The Chemistry of Explosions

The Explosive Collection
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One more important section related to combustion is the "explosive" chemical. Explosives are substances that undergo a rapid oxidation reaction with the production of large quantities of gases.

It is the sudden buildup of gas pressure (see products above) that constitutes the nature of an explosion. The speed at which explosives decompose permits their classification as high or low explosives. The shock wave produced by the expanding gases causes much of the damage associated with an explosion.

TNT is explosive for two reasons:

1. TNT is composed of the elements carbon, oxygen and nitrogen. When TNT explodes it forms several covalent gases: CO, CO₂ and N₂ that are very stable. The production of these very low energy (stable) bonds means that a great deal of energy is released. It should be noted that most explosives contain these same elements.
2. TNT itself is high energy and unstable. The structure of TNT (above) shows three fairly large nitro groups (NO₂) bound to Toluene. Because these groups are fairly large and in close proximity to each other they cause strain on the structure of the Toluene. Remember that groups of electrons repulse each other. Other compounds under similar conformational strain are also explosive for this same reason.

Dynamite and TNT
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There are 3 major classes of explosives:

1. Low
2. Primary High
3. Secondary High

The classification of explosives is based on the speed by which the reaction takes place.

Low Explosives

The Chemistry of Gun Powder: What Makes It Burn So Fast
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Low explosives are those that burn only at their surface. This means that the burning of the compound follows a path. In a cartridge for a gun, the spark at one end of the cylinder starts the burning process and the burn follows down the length of the tube. This process takes place very rapidly, however, and is complete in just a few thousandths of a second. While this reaction is very fast it is sequential rather than everything all at once and thus slower than many other explosives. The sequential/slower combustion of these low explosives works well for guns and artillery because too rapid an explosion could cause the weapon itself to blow up. The slow reaction allows the gases to build up pressure and smoothly fire the bullet from the gun. Gun powder and fireworks are the most common low explosives. Low explosives only explode when contained. This means that if not placed in a sealed container like a bullet, the materials will just burn very rapidly. It is only if the gases they produce are contained that they cause explosions.

Primary High Explosives

Among the high explosives, primary explosives are those that ultrasensitive to heat, shock, or friction and provide the major ingredients found in blasting caps or primers used to detonate other explosives. Examples include nitroglycerine, and mercury fulminate & lead styphnate. Each of these molecules share the property of being highly strained structurally.

Secondary High Explosives

Secondary high explosives are those chemicals that do not have to be contained to explode and are relatively stable and safe to handle. They require an electrical spark, fuse, intense heat, or sharp blow to initiate their explosion. Secondary explosives are relatively insensitive to heat, shock, or friction and will normally burn rather than detonate if ignited in small quantities in the open air. Dynamite, TNT, Semtex (plastics) are some common examples of this type of explosive.

Huge Explosion! TNT!
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Collecting Evidence at the Scene of an Explosion

The process for examining evidence at the site of an explosion is very similar to the process followed at the scene of a fire.

1. Search systematically the entire scene and try to locate the epicenter (origin) of the explosion.
2. Objects located at or near the origin of the explosion must be collected for laboratory examination. Each object should be placed individually into a sealed airtight metal canister. Some explosive residues are known to be able to seep through plastic containment and contaminate other evidence.
3. Often a crater is located at the origin and loose soil and other debris must be preserved from its interior for laboratory analysis.
4. One approach for screening objects for the presence of explosive residues in the field or laboratory is the ion mobility spectrometer (IMS).

Ion Mobility Spectrometer

An Ion-mobility spectrometer (IMS) is an instrument that detects and separates ions in the gas phase. The technique passes absorbed gases through a tube using a carrier gas to promote flow. The walls of the tube are electrified so that ions with greater charge are more attracted and move slower through the tube. Large ions also move more slowly through the tube. A detector at the end of the tube recognizes the ions based on their charge and shape. The technique is heavily used by both military and law enforcement purposes; not only detecting explosives but also having use for identification of drugs.

Field Detection of Odor Signatures 2011 : 05 : Static and Dynamic Demo
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