# Looking for an isomerization reaction that involves a color change for classroom demonstration purposes

I am in the process of designing a chemical kinetics course for undergraduates. I would like to demonstrate isomerization equilibria, and therefore am looking for a reaction A <-> B fulfilling the following criteria:

1. At least one of the reactants has a color;
2. The equilibrium is reached in a few minutes so that the color change is easy to follow (not too fast or slow);
3. The reaction is safe and could be demonstrated in a beaker on the table;
4. The reaction is reversible and e.g. by gentle heating the equilibrium could be shifted easily.

Titration of some weak acids/bases with suitable indicators might also be good, but of course these are not isomerisations and are a bit more complex than a simple A <-> B reaction.

Any creative ideas are welcome. Thanks!

• If you can use gases, how about dimerisation of $\ce{NO2}$ to $\ce{N2O4}$ (decreasing temperature). – Jan Sep 30 '15 at 18:24
• The photoisomerization / thermoisomerization of various azobenzenes dyes seems like a place worth investigating. It could be tough to find a combination of "long" (several minute" equilibration times, aqueous solubility, and a molecule you can buy easily, but there is probably one somewhere. – Curt F. Sep 30 '15 at 18:25
• @Jan Good idea with NO2 but there are safety concerns. We had a setup at the university: two sealed glass bulbs with NO2 in them, connected by a glass tube. Warm one of the bulbs in a water bath, put the other bulb in an ice bath... looked nice. – Laryx Decidua Oct 1 '15 at 8:27
• @CurtF: thanks for the tip, I'll be looking into it and report back. – Laryx Decidua Oct 1 '15 at 8:27
• You might use thermochromic cobalt chloride in a water/ethanol (or isopropanol) mixture... though this color change is due to complex formation and change in the amount of water of hydration. See chemistry.about.com/od/colorchemistryprojects/a/…. – DrMoishe Pippik Oct 1 '15 at 21:06