LECTURE DEMOS
Dichromate Decomposition

The Decomposition of Ammonium Dichromate

A simple exercise in stoichiometry

If you search the web for the phrase "dichromate volcano," you will find many references to this classic demonstration of reaction stoichiometry. Some are good, some are less so; some have movies of the demo, some do not. A particularly good one is on the site for the American Chemical Society's Division of Chemical Education's journal, The Journal of Chemical Education. (But don't try the movie link on this page. It doesn't actually work. They want you to buy their video! There are several good still photos, however, along with other questions and discussions about this reaction that won't make sense to you now, but will by the end of Chem 5!)

In this reaction, the orange crystalline (and toxic - it contains chromium in the carcinogenic "chromium(VI)" ion state made famous in the film Erin Brockovich) compound, ammonium dichromate, (NH₄)₂Cr₂O₇, spontaneously decomposes when heated strongly to generate nitrogen gas, N₂, gaseous water, H₂O, and the green crystalline compound chromium(III) oxide, Cr₂O₃.

The lecture handout walked us through the stoichiometric logic behind the balanced net reaction and what it could tell us about the final mass of solid Cr₂O₃ product given an initial amount of (NH₄)₂Cr₂O₇. Here, we look at the compounds involved a bit more.

First, the easy ones: water and nitrogen. Exactly why nitrogen gas is N₂ and not N, or N₃, etc., is a topic for Chem 6, as is why water is H₂O. For now, we can say that at least H₂O makes sense from what we have mentioned about the Periodic Table so far: oxygen is commonly found as the oxide ion O^2– and hydrogen is in the same column as metals like Na that form cations with a +1e charge: H⁺. While conservation of charge applied to these facts suggests that water should have the stoichiometry it does, the reasoning is flawed. In the water molecule, there is not the electron transfer from H to O that this argument implies. Water is a "covalent" compound, not an "ionic" compound. (See section 2.9 in the text on nomenclature.)

Chromium oxide, however, is a simple ionic compound along the lines of the iron oxides we discussed in class. Just as for Fe₂O₃, in which the Fe ion was iron(III), Fe³⁺, so too is chromium in the chromium(III) state in Cr₂O₃.

Finally, let's take apart ammonium dichromate. As the name and the way the formula is written both suggest, there is an "ammonium" part and a "dichromate" part. Ammonium we know to be NH₄⁺, and there are two such cations in the formula: +2e total. Thus, to conserve charge, the "dichromate" ion must be an anion with a net charge of –2e: Cr₂O_7^2–. Now that we have this figured out, we can deduce that chromium really is "chromium(VI)" as claimed above. Each oxygen carries the usual –2e charge, and there are seven of them, for a total of –14e. The entire dichromate anion has a net charge of –2e; so, the two chromium ions must provide a total charge of +12e, or +6e each: Cr(VI), as advertised!