An endothermic reaction occurs when energy is absorbed from the surroundings in the form of heat. Conversely, an exothermic reaction is one in which energy is released from the system into the surroundings. The terms are commonly used in the physical sciences and chemistry.

Comparison chart

Endothermic versus Exothermic comparison chart
Edit this comparison chartEndothermicExothermic
Introduction A process or reaction in which the system absorbs energy from its surroundings in the form of heat. A process or reaction that releases energy from the system, usually in the form of heat.
Result Energy is absorbed from the environment into the reaction. Energy is released from the system into the environment.
Form of Energy Energy is absorbed as heat. Energy is usually released as heat, but can also be electricity, light or sound.
Application Thermodynamics; physics, chemistry. Thermodynamics; physics, chemistry.
Etymology Greek words endo (inside) and thermasi (to heat). Greek words exo (outside) and thermasi (to heat).
Examples Melting ice, photosynthesis, evaporation, cooking an egg, splitting a gas molecule. Explosions, making ice, rusting iron, concrete settling, chemical bonds, nuclear fission and fusion.


What is an endothermic reaction?

An endothermic reaction or process takes place when the system absorbs heat energy from the surrounding environment.

What is an exothermic reaction?

In an exothermic reaction or process, energy is released into the environment, usually in the form of heat, but also electricity, sound, or light.

Exothermic vs Endothermic Processes in Physics

Classifying a physical reaction or process as exothermic or endothermic can often be counterintuitive. Making an ice cube is the same type of reaction as a burning candle — both have the same type of reaction: exothermic. When considering whether a reaction is endothermic or exothermic, it is vital to separate the reaction system from the environment. What matters is the change in temperature of the system, not how hot or cold the system is in general. If the system cools down, that means heat is being released, and the reaction taking place is an exothermic reaction.

The fire example above is intuitive, as energy is clearly being released into the environment. Making ice, however, might seem like the opposite, but water sitting in a freezer is also releasing energy as the freezer draws the heat out and expels it in the back of the unit. The reaction system to be considered is only the water, and if water is cooling down, it must be releasing energy in an exothermic process. Sweating (evaporation) is an endothermic reaction. Wet skin feels cool in a breeze because the evaporative reaction of the water absorbs heat from the surroundings (skin and atmosphere).

In Chemistry

In chemistry, endothermic and exothermic only consider the change in enthalpy (a measure of the total energy of the system); a full analysis adds additional term to the equation for entropy and temperature.

When chemical bonds are formed, heat is released in an exothermic reaction. There is a loss of kinetic energy in the reacting electrons, and this causes energy to be released in the form of light. This light is equal in energy to the stabilization energy required for the chemical reaction (the bond energy). The light released can be absorbed by other molecules, giving rise to molecular vibrations or rotations, from which comes the classical understanding of heat. The energy needed for the reaction to occur is less than the total energy released.

When chemical bonds break, the reaction is always endothermic. In endothermic chemical reactions, energy is absorbed (drawn from outside the reaction) to place an electron in a higher energy state, thus allowing the electron to associate with another atom to form a different chemical complex. The loss of energy from the solution (the environment) is absorbed by reaction in the form of heat.

The splitting of an atom (fission), however, should not be confused with the "breaking of a bond." Nuclear fission and nuclear fusion are both exothermic reactions.

Everyday Examples

Endothermic and exothermic reactions are frequently seen in everyday phenomena.

Examples of endothermic reactions:

Examples of exothermic reactions:


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