In a chemical reaction, reactants are converted into products by the breaking and making of chemical bonds. An example is the burning of carbon in oxygen to make carbon dioxide. Bond breaking requires energy, while bond making releases energy. The balance between the two gives a positive or negative energy change for the reaction. Chemical reactions are classed as being either endothermic, with a positive energy change, or exothermic, with a negative energy change. In an endothermic reaction, more energy is taken breaking bonds than is released making them, so the reaction proceeds with a net absorption of energy. In an exothermic reaction, the reverse is true and energy is released. The issue of whether a reaction is endothermic or exothermic is important in the forensic investigation of explosions.
Endothermic reactions are in the minority—most chemical reactions release energy. An endothermic reaction usually needs some energy to get it going. However, some endothermic reactions occur without this boost; the dissolving of ammonium nitrate in water occurs spontaneously with a marked decrease in temperature due to heat being taken in from the surroundings. Compounds formed by endothermic reactions have stored or potential chemical energy in their bonds, which may be released spontaneously in an explosion. Such compounds include chlorates, perchlorates, and nitrates. These may explode spontaneously, which makes them dangerous to handle whether or not there is any criminal intent. If combined with other components, they will make a high explosive mixture. Explosions of endothermic compounds typically produce a lot of gas, creating a destructive pressure wave spreading through the surrounding area.
The explosion of an endothermic substance is in itself exothermic, with the chemical potential energy of the compound being released in the form of heat, light, and sound. Endothermic explosions are possible too, but need some kind of primer explosive to give them the energy required to set them off. A knowledge of the chemistry of explosives can help the forensic investigator understand the devastation that occurs at the scene of an explosion.
see also Exothermic reactions.