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Exergonic Reactions

Andria Emerson, John Williams
  • Author
    Andria Emerson

    Andria Emerson has taught high school science for over 17 years. She has a M.S from Grand Canyon University in Educational Leadership and Administration, M.S from Grand Canyon University in Adult Education and Distance Learning, and a B.S from the University of Arizona in Molecular and Cellular Biology.

  • Instructor
    John Williams
What is an exergonic reaction? Find the definition of "exergonic" and exergonic reaction examples. Learn the properties of exergonic chemical reactions and exergonic processes. Updated: 10/25/2021

Table of Contents


What is an Exergonic Reaction?

An exergonic reaction is a reaction that releases free energy. Free energy, also called Gibbs free energy ({eq}\Delta G\ {/eq}) is the free energy difference between the products and reactants in a chemical reaction. {eq}\Delta G\ {/eq} is the change in free energy as reactants undergo reactions to form products. In an endergonic reaction, {eq}\Delta G<0\ {/eq}. This is because an endergonic reaction releases free energy.

Exergonic reactions are also referred to as spontaneous, favorable, and exoergic reactions. These reactions do not always occur immediately; they just occur without interventions. The reaction that occurs as iron rusts is a spontaneous reaction. This reaction is not sudden and does not occur quickly.

Energy is stored in chemical bonds. When molecules have a high amount of free energy in chemical bonds, they are unstable. This is because free energy prefers to be released rather than stay in a bond. This encourages the reaction to be spontaneous.

Photosynthesis and cellular respiration provide examples of how organisms store energy and release energy in biochemical systems. Photosynthesis is a type of endergonic reaction where energy is stored. During photosynthesis, photosynthetic organisms convert energy from the sunlight into energy-storing compounds such as glucose (shown below). During cellular respiration, otherwise known as an endergonic reaction, cells in both plants and animals release the energy stored in glucose molecules.

Glucose Molecule

Image of a glucose molecule

The diagram below shows the course of an exergonic reaction. In order for a reaction to occur, two events must occur: molecules need to collide and the molecules need to be held in a specific orientation to ensure the reaction occurs. Energy is required for these events to occur. This creates an energy barrier that must be overcome. The smallest amount of energy required to overcome the energy barrier and enable the reaction to occur is called activation energy. Activation energy is not the same as Gibbs free energy. The free energy from the system must overcome the activation energy. Once this occurs, the reaction can proceed.

Endergonic Reaction

Image of the energy changes that occur during an endergonic reaction

In an endergonic reaction, energy is released as chemical bonds are broken. Therefore, the products of an exergonic reaction have less energy than the reactants.

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Exergonic Reactions vs Endergonic Reactions

The difference between exergonic and endergonic reactions are shown below:

Endergonic vs. Exergonic Reactions

Two graphs showing energy changes during exergonic and endergonic reactions

Exergonic Reactions

  • Referred to as spontaneous, favorable, and exoergic
  • Outside energy input is not required for the reaction to occur
  • Energy is released
  • Free energy of the system decreases
  • Change in free energy is negative: {eq}\Delta G < 0\ {/eq}
  • Chemical bonds are broken during reaction and energy is released
  • Change in entropy (disorder) increases
  • Energy is released to surroundings
  • Temperature of the surroundings increases

Endergonic Reactions

  • Referred to as non-spontaneous or unfavorable
  • Outside energy input is required for the reaction to occur
  • Energy is absorbed
  • Free energy of the system increases
  • Change in free energy is positive: {eq}\Delta G>0\ {/eq}
  • Chemical bonds are created during the reaction and energy is stored
  • Change in entropy (disorder) decreases
  • Energy is absorbed from surroundings
  • Temperature of the surroundings decreases

Exergonic Reaction Examples

Cells require a constant supply of energy for maintenance and to perform needed functions. Exergonic reactions are involved in many of the processes cells use to obtain energy. During exergonic reactions, chemical bonds in food molecules are broken releasing energy and fueling cells. Energy is released in the form of heat.

Cellular Respiration

Cellular respiration is a process organisms use to obtain energy. During cellular respiration, the energy stored in glucose molecules is transferred to an energy-storing molecule called adenosine triphosphate (ATP). Glucose is to humans as gasoline is to cars. ATP is referred to as the energy-storing compound in cells. Energy must be stored in ATP for cells to have access to the energy released in glucose.

The equation below shows the reactants and products involved in cellular respiration. During cellular respiration, glucose, in the presence of oxygen, produces carbon dioxide, water, and energy (ATP).

{eq}C_{6}H_{12}O_{6}+6O_{2}\rightarrow 6CO_{2}+6H_{2}O+energy {/eq}

Cellular respiration occurs in three steps: glycolysis, Krebs cycle, and the electron transport chain.

During glycolysis, one molecule of glucose is broken into two smaller molecules called pyruvate. Two molecules of ATP (energy) are needed to break one glucose molecule, however, four molecules of ATP are produced creating a net gain of two ATP molecules for the cell to use as energy. Glycolysis comes from the Greek glukos (sweet) and lusis (rupture).

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Frequently Asked Questions

What is exergonic in organic chemistry?

Exergonic means energy is released. Energy is stored in the bonds of molecules. During an exergonic reaction, the energy of the reactants is higher than the energy of the products.

What are some types of exergonic reactions in the human body?

Cellular respiration and lipid metabolism are exergonic reactions found in the human body. Both of these reactions release energy stored in chemical bonds.

What is an exergonic reaction in biology?

In cellular respiration, organisms break bonds in glucose molecules. This releases energy for cells to use. Energy is found in chemical bonds.

What does the word "exergonic" mean?

An exergonic reaction is a reaction that releases free energy. Ex means to exit or to release. An exergonic reaction is spontaneous.

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