"Serpents in the grass" reaction

[5614]

If you go to here:

http://www.webelements.com/webelements/elements/text/Cr/key.html

(it's data about Cr, but that's not really what this is about)

 

And scroll down a bit to the "WebElements Movie" there's an interesting video given to us by Sheffield Uni.

 

The picture above [video/image on site][/i']shows the result of burning ammonium dichromate, (NH4)2Cr2O7, containing pellets of mercuric thiocyanate (HgCNS)

I would like to know more about this reaction, focusing on the why part of the 'snakes' being formed (watch the video if you don't know what I mean).

 

I know that traditionally ammonium dichromate makes a volcano effect which is visible here and I'm guessing there were 3 "mercuric" (wouldn't it just be mercury?) thiocyanate pellets as there were 3 'snakes'.

 

Questions:

 

1) Why do the snakes form? What would the chemical formula of the 'snakes' be?

 

2) It says this is a dangerous experiment, I see it says this on basically every experiment, but is there actually anything seriously dangerous here, like some gas produced?

 

Or I spose if anyone can give me the full equation of the whole reaction I could answer that for myself!

 

I know we're dealing with 2 toxic chemicals and the ammonium dichromate is corrosive/oxidising, but I know that so I can work with it, I was wondering if there were other dangers e.g. poisonous gas being produced etc.

 

I found:

[math](NH_4 )_2 Cr_2 O_7 --> N_2 + 4H_2 O + Cr_2 O_3 [/math]

but that doesn't include the HgCNS side of things

[akcapr]

perhaps some cyanide is realeased along wit hthe gases that form the faomyness

[budullewraagh]

well we know that

(NH4)2Cr2O7 --> Cr2O3 + 4 H2O + N2

the oxygen of the dichromate attack the hydrogen of the ammonium. so...

 

im pretty sure the green stuff is chromic oxide. perhaps the sulfur attacks the ammonium? perhaps the sulfur is attacked by the oxygen? i really dont know

[YT2095]

for the mercury thiocyanate, the foam is composed of carbon and mercury sulphide, mercury vapor is also given off, so be Very careful where you do this!

[woelen]

If you perform the experiment with ammonium dichromate only, then the products formed are not that toxic. However, with this reaction one must be careful as well, because of the fact that not all ammonium dichromate reacts. The green stuff (which is Cr2O3 and which is only slightly toxic on its own) usually is contaminated with remains of (NH4)2Cr2O7 and the latter is very toxic. Dichromate ion, Cr2O7(2-), is a carcinogen.

 

If mercury thiocyanate, Hg(SCN)2, is mixed in, then the reaction becomes much more dangerous. The serpents consist of a mercury compound of unknown and variable composition and no single formula can be given to describe that compound. Besides that, fumes of metallic mercury and volatile mercury compounds are released into the air. This makes the experiment absolutely unfit for demonstration in confined spaces without a good fume hood.

[5614]

the foam is composed of carbon and mercury sulphide, mercury vapor is also given off, so be Very careful where you do this!

 

fumes of metallic mercury and volatile mercury compounds are released into the air

 

Okay, I think it was kinda good that I asked first!!! Thanks a lot.

 

YT, by the "foam" do you mean the 'snakes'? (assuming yes) why do these carbon/mercury sulphide 'snakes' form? Like obviously it's a reaction, but why is this unusual snake like product formed?

(I can imagine a long carbon chain, not so familiar with HgS.)

[YT2095]

I`m not entirely sure Why it does it either, the gasses that are formed space the carbon out into this foam/snake, but as to why it sticks together in one shape and not just turn directly into a powder or something I`ve not the remotest idea, sorry dude.

[5614]

No problem, the info you have given me is great... you chemistry guys just like look at reactions and can tell soo much about them, it really is impressive, thanks a lot.