Why is nitroglycerin explosive?


Vocabulary
exothermic*

Nitroglycerin is a dense, oily liquid that detonates if heated to 218°C or if subjected to mechanical shock. The molecule is explosive for three reasons:

  1. Nitroglycerin contains both oxidant and fuel. Nitroglycerin molecules contain three nitrate groups (that act as powerful oxidizing agents) bound directly to a hydrocarbon fragment (which acts as a fuel):

    Nitroglycerin, the nitric acid triester of glycerol. Click on either image for a 3D Chime model.

    Hydrocarbons like gasoline usually burn rather than exploding because oxygen must come into contact with the fuel in the combustion reaction. Nitroglycerine contains its own oxidant; oxygen doesn't have to diffuse to the fuel to keep the reaction going.

  2. The decomposition is highly exothermic. Many reactions occur when nitroglycerin detonates, but the overall process can be written

    4 C3H5(ONO2)3(ell) rightarrow 12 CO2(g) + 10 H2O(g) + 6 N2(g) + O2(g)

    The reaction releases an enormous amount of heat because many strong bonds in the product gas molecules replace the fewer, weaker bonds in nitroglycerin. Every mole of nitroglycerin that detonates releases about 1.5 MJ of heat!
  3. A large volume of gas is produced when nitroglycerin detonates. Nitroglycerin has a density of 1.6 g/mL around room temperature. Four moles of nitroglycerin (MW 227.09) occupy about 570 mL; the gases produced by detonation of this amount of nitroglycerin would have a volume of about 710 L at room temperature; the actual volume would be much higher if the temperature rise accompanying the reaction were considered.
Nitroglycerin mixed with inert materials is much less sensitive to concussion. For example, it is mixed with diatomaceous earth to make dynamite; dynamite won't explode if dropped. Nitroglycerin tablets and sprays prescribed as heart medication [2] contain tiny amounts of nitroglycerin diluted by inert matter and are completely nonexplosive.

References and notes

  1. The Merck Index, 8th ed., Merck & Co., Rahway NJ, 1968.
  2. A tiny amount of nitroglycerin placed under the tongue causes blood vessels to dilate, instantly lowering blood pressure. It is often prescribed to people with blocked coronary blood vessels to relieve chest pains. Each nitroglycerin tablet contains under a milligram of nitroglycerin.

Author: Fred Senese senese@antoine.frostburg.edu



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