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Exergonic Reaction

What is an Exergonic Reaction

An exergonic reaction is a chemical reaction in which the reaction (system) gives out energy to the surroundings. It is the opposite of an endergonic reaction. In this process, old bonds are broken, and new bonds are formed, thereby releasing energy. The change in Gibbs’ free energy (ΔG) is negative, i.e., the reactants have more free energy than the products. Exergonic reactions are spontaneous reactions since they can occur without adding external energy to the system. All exothermic reactions are exergonic, but not all exergonic reactions are exothermic [1-5].

General Equation

Reactants → Products + Energy

Exergonic Reaction

What Happens to Energy in an Exergonic Reaction

The energy required to initiate an exergonic reaction is called activation energy. After overcoming this activation barrier, the reactants react and form products. During the reaction, the Gibbs’ free energy of the reactants (ΔG) is higher than that of the products (ΔGP). Therefore, the change in free energy is negative [6].

ΔG = GP – GR < 0

Where,

GR: Gibbs’ free energy of the reactants

GP: Gibbs’ free energy of the products

Exergonic Reaction Graph

Exergonic Reaction Examples

Here are some examples of the exergonic reaction.

1. Mixing of sodium (Na) and chlorine (Cl) to form sodium chloride (NaCl)

    2 Na + Cl2 → 2 NaCl + energy

2. Hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ATD)

    ATP + H2O → ADP + Pi + energy

3. Combustion of Propane (C3H8) with Oxygen (O2)

    5 O2 + C3H8 → 4 H2O + 3 CO2 + energy

Exergonic Reaction Example

Endergonic and Exergonic Reactions

Exergonic reactions can be coupled to endergonic reactions. An example of such coupling is the oxidation-reduction (redox) reaction. Both reactions require energy to proceed. They use enzymes to lower the activation energy needed to complete the reaction. Enzymes often act by coupling an endergonic reaction to the exergonic reaction during the hydrolysis of ATP [7].

Endergonic and vs Exergonic Reactions

Exergonic vs. Endergonic Reactions

The following table list the basic differences between exergonic and endergonic reactions [8].

Exergonic ReactionEndergonic Reaction
DefinitionGives off energyAbsorbs energy
Input energyNot requiredRequired
Energy of reactantsHigher than the productsLower than the products
Change in Gibbs’ free energy (ΔG)NegativePositive
EntropyIncreasedDecreased
SpontaneityFavorable and spontaneousLess favorable and non-spontaneous
Exothermic and endothermic reactionsExothermic reactions are exergonicEndothermic reactions are endergonic
At normal conditionsOccursDoes not occur

References

  1. Bio.libretexts.org
  2. Estrellamountain.edu
  3. Chem.ucla.edu
  4. Openoregon.pressbooks.pub
  5. Projects.ncsu.edu
  6. Biology.arizona.edu
  7. Bcrc.bio.umass.edu
  8. Thoughtco.com