chemical volcano

One of the most spectacular chemical reactions is the process of decomposition of ammonium dichromate (VI) (NH4) 2Cr2O7, known as the "chemical volcano". During the reaction, a large amount of porous substance is released, ideally imitating volcanic lava. In the early days of cinema, the decomposition of (NH4)2Cr2O7 was even used as a "special effect"! Experimenters wishing to conduct the experiment are asked not to do it at home (due to the release of flying dust that can pollute the apartment).

To perform the test, you will need a porcelain crucible (or other heat-resistant vessel) filled with ammonium (VI) dichromate (NH₄)₂Cr₂O₇ (photo 1). Place the crucible on top of a mound of sand simulating a volcanic cone (Pic 2) and light the orange powder with a match (Pic 3). After some time, a rapid process of decomposition of the compound begins, leading to the release of a large amount of gaseous products, which disperse the porous chromium oxide (III) Cr.₂O₃ (photos 4, 5 and 6). After the end of the reaction, everything around is covered with dark green dust (photo 7).

The ongoing decomposition reaction of ammonium dichromate (VI) can be written by the equation:

The transformation is a redox reaction (the so-called redox reaction), during which the oxidation state of the selected atoms changes. In this reaction, the oxidizing agent (a substance that gains electrons and reduces its oxidation state) is chromium (VI):

The reducing agent (a substance that donates electrons and, therefore, increases the degree of oxidation) is nitrogen contained in the ammonium ion (we take into account two nitrogen atoms due to N₂):

Since the number of electrons donated by the reducing agent must be equal to the number of electrons accepted by the oxidizing agent, we multiply the first equation by 2 on both sides and balance the number of remaining oxygen and hydrogen atoms.