Naked Science Forum
General Science => General Science => Topic started by: chenhongxia on 14/08/2008 04:07:10
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Chemical explosions may be either decomposition or combination reactions. In either case, the reaction is exothermic and the energy released by the reaction is partially converted to work. Decomposition reactions occur in materials such as trinitrotoluene (TNT) and nitroglycerine. The molecules of these materials contain oxygen. When the molecule decomposes, the products are combustion gases, which are produced at high temperatures. The volume of the gases is much larger than the volume of the explosive, generating high pressures at the reaction zone. The rapid expansion of the gases forms the shock wave that provides the explosive effect. Even some hydrocarbons that have no oxygen in their molecules, such as acetylene, can decompose explosively.
Combination reactions require that two or more components react together exothermically to produce hot gases. Some examples are ammonium nitrate and fuel oil (ANFO), gunpowder (potassium nitrate, carbon, and sulfur), and fireworks. In these explosions, the reactants that make up the explosive must be carefully mixed to assure that the reaction will continue.
The damage caused by an explosion depends partly on how fast the explosive reaction occurs. Decomposition reactions generally occur much faster than combination reactions. They are more likely to be used for military applications because they are more destructive. They also have a stronger shattering effect (called brisance) than combination reactions. Combination explosions are frequently used in mining operations because they have lower brisance and occur at slower rates.
There is a special case of explosion known as a vapor cloud explosion that can occur when a fuel (such as ordinary propane) is mixed with the atmosphere. If the cloud is ignited, the burning rate may be fast enough to form a shock wave. Although the overpressure in the shock wave may not be very high compared to other explosions, it is strong enough to damage or destroy structures. In a vapor cloud explosion, the result is called a deflagration if the shock wave moves slower than the speed of sound and a detonation if the shock wave moves faster than the speed of sound. Detonations are more destructive than deflagrations because of their stronger shock waves.
It should be kept in mind that explosives are generally unstable compounds. Once the exothermic decomposition or combination reaction is started, the temperature rises and the reaction accelerates; it usually cannot be stopped.
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