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These terms are usually applied to chemical reactions. A chemical reaction can only be one of these three terms at once. A reaction that is exothermic will be endothermic if run backward and vice-versa.
Exothermic chemical reactions liberate heat. A simple and familiar example is the combustion of methane gas (CH4). The balanced chemical reaction for this process is:
CH4(g) + 2 O2 (g) CO2(g) + 2 H2O(g).
We could write "heat" as one of the products on the right (products) side of the reaction if we wished. The term enthalpy, H, is used by chemists to describe how heat flows into or out of a system.
For an exothermic reaction, the change in enthalpy, ΔH, as we go from reactants (methane and oxygen) to products (carbon dioxide and water) is a negative quantity. For an endothermic reaction, ΔH is greater than zero. And for a thermoneutral reaction ΔH = 0.
ΔH tells us how much heat energy will be given off or will be required for a particular chemical reaction. It doesn't tell us whether the reaction will occur or how fast.
Endothermic reactions are usually not a great safety hazard. However, because the reaction draws heat from its surroundings, the reaction container may become cold and cause condensation or ice to form. This can be a safety hazard if the materials involved react with water.
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Many common chemicals undergo exothermic reactions. For example, simply dissolving sodium hydroxide (NaOH) in water produces enough heat that if this is not done carefully it could melt a plastic container! Our entry on oxidation has an example of an even more exothermic reaction.
The heat that a chemical reaction gives off can quickly heat the surrounding area (or rest of the chemicals in the container) to very high temperature. As temperature increases, the rate of chemical reactions generally increase as well. Thus, once an exothermic reaction begins, it can quickly "run away" -- accelerating its rate because of the heat produced. This can be especially dangerous if the material reaches its flash point or autoignition temperature, at which point a fire or explosion could occur.
Therefore, it is very important to know when a chemical reaction can generate excess heat and to take appropriate measures to deal with this. Examples include slow mixing, using a cooling bath, or avoiding that reaction. Most mixtures of incompatible chemicals involve violently rapid exothermic reactions.
See also: autoignition temperature, flash point, oxidation.
Additional definitions from Google and OneLook.
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