Classical chemical volcano refers to the eruption of ammonium dichromate [(NH4)2Cr2O7] in a classical chemistry illustration. The reaction is termed as one of the classics of chemistry demonstrations. During the presentation, ammonium dichromate glows and sparks due to decomposition. The decomposition of ammonium dichromate begins at 1800C but becomes self-sustaining at 2250C since it is exothermic. Cr+3 ions act as the oxidant whereas N3-
is the reducing agent in the reaction. The reaction is termed as ‘classical chemical volcano’ since it resembles a volcanic eruption and the products builds a mountain and produces sparks. The end product is substantial amounts of chromium (III) oxide ash, which is green in color.
Materials and apparatus required
Ammonium dichromate solid
Sand tray/ ceramic tile
Bunsen burner
Box of matches
Ethanol.
Fume chamber
Large spatula
Tongs
Note: The experiment can be performed either in a lightened or darkened room.
Procedure
When performed in a fume chamber:
1. Form a volcanic cone (pile) on the ceramic tile/ sand tray
2. Using a Bunsen burner, heat the tip of the volcanic pile until the reaction starts. Alternatively, the tip of the pile can be dampened with ethanol and lit using a match.
When the experiment is performed in open room:
1. Put the ammonium dichromate into a large flask.
2. Cover the flask with a filtration funnel to prevent chromium oxide from escaping when heating starts.
3. Heat the bottom of the flask until decomposition begins.
Results
Ammonium dichromate decomposes to produce a large amount of ash, which resembles a volcanic eruption. The solid forming the mountain is dark green ash and contains sparks. Besides, steam and a colorless gas are also released during decomposition.
Discussion
The orange crystals of ammonium dichromate decompose exothermically to produce sparks. Heating is required to start up the reaction but the reaction becomes self-supporting after heated to a sufficient temperature. External energy applied allows overcoming the barrier by activation energy, therefore allowing decomposition to continue spontaneously. As a result, voluminous green chromium (III0 oxide (ash), steam and nitrogen are produced. During the reaction, the dichromate ions oxidize the ammonium ions to form nitrogen gas and water. On the other hand, chromium in the dichromate is reduced from the oxidation of +6 to the oxidation of +3 in the chromium trioxide. The reaction occurs as shown in the equation below:
(NH4)2Cr2O7 (s) → Cr2O3 (s) + 4H2O (l) + N2 (g)
The experiment is carried out in a fume chamber or well-ventilated area since the chemicals involved in the reaction are irritants. Chromium (III) oxide and chromium (VI) compound are irritants especially to the skin, eyes, and respiratory system in case of contact. Also, ammonium dichromate is suspected to be carcinogenic and therefore should be handled when one is wearing gloves and safety goggles.
Conclusion
We can term ammonium dichromate as an unstable and reactive compound which decomposes easily. This occurs since the salt anion has an atom of high oxidation state which has the capability of oxidizing the nitrogen atom from the ammonium cation. All hexavalent chromium salts such as ammonium chlorate and ammonium perchlorate have the capability to undergo the same. Also, ammonium nitrate can undergo classical chemical volcano. It is, however, necessary to carry out the demonstration in a fume chamber or well-ventilated area so as to avoid inhalation or coming into contact with the chemicals due to their toxicity nature.
