30% hydrogen peroxide is added to a glass cylinder containing a concentrated aqueous mixture of potassium iodide and dishwashing soap. The iodide ion catalyzes the decomposition of hydrogen peroxide, generating oxygen gas which causes the soap to foam up and shoot up out of the cylinder.
This demonstration is usually used to illustrate catalysis in a kinetics unit. It could also be used as an example of a disproportionation reaction. Allow about 10 minutes for this demo.
This reaction is particularly effective not only because it is amusing, but any student who has put household 3% hydrogen peroxide on a cut or scrape is familiar with the catalytic decomposition of the substance:
2 H2O2(aq) --> 2 H2O(l) + O2(g)
In the case of cuts and scrapes, the catalyst is an enzyme called catalase which is present in our tissue.
In this demonstration it is the iodide ion which catalyzes the decomposition.
It should be noted that oxygen is both oxidized and reduced during this reaction, going from a -1 oxidation state in the peroxide to -2 in water and 0 in elemental oxygen. This is an example of disproportionation.
H2O2(aq) + 2I-(aq) --> 2 OH-(aq) + I2(s)
- large glass cylinder
- small stoppered flask containing 50 ml of 30% hydrogen peroxide solution
- small stoppered flask containing KI/soap solution
- large basin to catch overflow
- neoprene gloves
Place the glass cylinder in the basin. Pour the KI/soap solution into the cylinder. Wearing the gloves, carefully but quickly add the hydrogen peroxide solution to the cylinder. A steaming column of suds spurts out of the top of the cylinder.
30% hydrogen peroxide solution is a strong oxidizer. Splash goggles and neoprene gloves should be worn during this demonstration.
To prepare the KI/soap solution, dissolve 5 g of KI in a minimal amount of water in a 50 mL Erlenmeyer flask. Add about 10 mL of dishwashing liquid and swirl to mix. Be careful handling the 30% hydrogen peroxide. Be careful during clean-up, too; there could be a lot of unreacted hydrogen peroxide in the foam.