Celebrating the 90s - 50th Celebration of Chem 13 News
I begin by asking my class, “Does anybody know what I have in this flask?” The students shuffle their feet and glance at each other, trying to see if anybody’s taking notes. “You know, I could have anything in here, anything at all”, I continue while holding up the flask. “This could be Aladdin’s Lamp.”
Definitely not even worth watching. Then, all of a sudden, with a WHOOSH, a two-metre cloud of steam begins pouring from the flask. The students’ eyes widen, and they start to laugh and nudge each other. Not bad. Not bad, at all.
Aladdin’s Lamp is an example of the startling effect produced by rapid decomposition of hydrogen peroxide, catalyzed by iodide ions. It’s one of the fascinating experiments described in Chemical Demonstrations, Volume 2, published by the American Chemical Society.
In this case, the net reaction is:
2 H2O2(l) → 2 H2O(l) + O2(g) + heat
It’s believed to be a two-step mechanism, involving the production of IO- ions, which are consumed in the second step:
H2O2 + I- → H2O + IO-
H2O2 + IO- → H2O + O2 + I-
How do you do it? Empty a tea bag and refill it with ¼ teaspoon of fine granular KI. Suspend the tea bag in the neck of a 500 mL flask, using a stopper to hold the string. The flask contains 50 mL of 30% H2O2. When the stopper is removed the tea bag falls into the peroxide. First the KI dissolves to supply I-, so there’s a delay before the steam appears. But once the reaction begins, there’s plenty of heat available to boil the water and to speed the reaction.
A brief consideration of the enthalpies of formation of the participants and the use of Hess’ Law shows that the decomposition liberates about 82.0 kJ/mol.
Our 50 mL of 30% H2O2 contains about 0.5 mol of H2O2, so we can make a rough estimate of the amount of steam produced. Ignoring elevation of the boiling point and the energy required to heat the flask, and taking the specific heat of pure H2O2 as 1.3 J g-1 ⁰C-1, I calculate that 14.5 kJ are needed to heat the reagents to boiling.
The reaction generates 39.4 kJ, so 24.9 kJ are available to make steam at 100 oC. About 11 g of steam would be produced, and this occupies about 19 L at 100 ⁰C.