

This is a cerium clock reaction:
http://www-chem.ucsd.edu/academic/Instruc/lab/demos/demos.cfm?page=bel-zhab
Huh? What?

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meat.paste
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Re: Chemistry Demonstrations

Postby meat.paste » Tue Mar 17, 2009 6:21 pm UTC

Meteorswarm wrote:The iodine clock reactions (the ones that cycle) were actually discovered by a high school teacher who mixed the right reagents by accident, preparing for a lab.

-Cool. I did not know that.

Meteorswarm wrote:Or make TATP and explode it.

That is a nasty and unstable high explosive. It would scare me to try. Maybe do some NI3? It's expensive because of the elemental iodine required, bit it is easy to do. (Mix I2 with a concentrated aqueous ammonia solution. Filter.)
Huh? What?

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BlackSails
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Re: Chemistry Demonstrations

Postby BlackSails » Tue Mar 17, 2009 6:24 pm UTC

meat.paste wrote:That is a nasty and unstable high explosive. It would scare me to try. Maybe do some NI3? It's expensive because of the elemental iodine required, bit it is easy to do. (Mix I2 with a concentrated aqueous ammonia solution. Filter.)


You could probably get some iodine out of iodine tincture.

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Re: Chemistry Demonstrations

Postby danpilon54 » Tue Mar 17, 2009 8:22 pm UTC

I'm no chemist, but if you take pure sodium and put it in a beaker full of water (put it in a big sink), the beaker will shatter if you put enough sodium in. Don't know if they want you breaking their lab equipment though. If you do it with a small amount the sodium will just burn and spin around in the beaker.
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Re: Chemistry Demonstrations

Postby sgt york » Tue Mar 17, 2009 8:34 pm UTC

Oh, I love chemistry demos....

1. Bubble bombs. Take a 2L soda bottle and bore two holes a little ways from the bottom on opposite sides. Run a pair of electrodes through the holes and seal with hot glue/epoxy. Fill bottle most of the way with water. Add some baking soda and liquid soap. Mix well. Run a current through the eletrodes and wait. The water will bubble; let a few go; they will have a good bit of air in them and won't do much. After a while, light the bubbles. Works better in a dark room.

2. Liquid CO2. Get a big syringe and some dry ice. Get a plug that fits the opening of the syringe. Remove the plunger and put a small piece of dry ice in there and replace the plunger. Push it most of the way in. Hold it so the plunger faces down and let some of the dry ice sublime. Depress it the rest of the way; slowly. The CO2 is heavy and will settle at the bottom; this makes the gas part mostly CO2. Put the plug in. The dry ice will sublime and push the plunger back. When it fills, hold the plug in tight and push the plunger as hard as you can; you'll get something very few people see; liquid CO2.

3. Sodium acetate hand warmer. Mix vinegar and baking soda, try not to lose too much of it. Boil it down and make crystals. Break up the crystals and make a saturated solution in a ziplock bag. Cut out a metal disc from the concave bottom of a soda can; cut out the very middle of the bottom. Put it in the bag. Flex the disc so it snaps into its inverted shape and watch the crystals form....a handwarmer. (disclaimer: I've never tried this last one, just read about it, can't vouch for it working)

More fun ones I'm sure you can find online...electroplating coins (make a golden nickel; kind of fun to try), "pouring" CO2 to put out a candle, cornstarch superfluids, and borax superballs.

Of course, liquid nitrogen ice cream is always a big hit. Take the ingredients for ice cream, put them in a big bowl, and pour liquid nitrogen over them. You'll need about 3-4 times as much volume of lN2 as you have ingredients. If in doubt, guess high; I've never done the actual titration on it. Swirl it around a bit if you're making a big batch. Makes the best damn ice cream in the world. lN2 is so cold ice doesn't crystalize at all, so it's incredibly smooth. Oh, and be careful when you eat it. Make sure it's not -50C when you put it in your mouth.

And when I say "a big bowl" I really mean a bucket. Be sure you make it in a container that is WAY too big. If when selecting your container, you see one that makes you say "Well, that's just crazy. There's no way I'd need a container this big to make a quart of ice cream," get a bigger one. This can get really, really messy.

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Re: Chemistry Demonstrations

Postby V4mp » Tue Mar 17, 2009 8:42 pm UTC

These sound good so far, i like the sound of the clock one, that sounds quite interesting :-)

Bit more guidance on the budget - there is definitely available elemental iodine, so that could be something... All the explosive compounds i could think of previously tended to be organic and unstable, so weren't possible simply because of the time organic prep takes - but these new ones seem quick and easy :)

Ohhh liquid nitrogen ice-cream, the only ice-cream i've made was in physics on a much lower budget, using ice + salt to cool it...

Also, am I missing the chem behind 1) - we would be making hydrogen/air bubbles there right? We have sufficient apparatus to make more efficient hydrogen generators, and also oxygen generators - what % mixture would we be looking at for maximum bubble bang?
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Re: Chemistry Demonstrations

Postby sgt york » Tue Mar 17, 2009 8:49 pm UTC

V4mp wrote:Also, am I missing the chem behind 1) - we would be making hydrogen/air bubbles there right? We have sufficient apparatus to make more efficient hydrogen generators, and also oxygen generators - what % mixture would we be looking at for maximum bubble bang?

Hydrogen-oxygen bubbles. The first few will contain some air as contamination, but you let those go.

The beautiful part of the setup is that the water you are splitting is exactly the right ratio of hydrogen & oxygen already (think about it for a minute).

Just let the gasses mix as they come off the electrodes; that way the gas filling the bubbles is already the perfect mix.

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Re: Chemistry Demonstrations

Postby BlackSails » Tue Mar 17, 2009 9:00 pm UTC

Sugar in sulfuric acid forms a tall, smoking hot graphite stick. Make sure you do it with a fume hood, gloves and goggles though.

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Re: Chemistry Demonstrations

Postby alexh123456789 » Tue Mar 17, 2009 11:13 pm UTC

Sgt York wrote:cornstarch superfluids

what is that? I assume it's not an actual superfluid? http://en.wikipedia.org/wiki/Superfluid

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Re: Chemistry Demonstrations

Postby IHOPancake » Tue Mar 17, 2009 11:17 pm UTC

BlackSails wrote:Sugar in sulfuric acid forms a tall, smoking hot graphite stick. Make sure you do it with a fume hood, gloves and goggles though.


You need to use really concentrated H2SO4 (> 10M) though.

There is an experiment I've seen a few times where magnesium is burned between two bricks of dry ice. The reaction gets very bright.

http://www.chem.umn.edu/services/lecturedemo/info/Magnesium_and_dry_ice.html
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Re: Chemistry Demonstrations

Postby Minerva » Wed Mar 18, 2009 1:52 am UTC

Meteorswarm wrote:
alexh123456789 wrote:
Sgt York wrote:cornstarch superfluids

what is that? I assume it's not an actual superfluid? http://en.wikipedia.org/wiki/Superfluid


cornstarch in water forms a shear-resistant fluid, it's neat.


The term you're looking for is non-newtonian fluid.
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Re: Chemistry Demonstrations

Postby The-Rabid-Monkey » Wed Mar 18, 2009 3:08 am UTC

The barking dog experiment! http://www.youtube.com/watch?v=mGMR72X8V-U
If you're feeling dangerous, Mercury Nitride is amusing. It's like Nitrogen Triiodide but much much more explosive and sensitive.
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Re: Chemistry Demonstrations

Postby sgt york » Wed Mar 18, 2009 5:17 am UTC

alexh123456789 wrote:
Sgt York wrote:cornstarch superfluids

what is that? I assume it's not an actual superfluid? http://en.wikipedia.org/wiki/Superfluid
You're right; I brainfarted all over this thread, pardon me. I meant a type of non-Newtonian fluid. I don't know why the hell I wrote something as stupid as superfluid.

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Re: Chemistry Demonstrations

Postby Rentsy » Wed Mar 18, 2009 9:26 am UTC

Dry ice + Magnesium

The magnesium steals the oxygen from the CO2, releasing lots of heat, and hot carbon burns.

Unbelievable, white-hot in the middle of cold.

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Re: Chemistry Demonstrations

Postby wst » Wed Mar 18, 2009 10:17 pm UTC

V4mp wrote:Bit more guidance on the budget - there is definitely available elemental iodine, so that could be something... All the explosive compounds i could think of previously tended to be organic and unstable, so weren't possible simply because of the time organic prep takes - but these new ones seem quick and easy :)
Be so fucking careful with Nitrogen Triiodide. You sneeze on it, it'll go bang. Citation

Good simple fun is just a balloon with oxygen in (bang), then one with hydrogen in (boom), then one with both in (BOOM!). It wakes people up well :)
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Re: Chemistry Demonstrations

Postby Jorpho » Thu Mar 19, 2009 4:01 am UTC

I've heard that the "solvated electron" - sodium in liquid ammonia - is really pretty, and not just mathematically. How exactly one handles liquid ammonia is quite beyond me.

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Re: Chemistry Demonstrations

Postby Alexius » Thu Mar 19, 2009 12:43 pm UTC

Ammonium dichromate can be very impressive. It simultaneously oxidises and reduces itself when ignited, somewhat like ammonium nitrate. The result is a huge fluffy pile of green powder.

You can do all sorts of cool things with liquid oxygen. In addition to freezing things and burning things, it's magnetic. Yes, really.

And I second both the barking dog and sugar with concentrated sulphuric acid. The barking dog (ignite a mixture of carbon disulphide and nitrogen monoxide), can be dangerous and needs a long glass tube, but both looks (it gives off blue light) and sounds spectacular.

The iodine clock is cool, if not as explosive- I was demonstrating it to primary-school children at a science festival a few days ago.

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Re: Chemistry Demonstrations

Postby Kow » Fri Mar 20, 2009 3:52 pm UTC

My chem teacher used a pickle as a light bulb.
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Re: Chemistry Demonstrations

Postby BlackSails » Fri Mar 20, 2009 5:04 pm UTC

You could try making flash paper. You just need some cellulose, a nitrating agent (concentrated nitric acid) and a dehydrating agent (concentrated sulfuric acid).

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Re: Chemistry Demonstrations

Postby Minerva » Fri Mar 20, 2009 6:57 pm UTC

The-Rabid-Monkey wrote:The barking dog experiment! http://www.youtube.com/watch?v=mGMR72X8V-U
If you're feeling dangerous, Mercury Nitride is amusing. It's like Nitrogen Triiodide but much much more explosive and sensitive.


There's no such compound as "mercury nitride". Perhaps you're thinking of mercury fulminate? In which case, (a) it's not at all more sensitive than NI3.NH3, (b) it's far too dangerous to be in a school lab, simply because it's a primary explosive, and (c) you won't find highly toxic mercury salts in a school lab, either.

NI3.NH3 is about the only high explosive (a low explosive such as nitrocellulose is OK, I think) I think you can justify having in a school lab, because it's so very sensitive, it's practically impossible to acquire a large quantity of the stuff in one place, and it's not enormously powerful.

Jorpho wrote:I've heard that the "solvated electron" - sodium in liquid ammonia - is really pretty, and not just mathematically. How exactly one handles liquid ammonia is quite beyond me.


There's no way you'd be able to get away with setting up a Birch reduction in a high-school lab. Condensed liquid ammonia? With kids? This would be declared far too dangerous; it would never be permitted.

Alexius wrote:Ammonium dichromate can be very impressive. It simultaneously oxidises and reduces itself when ignited, somewhat like ammonium nitrate. The result is a huge fluffy pile of green powder.


Like all dichromates, it's also carcinogenic, meaning that it's another one of these chemicals that has been banished from high schools over recent years.
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Re: Chemistry Demonstrations

Postby V4mp » Fri Mar 20, 2009 9:31 pm UTC

Dichromates are carcinogenic? We're still using potassium dichromate regularly...
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Re: Chemistry Demonstrations

Postby meat.paste » Sat Mar 21, 2009 6:38 pm UTC

V4mp wrote:Dichromates are carcinogenic? We're still using potassium dichromate regularly...


The Cr6+ ion is the culprit. It is one of the 6 substances banned in the Reduction of Hazardous Substances (RoHS) act in the EU (~2% chance I am wrong. I have to go catch a plane, so I don't have time to check)
Huh? What?

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Re: Chemistry Demonstrations

Postby Alexius » Sun Mar 22, 2009 7:16 pm UTC

Meat.Paste and Minerva: You are somewhat wrong. The list I could find available online :http://chemistry-schools.dept.shef.ac.uk/pdf/banlist.pdf states that solid dichromates "can be used in a demonstration with suitable precautions and risk assessments". As they're carcinogenic by inhalation only, that probably means a fume cupboard.

V4mp: The list only applies to solids, not the solutions or impregnated paper that you usually use in schools. Solutions make things a lot safer- I know someone who did a school experiment 2 years ago which involved picric acid solution!

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Re: Chemistry Demonstrations

Postby V4mp » Sun Mar 22, 2009 8:07 pm UTC

Ahh ok, thats more reassuring :)

I think i have a good list here for an exciting last lesson now :D

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Re: Chemistry Demonstrations

Postby zaqwithaq » Sat May 02, 2009 6:58 pm UTC

I saw an instruction manual on making aerogel before

http://hackaday.com/2008/03/23/make-your-own-aerogel/

then you could do a demonstration like this:

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Spoiler:
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meat.paste
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Re: Chemistry Demonstrations

Postby meat.paste » Tue May 05, 2009 10:10 pm UTC

The aerogel is a cool trick, but the final step in the preparation (supercritical fluid extraction of the ethanol) is difficult / dangerous without the proper equipment. If you are happy with the solvated form, then it is easy.
Huh? What?

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Re: Chemistry Demonstrations

Postby Jorpho » Wed May 06, 2009 3:13 am UTC

meat.paste wrote:The aerogel is a cool trick, but the final step in the preparation (supercritical fluid extraction of the ethanol) is difficult / dangerous without the proper equipment.
The link suggests that said fluid extraction is not necessarily as difficult as it once was - though mucking about with tanks of compressed CO2 is hardly without its risks.
Last edited by Jorpho on Wed May 06, 2009 2:29 pm UTC, edited 1 time in total.

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Re: Chemistry Demonstrations

Postby Tass » Wed May 06, 2009 9:27 am UTC

meat.paste wrote:The aerogel is a cool trick, but the final step in the preparation (supercritical fluid extraction of the ethanol) is difficult / dangerous without the proper equipment. If you are happy with the solvated form, then it is easy.


Solvated aerogel? But thats just gel. Anybody can make gel.

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Re: Chemistry Demonstrations

Postby teacupthesauceror » Wed May 06, 2009 4:00 pm UTC

1. Screaming Jelly Baby: drop one jelly baby into boiling tube of your favourite catalyst/oxidising agent, light blue touch paper, live out sadistic fantasies.

2. "Let's See If We Can Burn the Ceiling!": run your gas tap through a beaker of washing-up liquid. When the bubbles reach a reasonable height, knock them off with a metre rule and light them with a burning splint taped to the end of said metre rule. Since methane is lighter than air (12+4x1 < 16*), the bubbles and therefore the fire should float. This is the reason all our labs have soot on the ceiling.

3. Slime: never gets old. 50 of watered down PVA glue, 10 of 1% borax, mix thoroughly and try to resisst throwing at each other.

You can also make some pretty violent polymers, one of which is like a red honeycomb structure and uses conc sulphuric acid in the making, but I've misplaced my activity sheets, and I'm not sure if your school/college allows you to even look at conc acid for fear of legal action.

Oh yes! If you mix lead nitrate and potassium iodide you make spangles, which are pretty but not explosive.

And this one is banned in the UK, but making hydrogen chloride in a plastic (not glass, as glass shatters) tube with two bungs stuck in either end. They shoot at amusing speed, apparently.

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Re: Chemistry Demonstrations

Postby teacupthesauceror » Wed May 06, 2009 4:08 pm UTC

Also, with the dichromate, I'm fairly sure we used that in our oxidation, and dissolved it ourselves. Then again, we also used conc (well, school-conc) sulfuric acid for that one, and our chemistry teacher believes safety is having goggles on and a bottle of thio lying around.

And there is no Cr6+ There is dichromate(VI), where the VI is the ox. no, but anything bigger than a 3+ ion is rare (i.e. "impossible" until degree level), and 6+ is all kinds of silliness. Although seeing as ox. nos are linked to ion charges anyway, it's easy to get confused.

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Re: Chemistry Demonstrations

Postby oxoiron » Wed May 06, 2009 6:18 pm UTC

teacupthesauceror wrote:And there is no Cr6+ There is dichromate(VI), where the VI is the ox. no, but anything bigger than a 3+ ion is rare (i.e. "impossible" until degree level), and 6+ is all kinds of silliness. Although seeing as ox. nos are linked to ion charges anyway, it's easy to get confused.
Apparently, I'm easily confused. Please, explain to me how chromate and dichromate don't contain Cr6+.
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Re: Chemistry Demonstrations

Postby teacupthesauceror » Wed May 06, 2009 6:46 pm UTC

oxoiron wrote:
teacupthesauceror wrote:And there is no Cr6+ There is dichromate(VI), where the VI is the ox. no, but anything bigger than a 3+ ion is rare (i.e. "impossible" until degree level), and 6+ is all kinds of silliness. Although seeing as ox. nos are linked to ion charges anyway, it's easy to get confused.
Apparently, I'm easily confused. Please, explain to me how chromate and dichromate don't contain Cr6+.


They are covalently bonded. Altogether they're an ion, but the Chromium has an oxidation number of +6, not a charge, as it is covalently bonded with the oxygen. Like I say, easily mistaken for each other, expecially as you don't expect metals to be covalently bonded.

EDIT: I also refer you to the bottom of your own wikipedia page, so there, nyer nyer nyi nyer nyer :P
Last edited by teacupthesauceror on Wed May 06, 2009 6:55 pm UTC, edited 1 time in total.

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Re: Chemistry Demonstrations

Postby Technical Ben » Wed May 06, 2009 6:53 pm UTC

The-Rabid-Monkey wrote:The barking dog experiment! http://www.youtube.com/watch?v=mGMR72X8V-U
If you're feeling dangerous, Mercury Nitride is amusing. It's like Nitrogen Triiodide but much much more explosive and sensitive.


In that video they keep liquid oxygen in a THERMOS FLASK! Is that safe?
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Re: Chemistry Demonstrations

Postby Jorpho » Wed May 06, 2009 7:05 pm UTC

Technical Ben wrote:
The-Rabid-Monkey wrote:The barking dog experiment! http://www.youtube.com/watch?v=mGMR72X8V-U
If you're feeling dangerous, Mercury Nitride is amusing. It's like Nitrogen Triiodide but much much more explosive and sensitive.


In that video they keep liquid oxygen in a THERMOS FLASK! Is that safe?
It has a label! It must be safe!

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Re: Chemistry Demonstrations

Postby oxoiron » Wed May 06, 2009 7:38 pm UTC

teacupthesauceror wrote:
oxoiron wrote:
teacupthesauceror wrote:And there is no Cr6+ There is dichromate(VI), where the VI is the ox. no, but anything bigger than a 3+ ion is rare (i.e. "impossible" until degree level), and 6+ is all kinds of silliness. Although seeing as ox. nos are linked to ion charges anyway, it's easy to get confused.
Apparently, I'm easily confused. Please, explain to me how chromate and dichromate don't contain Cr6+.
They are covalently bonded. Altogether they're an ion, but the Chromium has an oxidation number of +6, not a charge, as it is covalently bonded with the oxygen. Like I say, easily mistaken for each other, expecially as you don't expect metals to be covalently bonded.
Maybe you don't expect metals to be covalently bonded, but I know hundreds of chemists (myself included) who work day in and day out with covalently bonded metals. I'm still trying to figure out what you are saying about Cr6+ not existing, when you state that Cr(VI) does exist. Are you trying to say that Cr(VI) and Cr6+ are different?
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Re: Chemistry Demonstrations

Postby teacupthesauceror » Wed May 06, 2009 7:51 pm UTC

oxoiron wrote:
teacupthesauceror wrote:
oxoiron wrote:
teacupthesauceror wrote:And there is no Cr6+ There is dichromate(VI), where the VI is the ox. no, but anything bigger than a 3+ ion is rare (i.e. "impossible" until degree level), and 6+ is all kinds of silliness. Although seeing as ox. nos are linked to ion charges anyway, it's easy to get confused.
Apparently, I'm easily confused. Please, explain to me how chromate and dichromate don't contain Cr6+.
They are covalently bonded. Altogether they're an ion, but the Chromium has an oxidation number of +6, not a charge, as it is covalently bonded with the oxygen. Like I say, easily mistaken for each other, expecially as you don't expect metals to be covalently bonded.
Maybe you don't expect metals to be covalently bonded, but I know hundreds of chemists (myself included) who work day in and day out with covalently bonded metals. I'm still trying to figure out what you are saying about Cr6+ not existing, when you state that Cr(VI) does exist. Are you trying to say that Cr(VI) and Cr6+ are different?


I'm sorry if I insulted your intelligence; I tend to assume people have the same level of knowledge as myself. Yes, I am saying they are different. You wouldn't call the Oxygen in H2O O2- (or maybe you would, A-level chemistry tends not to, as it is a covalent bond), so I reserve the right to call the Chromium in chromates Chromium(VI) instead of Cr6+, because it cannot be isolated as Cr6+ without insane amounts of energy. And the charge on the Chromium is not, in fact, 6+, as the electrons are shared with oxygen, even if oxygen is more electronegative and gets more than its fair share, the electrons are still spread over the chromium as well, due to the weirdness that is quantum mechanics.

And it was an impersonal you, get over yourself because "one" just sounds silly on the internet.

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Re: Chemistry Demonstrations

Postby d0nk3y_k0n9 » Wed May 06, 2009 8:58 pm UTC

My chemistry teacher is going to have us (probably me) blow up an egg with hydrogen next week, after we take the AP test. Apparently, she doesn't have enough hydrogen to fill a large enough balloon to be exciting and she doesn't want to generate more, so I get to hollow out an eggshell and we're blowing that up instead. Should I be at all concerned about shrapnel?

Another fun one we did is to take Teddy Grahams and dip them into liquid nitrogen, then take them out, shake them off, and eat them. The really cold temperature causes the water vapor in your mouth to condense, and you can see your breath despite being indoors at normal room temperature. Also, eating freezing cold Teddy Grahams is fun.

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oxoiron
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Re: Chemistry Demonstrations

Postby oxoiron » Wed May 06, 2009 9:06 pm UTC

teacupthesauceror wrote:
oxoiron wrote:Are you trying to say that Cr(VI) and Cr6+ are different?
Yes, I am saying they are different. You wouldn't call the Oxygen in H2O O2- (or maybe you would, A-level chemistry tends not to, as it is a covalent bond), so I reserve the right to call the Chromium in chromates Chromium(VI) instead of Cr6+, because it cannot be isolated as Cr6+ without insane amounts of energy. And the charge on the Chromium is not, in fact, 6+, as the electrons are shared with oxygen, even if oxygen is more electronegative and gets more than its fair share, the electrons are still spread over the chromium as well, due to the weirdness that is quantum mechanics.
Cr(VI) and Cr6+ are the same thing, and yes, I would call oxygen in H2O, O2-. I would also call chromium in CrO42-, Cr6+. While the oxygen and chromium atoms are covalently bonded, the chromium atom has virtually no electron density in its d-orbitals. It's the same thing as the metal atoms in my avatar.

My avatar has a +2 charge. There is one alkoxo ligand (-1), one phosphinato ligand (-1) and one bidentate peroxo ligand (-2). This means that to balance the charge, each iron atom has to be Fe(III), unless you want to posit a mix-valent system (highly unlikely to exist with this ligand set). Now, this is where you tell me that they are not formally Fe3+ because they are sharing electrons with the N and O ligands. Yes, I agree that they are sharing electrons, but they still have only five d-electrons, meaning they are formally Fe3+ ions. This can be shown using a variety of spectroscopic techniques, including:

1) XAS (edge energy)
2) Paramagnetic 1H NMR (not 31P NMR, because the phosphorus atom is too close to the iron centers, so the paramagnetic shift is unmeasurable)
3) Mossbauer spectroscopy (isomer shift)
4) EPR (if not anti-ferromagnetically coupled)
5) Magnetic circular dichroism
6) Others not on this list

All of these methods will reveal that each iron atom has only five d-electrons. The iron exists as +3 ions, even though they are covalently bonded to six other atoms. I'm not trying to be insulting or dismissive, but I think you need to look into this issue in more detail before making blanket pronouncements regarding the unavailability of Cr6+. If you have enough background knowledge to understand these, I suggest reading Physical Methods in Bioinorganic Chemistry, edited by L. Que, Jr. and/or Physical Methods for Chemists by R. Drago. If you can't understand that material, take a physical inorganic chemistry course and then decide if you still believe high-valent metals can't exist in ligand frameworks.
"Whenever you find yourself on the side of the majority, it is time to reform (or pause and reflect)."-- Mark Twain
"There is not more dedicated criminal than a group of children."--addams


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