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

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. The Gibbs’ free energy of the reactants (G_R) is higher than that of the products (G_P). Therefore, the change in free energy is negative ^[6].

ΔG = G_P – G_R < 0

Where,

G_R: Gibbs’ free energy of the reactants

G_P: Gibbs’ free energy of the products

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

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].

Exergonic vs. Endergonic Reactions

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

	Exergonic Reaction	Endergonic Reaction
Definition	Gives off energy	Absorbs energy
Input energy	Not required	Required
Energy of reactants	Higher than the products	Lower than the products
Change in Gibbs’ free energy (ΔG)	Negative	Positive
Entropy	Increased	Decreased
Spontaneity	Favorable and spontaneous	Less favorable and non-spontaneous
Exothermic and endothermic reactions	Exothermic reactions are exergonic	Endothermic reactions are endergonic
At normal conditions	Occurs	Does not occur