Cannons to Carbohydrates: A Combustion Primer

Explosives are compounds that rapidly liberate energy when chemically rearranged through combustion. Energy (typically in the form of a flame or an electrical discharge) ignites the explosive and breaks weaker chemical bonds in the explosive molecule, rearranging its atoms to form new, stronger chemical bonds. This new bond formation liberates energy that makes the reaction self-sustaining. Overall, forming new bonds liberates much more energy than what is needed to break the original bonds, resulting in a net release of energy, including heat—a so-called exothermic reaction (exo meaning out; therme meaning heat).

For example, in the combustion of gasoline in air, carbon-carbon and carbon-hydrogen bonds are broken, and carbon-oxygen and hydrogen-oxygen bonds are formed, producing carbon dioxide, carbon monoxide, and water vapor. Combustion is rapid, and energy is liberated as an explosion within a car engine's cylinders, with the heated gases expanding and exerting a force on the pistons.

When your body cells burn food for energy, fundamentally the same exothermic chemistry occurs with the bonded atoms in food molecules such as fat, which is mostly carbon and hydrogen. The oxygen that we breathe and that is distributed by the bloodstream to all our body cells ultimately supplies the oxygen atoms to create carbon dioxide and water. But the bonds are broken and formed gradually, in many carefully controlled steps, so that energy is liberated slowly rather than explosively. Nonetheless, we correctly speak of "burning fat," a nonexplosive form of combustion known as cellular respiration. That this is an exothermic process is demonstrated by body heat. The faster we "run" our metabolism (such as when we burn more fats or carbohydrates during exercise), the warmer our bodies get. Sweating dissipates the heat and regulates body temperature.

Powered by sunlight, trees perform this metabolic process in reverse (thermodynamically, but not in terms of the details) in photosynthesis. The resulting carbohydrate, cellulose, is burned as wood to form the charcoal used to make gunpowder—first documented in the Middle East in 1200. It has taken 800 years for chemists to discover viable gunpowder alternatives in the form of high-nitrogen energetic compounds.

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