Ammonium dichromate is an orange colored crystalline substance, salt of “ammonium hydroxide” and dichromic acid. Because dichromic acid is a powerful oxidizing agent (oxidation state of chromium is +6) and oxidation state of nitrogen atom in ammonia is – 3, it can be imagined that ammonium dichromate is an unstable and reactive compound, that decomposes easily. The same conclusion can be made in case of ammonium nitrate, ammonium perchlorate, ammonium chlorate and other similar compounds. In all these cases, the salt anion has an atom in a high oxidation state which is capable to oxidize nitrogen atom from ammonium cation, and vice versa. In the case of ammonium dichromate, ammonium nitrogen is oxidized to elemental nitrogen, while chromium atom is being reduced to trivalent state, according to the following equation:
As can be observed from the reaction enthalpy, the reaction is exothermic, which means that energy in the form of heat is released in the reaction. Although this is energetically/thermodynamically feasible, it is necessary to initiate the decomposition of ammonium dichromate by applying some external energy. This energy need to be enough to overcome the activation energy barrier, after which the decomposition reaction continues spontaneously.
The volcano effect can be realized if we analyze the products of the reaction. Namely, elemental nitrogen and water produced in the reaction are both in gaseous state, while chromium(III) oxide is a green solid. The liberation of nitrogen and water vapor agitate the incandescent particles of chromium(III) oxide, which gives an effect of a volcano eruption.
Chromium(III) oxide that is formed in this reaction is very voluminous, so the volume is much greater at the end of the reaction than in the beginning.
All hexavalent chromium salts are highly toxic and carcinogenic, and it is necessary to conduct the whole experiment in a fume hood.