How To Perform the Nitrogen Triiodide Chemistry Demonstration

Easy and Dramatic Nitrogen Triiodide Demonstration

Iodine crystals readily change to the gas phase.
Iodine crystals readily change to the gas phase. Matt Meadows, Getty Images

In this spectacular chemistry demonstration, crystals of iodine are reacted with concentrated ammonia to precipitate nitrogen triiodide. The NI3 is filtered out. When dry, the compound is so unstable that the slightest contact causes it to decompose into nitrogen and iodine gas, producing a very loud 'snap' and a cloud of purple iodine vapor.

Difficulty: Easy

Time Required: Minutes

Materials

  • up to 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.
  2. The reaction for producing the nitrogen triiodide from iodine and ammonia is:

    3I2 + NH3 → NI3 + 3HI

     

  3. You want to avoid handling the NI3 at all, so my recommendation would be to set up the demonstration in advance of pouring off the ammonia. Traditionally, the demonstration uses a ring stand on which a wet filter paper with NI3 is placed with a second filter paper of damp NI3 sitting above the first. The force of the decomposition reaction on one paper will cause decomposition to occur on the other paper as well.
  1. 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.
  1. To activate the decomposition, tickle the dry NI3 solid with a feather attached to a long stick. The decomposition occurs according to this reaction:

    2NI3 (s) → N2 (g) + 3I2 (g)
     
  2. In its simplest form, the demonstration is performed by pouring the damp solid onto a paper towel in a fume hood, letting it dry, and activating it with a meter stick.

Tips and Safety

  1. Caution: This demonstration should only be performed by an 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.
  2. NI3 in the ammonia is very stable and can be transported, if the demonstration is to be performed at a remote location.
  3. How it works: 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.
  1. The amount of energy released upon detonating NI3 exceeds that required to form the compound, which is the definition of a high yield explosive.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "How To Perform the Nitrogen Triiodide Chemistry Demonstration." ThoughtCo, Jul. 19, 2016, thoughtco.com/nitrogen-triiodide-chemistry-demonstration-606311. Helmenstine, Anne Marie, Ph.D. (2016, July 19). How To Perform the Nitrogen Triiodide Chemistry Demonstration. Retrieved from https://www.thoughtco.com/nitrogen-triiodide-chemistry-demonstration-606311 Helmenstine, Anne Marie, Ph.D. "How To Perform the Nitrogen Triiodide Chemistry Demonstration." ThoughtCo. https://www.thoughtco.com/nitrogen-triiodide-chemistry-demonstration-606311 (accessed January 18, 2018).