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Author Topic: Why does Mercury(II) thiocyanate decompose the way it does?  (Read 34732 times)

Offline thedoc

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Donny Kester  asked the Naked Scientists:
   
Hello, Naked Scientists! Could you explain WHY  Mercury(II) thiocyanate decomposes in the way it does? It's horrifying to behold!

How can something so large form from a powdery substance? You'd think it would be really thin and brittle, but it'sinsolublein water.

Would the same thing occur if, say, you ignited a barrel full of the stuff?

(It explains it, I think, but I don't understand it. At all, really.)
http://www.chem-toddler.com/redox-reactions/pharaohs-snakes.html

Thanks for your time!
-Donny from Indiana

What do you think?
« Last Edit: 13/07/2012 21:30:01 by _system »


 

Offline evan_au

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Some of the final products of the reaction are gases, like SO2 and CO2. At room temperature, these gases take up a much larger volume than the initial powder. A quick calculation suggests that 1 mole of the powder (weighing 316 grams) will produce around 2.5 moles of gas (taking up 56 litres).

If you cut up the "snakes", you will probably find that they have many internal bubbles, like a solidified foam.

This reaction releases energy when the powder reacts with oxygen in the air, so it will work better if it is spread out loosely as shown in the video, rather than being compacted in a closed container like a barrel.

Be careful - exposure to mercury vapour can cause brain damage.
 

Offline evan_au

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PS: Correction
The volume of a mole of gas is around 24.5 litres at 25C and 1 atmosphere pressure. http://en.wikipedia.org/wiki/Molar_volume#Ideal_gases

However, the volume is much greater at the temperature of the flame, and mercury will also be a gas following the reaction.

As for solubility in water, water is rather selective in the things it dissolves. Water is a "polar" molecule, which means that it has a small electric charge. Water tends to dissolve polar molecules much better than non-polar molecules.
The water molecule is shaped like a boomerang, and this asymmetry gives it a charge separation that linear molecules like CO2 do not have.
Water easily dissolves ionic solids like salt, Na+Cl- or organic molecules which also have a small charge separation, like alcohol. However, organic molecules like octane do not have this charge separation and do not dissolve nearly as well.

I assume that the C3N4 matrix is a covalently bonded solid, which would have low solubility in water.

 
 

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