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ExposeExciteIgniteMay2012

Figure 10.10 Approach

Figure 10.10 Approach the crystals with the feather mounted to the long pole and lightly, from a distance, dust the crystals with the feather . . . BANG ! ! ! Immediately step away from the purple iodine vapour. NOTE: If you do not get immediate detonation, leave the crystals for another 15 minutes to dry. Teaching Extension Here are two other fun activation methods: Buy a wind-up toy or pull-back car from a toy shop and make it walk a deadly destructive walk towards the touch sensitive triiodide. You may want to film this so it can be viewed in slow motion. Figure 10.11 Party poppers are available for sale even Figure 10.12 in places where fireworks are banned. Their pyrotechnic parts can be very handy! Remove the bottom cardboard lid and paper pop-out parts. Cut the plastic body with scissors so you can pull the rod shaped ‘detonator’ free from the bottom. (Not from the top . . oops!) Lengthen the detonator’s string with another piece and tape the detonator with masking tape to a sturdy surface. This all still has to be done in a safe and well-ventilated area. Place the wet nitrogen triiodide on top of the detonator to dry and wait . . . do not detonate before the triiodide is dry. 117

If it malfunctions, wait another 15 minutes and set all strewn triiodide off with the feather duster. Disposal & Clean-up If detonation went well then you won’t have much to discard. But if you set-off partially dry triiodide then the area may be “contaminated” with unreacted explosives. This is easily detected when walking around the area after the detonation. Sweep the area with a broom or duster. Clean the direct explosion surface area with water and discard all paper residues in a waste bin. Any purple iodine stains will sublime within a few days. For short term results, remove the stains with a sodium thiosulphate solution. Key Terms Detonation vs deflagration, explosives, reaction kinetics, activation energy, spontaneous reactions, Gibbs free energy, enthalpy, entropy References 1. Shakhashiri, B. Z.; Chemical Demonstrations: A handbook for teachers of chemistry, Vol. 1, The University of Wisconsin Press: Wisconsin, 1983; p 96 - 98 Appendix A: Thermodynamics: The driving force behind chemical reactions When chemists refer to a spontaneous process or reaction, it is one that occurs without the need for additional energy input after the reactants are mixed and the reaction is initiated. Most chemical reactions in nature are exothermic and are usually spontaneous at 25!, especially as many incorporate an increase in entropy ( !H 0 = neg. !S 0 = pos. ). !G 0 is the Gibbs free-energy change of a chemical reaction (after J W Gibbs who developed the concept in 1873). In chemistry it quantifies the maximum amount of energy available to perform useful work, from any chemical reaction. It allows one to determine whether a reaction will proceed and is minimized (! G = 0) when a system reaches equilibrium at constant temperature and pressure. 118