Exothermic vs Endothermic Chemical Reactions

A chemical reaction is a process in which one or more substances, called reactants, are transformed into one or more different substances, called products. Substances are either elements or compounds. An element is a substance that cannot be broken down into simpler substances by chemical means. A compound is a substance that is made up of two or more elements that are chemically combined.

Chemical reactions can be classified as exothermic or endothermic.

Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy in the form of heat or light. The energy released by an exothermic reaction is often used to power engines or to generate electricity.

Examples of Exothermic Reactions

  • The burning of wood or gasoline
  • The rusting of iron
  • The digestion of food
  • Hand warmers
  • Aerobic respiration


How Exothermic Reactions Work

In an exothermic reaction, the reactants have more energy than the products. When the reactants are combined, they release this excess energy in the form of heat or light.

exothermic reaction
endothermic reaction

Endothermic reactions are chemical reactions that require an input of energy (in the form of heat, light, or electricity) to proceed. The term “endothermic” is derived from the Greek words “endon” meaning “within” and “thermos” meaning “heat.”

During an endothermic reaction, energy from the surroundings is absorbed by the reactants to break their chemical bonds.

Common examples of endothermic reactions include:

  • Photosynthesis: The process by which plants convert light energy from the sun into chemical energy stored in glucose.
  • Mixing water and ammonium nitrate
  • Mixing water with potassium chloride
  • Ice pack