Answer to Question #33407 in Inorganic Chemistry for anissa

Question

hello,
I have seen a video of an experiment NH4Cr2O7 + HgSCN ignited by fire the result was a kind of tentacle comming out of it, could you please explain to me how this happens.
Thank you

Answer 1

The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion:

(NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4H₂O(g) (ΔH = −429.1 ± 3 kcal/mol)

Like the well-known explosive ammonium nitrate, it is thermodynamically unstable. Its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered chromium(III) oxide. Not all of the ammonium dichromate decomposes in this reaction. When the green powder is brought into water a yellow/orange solution is obtained from left over ammonium dichromate.

Observations obtained using relatively high magnification microscopy during a kinetic study of the thermal decomposition of ammonium dichromate provided evidence that salt breakdown proceeds with the intervention of an intermediate liquid phase rather than a solid phase. The characteristic darkening of (NH₄)₂Cr₂O₇ crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to Cr₃O₁₀^2-, Cr₄O₁₃^2−,etc., ultimately yielding CrO₃. The CrO₃ has been identified as a possible molten intermediate participating in (NH₄)₂Cr₂O₇ decomposition.

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. When the compound is in the presence of a strong enough heat source, a rapid exothermic reaction is started which produces a large mass of coiling serpent-like solid. An inconspicuous flame which is often blue but can also occur in yellow/orange accompanies the combustion. The resulting solid can range from dark graphite grey to light tan in color with the inside generally much darker than the outside. This property was discovered by Wöhler in 1821, soon after the first synthesis of mercury thiocyanate: "winding out from itself at the same time worm-like processes, to many times its former bulk, a very light material the color of graphite...". For some time, a firework product called "Pharaoschlangen" was available to the public in Germany, but was eventually banned when the toxic properties of the product were discovered through the death of several children mistakenly eating the resulting solid.