Why Is Photosynthesis An Endothermic Reaction?

Photosynthesis is an endothermic reaction since energy in the form of sunlight is absorbed by plants. Precisely, in an endothermic reaction, energy is absorbed from the environment.

During photosynthesis , the pigments present in photosynthesizers must absorb the energy of a photon and then use this energy to initiate a chain of chemical and photochemical events.

Description by drawing of the photosynthesis process

In contrast, exothermic reactions are reactions that release energy into the environment in the form of heat. These feel warm or hot, and can even cause an explosion.

In this kind of reaction, the enthalpy change (amount of energy contained)  has a negative value.

Photosynthesis and other examples of an endothermic reaction

Chemical reactions transfer energy to, or from, the environment. Endothermic reactions absorb energy from the environment, while exothermic reactions transmit energy to the environment.

What determines whether a reaction is endothermic or exothermic, it is the balance between the energy that must be supplied to break existing bonds and the energy that is released when new bonds are formed.

On the other hand, this type of reaction usually causes a change in temperature. Just as endothermic reactions absorb energy from the environment, it is generally transferred as heat energy, making the reaction mixture and its surroundings cooler.

This happens because the energy required to break existing bonds is greater than the energy released when new bonds are formed. In this way, global energy is transferred from the environment to the chemicals that react, absorbing heat.

In this sense, endothermic reactions are less common than exothermic ones, but there are a number that are quite well known.

One of the most important is photosynthesis. This is the process by which plants transform carbon dioxide and water into sugar and oxygen, using solar energy.

Also, any thermal decomposition reaction is endothermic, because the reaction only takes place if heat is introduced into the system. A clear example of this is the degradation of calcium carbonate into calcium oxide and carbon dioxide.

The reaction takes place only if the calcium carbonate is heated up to 800 ° C. Therefore, this reaction takes a large amount of energy from the surroundings. 

Also, when certain salts such as potassium chloride and ammonium nitrate dissolve in water, they absorb heat from the surroundings. Therefore, the temperature of the solution decreases

Other examples of endothermic reaction

-The reaction of crystals of barium hydroxide octahydrate with dry ammonium chloride.

-Evaporation of water (water in a liquid state is a compound, and heat is absorbed by breaking the bonds in water molecules).

-Dissolution of ammonium chloride in water.

-Electrolysis process (the molecules decompose into ions due to the passage of electric current).

-The reaction of thionyl chloride (SOCl2) with cobalt (II) sulfate heptahydrate.

-Fry an egg (the egg solidifies when absorbing heat from the pan).

-Mix of water with ammonium nitrate.

-Mix of water with potassium chloride.

-Ethanoic acid with sodium carbonate.


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  2. Hall, DO and Rao, KK (1999). Photosynthesis. New York: Cambridge University Press.
  3. Helmenstine, A. (2016, March 09). Exothermic Reactions – Definition and Examples. Retrieved on October 02, 2017, from sciencenotes.org.
  4. Energy changes in reactions (s / f). On BBC GCSE Bitesize. Retrieved on October 2, 2017, from bbc.co.uk.
  5. Fullick, A and Fullick, P. (2001). Chemistry for AQA. Oxford: Heinemann.
  6. Helmenstine, AM (2017, April 05). Endothermic Reaction Examples. In Thought Co. Retrieved on October 2, 2017, from thoughtco.com. 

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