Exothermic reactions release energy, usually heat, when chemical bonds are converted into heat energy. The term comes from the Greek prefix “ex-” and “thermein”. Exothermic reactions occur under constant pressure and volume, and are common in everyday life, such as in rain formation and lighting a candle. Nuclear fission is another example of an exothermic reaction. The opposite of an exothermic reaction is an endothermic one, which absorbs thermal energy.
Exothermic refers to a chemical process that releases energy as a byproduct. This energy is usually in the form of heat, but it can also be in the form of light, electricity, or sound. The release occurs when the bonds between the chemicals used in the reactions are converted into heat energy.
First used by the French chemist Marcellin Pierre Eugène Berthelot, the term derives from the Greek prefix “ex-” and the word thermein. Thermein refers to heating processes, while the prefix ex simply means external, indicating that this is the direction heat goes when it is produced. Berthelot believed that all chemical processes could be measured in some way, such as by the heat released by exothermic reactions.
Exothermic reactions occur under conditions where the chemical reactants are under constant pressure and volume. In the case of reactions that release heat, this manifests itself in an increase in temperature. This heat may come from an earlier release of another byproduct such as light. In this case, light is absorbed into the molecules of the chemical, causing the molecules to vibrate. This in turn generates heat.
Sometimes the heat is so small that scientific instruments are needed to measure the change in temperature. Other reactions are felt without the aid of scientific equipment. Exothermic reactions are common and occur when water condenses from vapor and forms rain or when snow forms in clouds. Lighting a candle causes a chemical, exothermic reaction felt in the heat of the flame, just as burning wood or coal in a stove produces enough heat to heat an entire house.
Nuclear fission is another example of an exothermic reaction. In nuclear fission, atoms of certain isotopes are split apart, releasing heat energy. When an atom is split with a neutron, it releases not only heat but an extra neutron which goes to split another atom, starting a chain reaction. It is this chain reaction that powers a nuclear reactor and the release of thermal energy that makes it such a valuable source of energy. However, the reactions are kept under control in a reactor; if left unchecked, the reaction turns into one that drives a nuclear bomb.
The opposite of an exothermic reaction is an endothermic one. In this type of chemical reaction, thermal energy is absorbed in the process; this is seen in reactions such as photosynthesis and evaporation. These chemical processes cannot take place without the existence of heat, and because exothermic reactions produce heat instead of needing it as a component of the reaction, exothermic processes occur more spontaneously.
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