How to Perform the Nitrogen Triiodide Chemistry Demonstration


The nitrogen triiodide demonstration produces a loud sound and a cloud of purple vapor.
The nitrogen triiodide demonstration produces a loud sound and a cloud of purple vapor.

The nitrogen triiodide chemistry demonstration is a dramatic explosive reaction that produces sound and colored vapor. Crystals of iodine react with concentrated ammonia to precipitate nitrogen triiodide (NI3). The NI3 is then filtered out. When dry, the compound is so unstable that the slightest contact causes it to decompose into nitrogen gas and iodine vapor, producing a very loud “snap” and a cloud of purple iodine vapor.

Materials

Nitrogen Triiodide Structure
Nitrogen Triiodide Structure

Only a few materials are required for this project. Solid iodine and a concentrated ammonia solution are the two key ingredients. The other materials are used to set up and execute the demonstration.

  • Less than 1 g iodine (do not use more)
  • Concentrated aqueous ammonia (0.880 S.G.)
  • Filter paper or paper towel
  • Ring stand (optional)
  • Feather attached to a long stick

How to Perform the Nitrogen Triiodide Demo

  1. The first step is to prepare the NI3. One method is to simply pour up to a gram of iodine crystals into a a small volume of concentrated aqueous ammonia, allow the contents to sit for 5 minutes, then pour the liquid over a filter paper to collect the NI3, which will be a dark brown/black solid. However, if you grind the pre-weighed iodine with a mortar/pestle beforehand a larger surface area will be available for the iodine to react with the ammonia, giving a significantly larger yield. The reaction for producing the nitrogen triiodide from iodine and ammonia is:
    3I2 + NH3 → NI3 + 3HI 
  2. You want to avoid handling the NI3 at all, so set up the demonstration before pouring off the ammonia. Traditionally, the demonstration uses a ring stand on which a filter paper of damp NI3 sits above a second filter paper of damp NI3. The force of the decomposition reaction on one paper causes decomposition to occur on the other paper as well.
  3. For optimal safety, set up the ring stand with filter paper and pour the reacted solution over the paper where the demonstration is to occur. A fume hood is the preferred location. The demonstration location should be free of traffic and vibrations. The decomposition is touch-sensitive and will be activated by the slightest vibration.
  4. To activate the decomposition, tickle the dry NI3 solid with a feather attached to a long stick. A meter stick is a good choice (don’t use anything shorter).

You can also perform the demonstration simply by pouring the damp solid onto a paper towel in a fume hood, letting it dry, and activating it with a meter stick.

See the nitrogen triiodide demonstration in real time and in slow motion. (Michael Bell, Creative Commons License)

How It Works

The decomposition occurs according to this reaction:

2NI3 (s) → N2 (g) + 3I2 (g)

NI3 is highly unstable because of the size difference between the nitrogen and iodine atoms. There is not enough room around the central nitrogen to keep the iodine atoms stable. The bonds between the nuclei are under stress and therefore weakened. The outside electrons of the iodine atoms are forced into close proximity, which increases the instability of the molecule.

The amount of energy released upon detonating NI3 exceeds that required to form the compound, which is the definition of a high yield explosive.

Tips and Safety

Caution: This demonstration should only be performed by a trained instructor, using proper safety precautions. Wet NI3 is more stable than the dry compound, but still should be handled with care. Iodine will stain clothing and surfaces purple or orange. The stain can be removed using a sodium thiosulfate solution. Eye and ear protection are recommended. Iodine is a respiratory and eye irritant; the decomposition reaction is loud.

NI3 in the ammonia is very stable and can be transported, if the demonstration is to be performed at a remote location.

References

  • Ford, L. A.; Grundmeier, E. W. (1993). Chemical Magic. Dover. p. 76. ISBN 0-486-67628-5.
  • Silberrad, O. (1905). “The Constitution of Nitrogen Triiodide”. Journal of the Chemical Society, Transactions. 87: 55–66. doi:10.1039/CT9058700055
  • Tornieporth-Oetting, I.; Klapötke, T. (1990). “Nitrogen Triiodide”. Angewandte Chemie International Edition. 29 (6): 677–679. doi:10.1002/anie.199006771

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