Nitrogen triiodide - a sensitive, contact explosive

Kit

• 50 ml beaker, 100 ml conical flasks
• filter funnel and filter paper
• glass stirring rod
• pestle and mortar
• 0.48 g iodine crystals
• 0.880 M concentrated ammonia solution
• 100 ml 5 M NaOH solution
• heat-proof mat; metre rule

Procedure 

Working in a fume cupboard, wearing a face shield and nitrile gloves, use a pestle and mortar to grind 0.5 g of iodine crystals into a fine powder and transfer to a small beaker. To the iodine powder add approximately 4 ml of concentrated ammonia solution and stir briefly using a glass rod. Allow the mixture to stand for ca  five minutes for the reaction to complete.  

To extract the solid explosive, swirl the mixture to suspend the solid in the ammonia solution and then pour onto a filter paper held over a conical flask. While the beaker is upturned over the filter paper use a plastic teat pipette to squirt concentrated ammonia solution to wash any residue NI₃ onto the filter paper.  

Turn off the fume cupboard to prevent air movement. Remove the wet filter paper containing the NI₃(s), open up and place on a heat-proof mat so that it lies flat. Leave to dry in the fume cupboard while taking adequate precautions to prevent students or other staff getting near the experiment - place stools or similar in front of the cupboard. 

To destroy any residual explosive on the glass rod or in the beaker, add 5 M NaOH solution (caution: corrosive) and leave until no black solid is visible. Once the filter paper is dry it is possible to detonate the crystals by tickling with a feather attached to a metre rule. Humidity and room temperature can prolong the drying process and reduce the shock-sensitivity of the explosive and so a greater 'strike' might be required to detonate it.  

Wearing a face shield and holding a metre rule at arms length hit the NI₃ to produce the explosion and then switch on the fume cupboard to remove the iodine vapour cloud. Tap the rule against a wall to detonate any remaining NI₃ that adheres. Place the filter paper remains into the 5 M NaOH solution, rinse until colourless and put in the refuse. 

Note: the photograph (right) shows six, stacked 0.5 g samples exploding, but these quantities far exceed what is permitted without a police certificate and are not recommended for a classroom demonstration, owing to the large volume of iodine vapour produced.  

Special tips 

Most iodine stains can be removed using a 1 M sodium thiosulfate solution. Cold, damp conditions will significantly increase the drying time. If the sample was still damp when struck, brushing the surrounding area will detonate and therefore remove any stray/dispersed crystals. 

Teaching goals

Nitrogen triiodide decomposes according to the following equation:

2NI₃(s) → N₂(g) + 3I₂(g) 

This reaction provides an excellent opportunity for post-16 students to consider the entropy changes during the reaction. The change from two moles of ordered, solid crystals into four moles of gas is consistent with the large ΔS^θ_system, and this can be easily visualised during the reaction, as can the exothermic nature of the reaction and thus the positive entropy change of the surroundings (ΔS^θ_surroundings = -ΔH/T ).  Therefore the total entropy change (ΔS^θ_total = ΔS^θ_system + ΔS^θ_surroundings) will be expected to be large and positive, indicating a spontaneous change which will go to completion.