April 4 2012 4 04 /04 /April /2012 15:39
This experiment involves reduction of copper(II) oxide, a black water-insoluble powder, with magnesium metal. The reaction is CuO + Mg -> MgO + Cu. The reaction is quite exothermic. Do this away from all flammable materials. Do not get too close to the reaction as burning metal can spray all over with larger quantities. This reactions is commonly known as a thermite.
Grind a magnesium rod or bar into shavings or, better yet, powder using a knife or a file.
Precipitate copper(II) carbonate from a solution of a copper(II) salt by adding sodium bicarbonate. Filter, wash, and dry by heating. It will turn black as it decomposes to copper(II) oxide. If you just have copper metal, electrolytically oxidize the copper in a sodium bicarbonate solution to form copper(II) carbonate. Heat this paste after filtering and drying.
Using a mortar and pestle, grind equimolar amounts (approximately 3 g CuO to 1 g Mg) of these two chemicals together until the mixture is uniform.
Place on a metal surface in a small pile. Ignition using a ferrocerium flint can be dangerous as the hands will be very close to the mixture. A better method of ignition would be electric ignition using a very thin piece of titanium foil and a nine volt battery or a magnesium ribbon.
Despite the dangers, I used a ferrocerium flint to ignite the mixture. Since there was some wind, I was afraid that the thermite mixture would blow away after placing it out in the open piece of metal. Therefore, I ignited it right away.
If you will try this at home, please be more careful than I was.
The "Shoe for Comparison" picture malfunctions in the video.
I then tried a copper(II) sulfate reduction. Aqueous copper sulfate reacts quite vigorously with magnesium metal, but the crystals react yet more vigorously. I shredded some magnesium and ground the copper(II) sulfate - magnesium shaving mixture together with the end of a pen. I then placed it on the same metal dish in a small pile and added some more magnesium shavings on top. Since grinding magnesium is so difficult, I hardly produced enough magnesium on top to ignite the mixture, so I had to strike the ferrocerium for a couple minutes before I actually got it ignited. The top layer burnt with a dazzling white flame, but the bulk was still unburnt. It would not ignite. Here is the video of the ignition.
I retried this with more finely powdered magnesium and dehydrated copper(II) sulfate. The result was much better.