What is an endothermic reaction?

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ncchemist | eNotes Employee

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Thermodynamics is the study of the flow of heat through a system.  In terms of chemical reactions, they can be described thermodynamically as endothermic and exothermic.  Exothermic reactions are chemical reactions that produce heat as they progress.  The reaction vessel feels warm to the touch.  Endothermic reactions absorb heat as they progress.  Since they absorb heat from their surroundings, the reaction vessel feels cold to the touch.  Other chemical processes besides reactions can also be endothermic.  Dissolving ammonium chloride in water, for example, is an endothermic process.  If you do this in a glass flask on a humid day, water vapor from the air will condense on the outside of the flask just like a cold beverage glass.