In 1866 Marcellin Berthelot was experimenting
with acetylene gas in various metal
salts including silver nitrate. What he
found was that as the gas bubbles through
the solution of ammonical silver
nitrate a white solid was produced. He
noticed that upon drying it became
extremely explosive. He called it
L'acétylure d'argent – silver acetylide.
Pure silver acetylide
is an unusual explosive
because in principle it produces no
gas when it explodes. The bang it
produces is almost completely down to the
rapid transfer of a lot of heat to the
surrounding air which in turn will
rapidly expand. To produce it safely in a
school environment we have to be careful
not to produce double salts which
produce gases as well
adding to the power of the explosion. So
its of the utmost importance that we
don't add the acetylene directly to the
silver nitrate and lazily chucking
calcium carbide into the solution
directly is a bad plan too. To make sure
we're only producing the single salt, a
multi-tube set up is used. In the first
tube there's water. Here calcium carbide
reacts with water to generate acetylene gas.
The gas is then bubbled through copper
sulfate solution to remove any hydrogen
sulfide impurities and in to a tube
containing ammoniacal silver nitrate to
form the white precipitate of silver
acetylide. Well I say white, it's sort of
white.
In the UK without an explosives certificate
it's illegal to generate more than half
a gram of any explosive so we're going
to use the concentration of silver
nitrate to control the mass of
acetylide we can produce. You make up the
ammoniacal silver nitrate solution by
adding 1 molar solution of sodium
hydroxide dropwise to 15 ml of a
0.25 molar silver nitrate solution
until the precipitate of silver oxide
forms.
Add a 2 molar ammonia solution drop-wise
stirring thoroughly until the precipitate
re-dissolves to form a colorless
solution. The filtered solid is stable
enough when moist so it can be safely
moved to a gauze over a Bunsen for
heating.
Once it dries out the bangs begin
Cleanup can be a pain in the neck
because during the reaction bits of
acetylide get thrown out, dry, and in a few
hours later is detonated as somebody walks
past. That's why some fine gauze like this
steel insect screen can be handy for
catching strays.
Once you're done, the area around the
experiment can be cleaned up by passing
over it with a Bunsen flame. If
you're not using the insect screen like
I've used a large sheet of hardboard is
super useful for this purpose so you
don't wreck your table surfaces when you
pass a Bunsen over them. You can
find further information and technician
sheets to support this demonstration on
the all-new Education in Chemistry
website. So head over there and check it
out.