Endothermic and Exothermic Reactions

Endothermic vs Exothermic

Photosynthesis is an example of an endothermic reaction.
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Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt.

This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)

Demonstrations You Can Perform

Many exothermic and endothermic reactions involve toxic chemicals, extreme heat or cold, or messy disposal methods. An example of a quick exothermic reaction is dissolving powdered laundry detergent in your hand with a bit of water.

An example of an easy endothermic reaction is dissolving potassium chloride (sold as a salt substitute) in your hand with water.

These endothermic and exothermic demonstrations are safe and easy:

Endothermic vs Exothermic Comparison

Here's a quick summary of the differences between endothermic and exothermic reactions:

heat is absorbed (feels cold)heat is released (feels warm)
energy must be added for reaction to occurreaction occurs spontaneously
disorder decreases (ΔS < 0)entropy increases (ΔS > 0)
increase in enthalpy (+ΔH)decrease in enthalpy (-ΔH)
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Your Citation
Helmenstine, Anne Marie, Ph.D. "Endothermic and Exothermic Reactions." ThoughtCo, Feb. 12, 2017, thoughtco.com/endothermic-and-exothermic-reactions-602105. Helmenstine, Anne Marie, Ph.D. (2017, February 12). Endothermic and Exothermic Reactions. Retrieved from https://www.thoughtco.com/endothermic-and-exothermic-reactions-602105 Helmenstine, Anne Marie, Ph.D. "Endothermic and Exothermic Reactions." ThoughtCo. https://www.thoughtco.com/endothermic-and-exothermic-reactions-602105 (accessed March 24, 2018).