The Ammonium Dichromate "Volcano" Reaction

This reaction involves the decomposition of ammonium dichromate. Nothing is required except heat to start the reaction; in this case a bunsen burner was used. The reaction throws off sparks and resembles an erupting volcano; the result is a pile of gray-green chromium(III) oxide. If you listen to the audio you can hear the hiss of gases escaping; this is the nitrogen gas and water vapor produced.

This reaction works becuse ammonium dichromate contains an oxidizing agent and a reducing agent in the same compound. The ammonium ion is a reducing agent; the production of nitrogen gas (where nitrogen has an oxidation state of 0) from the ammonium ion (where nitrogen has an oxidation state of -3) involves oxidation of the nitrogen. The dichromate ion is an oxidizing agent and is itself reduced; in this reaction the dichromate ion (where chromium has an oxidation state of +6) is reduced to chromium (III) oxide (where chromium has an oxidation state of +3). We can write the overall balanced equation for the reaction as shown below:

(NH₄)₂Cr₂O₇(s) = Cr₂O₃(s) + N₂(g) + 4H₂O(g)

Start of the reaction - a pile of orange ammonium dichromate	Starting the reaction with a bunsen burner
Reaction has started	The finished reaction

Caution: Reactions such as this should only be performed by qualified personnel. This demonstration has fallen into disfavor because chromium(VI) compounds have been deemed carcinogenic. It has been banned in many school districts. This reaction should be carried out only in a fully functional fume hood.

Download the video here
(Windows AVI format, Microsoft Video9 Compression)

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