Having a blast with nitrogen triiodide

Common ammonia and iodine are combined to make a seemingly inert dark brown paste. But when the paste dries, everything falls apart.

Ingredients: ammonia, iodine, potassium iodide, funnel, filter paper

Procedure: A complete recipe follows.

1. Mix ammonia, iodine and potassium iodide in water.

2. Drain the solution through a funnel with filter paper to collect the reddish-brown precipitate.

3. Spread the precipitate thin on smallish sheets of paper.

4. Separate the papers and allow to dry. The dry papers will explode on contact.

Understanding: This is a wonderful experiment that allows for the safe manufacture of an explosive. The reaction is

NH3(aq) + 3 I2(aq) → NI3(s) + 3 HI(aq)

where the reddish-brown precipitate is nitrogen triiodide. The wet paste is very stable. However, when it is dry, it becomes highly unstable. In fact, one can detonate the dry explosive with a light touch, a loud noise, a gentle breeze, or a feather. And there is the important characteristic shared by so many explosives, that a small volume of reactant (typically solid or liquid) leads to a large volume of product (typically gaseous).

2 NI3(s) → N2(g) + 3 I2(g)

This is an example of a reaction with a very large equilibrium constant. The products are favored by the great increase in disorder (or entropy) upon the formation of gaseous product. The reverse reaction has a very small equilibrium constant!

The wet paste is very stable, making it possible to paint it on objects such a door knobs, door frames, floors, and chairs. Of course, we advise you not to! However, most chemists can tell you stories of how they either set or fell for a prank involving nitrogen triiodide.

Be aware that the explosion will create a great deal of iodine dust that can stain clothes, skin, and other surfaces. Try to avoid inhaling the dust or allowing large quantities of the paste to dry in one location.

Stable paste

Question: Why is it that the wet nitrogen triiodide is initially stable while the dry nitrogen triiodide is not?

You can check your answers here.