What is an Exergonic Reaction?

The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.

Exergonic Reactions

Exergonic Reactions are spontaneous reaction. For a reaction to be spontaneous the change in Gibbs free energy, ∆G should be negative.  At constant temperature and pressure, ∆G= ∆H - T∆S where ∆H is the enthalpy change of the reaction, T is the temperature of the reaction and ∆S is the entropy change of the reaction.

Note: absorb or release of heat energy (∆H) - The energy going to disorder a system at a particular temperature (T∆S) = Energy left for work (∆G).

• Since exothermic reactions releases heat energy, their ∆H < 0 and ∆S > 0 after the reaction.  This makes ∆G <0. Thereby, exothermic reactions are spontaneous.

 For example, cellular respiration:

C₆H₁₂O₆ (glucose) + 6 O₂ -> 6 CO₂ + 6 H₂O

In this reaction energy is released with which the other cell activities proceed.

• Certain endothermic reactions even on absorption of heat, ∆H > 0 increases the entropy at high temperature, T∆S > 0, resulting in the negative Gibbs free energy, ∆G < 0. This happens only at high temperature, T i.e. when TΔS>ΔH, an endothermicreaction may become exergonic.  This reaction is also spontaneous.

For example, Water gas reaction in which water vapor is adsorbed on a solid carbon:

 H₂O+C↔CO+H₂.

The above mentioned reactions (a&b) are exergonic reactions in which the Products Gibbs free energy is lesser than that of the reactants Gibbs free energy.

Although exergonic reactions are spontaneous some may require a little input of energy to start but more energy is released by the reaction than what is required to initiate it. For example, to start a fire from woods, we need a little fire from any source but after the commencement of combustion, more light and heat energy is released than what was required to initiate.

Kinetics

For an exergonic reaction, the equilibrium constant, K_eq is greater than 1, i.e. the concentration of products is greater than the concentration of reactants at equilibrium.

K_eq = [products] / [reactants] > 1

However, we cannot predict the rate of the reaction just by knowing that the reaction is endergonic. For example, formation of rust (oxidation of iron) is an exergonic reaction that releases heat(exothermic), since the reaction occurs slowly it's hard to observe the release of heat to the environment.

Coupling Reactions

Exergonic reactions give/release Gibbs free energy which can be used as work energy by other reactions. This could be explained by an example of pair of weights joined by a rope with a pulley as given in the below mentioned diagram: when the heavier weight falls down, the lighter weight pulls up. Naturally, the lighter weight moves down due to gravitation which is a spontaneous process but because of its coupling to the heavier weight it raises above.

A reaction that consumes energy is known as ‘Endergonic’ reaction. They are non-spontaneous reactions with ∆G >0.Usually, in biological systems one reaction gives energy for the next and so endergonic reactions are coupled to exergonic reactions so that they get enough energy to proceed.

Examples of such coupled reactions are:

• Oxidation-reduction (redox) reactions of the coupling of exergonic and endergonic reactions. In the hydrolysis of ATP, enzymes act as a coupling agent for an endergonic reaction to the exergonic reaction.
• Bioluminescence of firefly is a result of endergonic reaction giving luminescence by luciferin (an organic substance which on oxidation by the enzyme luciferase generates light), this is coupled to the exergonic ATP (Adenosine triphosphate) release.

How Else Do Endergonic and Exergonic Relate to Each Other?

Endergonic reactions and exergonic reactions are sometimes also referred as reversible reactions because the quantity of the free energy released or absorbed is the same for both reactions, where endergonic reaction absorbs energy and exergonic reaction releases the energy.

For example, the synthesis of adenosine triphosphate (ATP) is an endergonic reaction.

ATP is synthesized by the reaction of an inorganic phosphate (P_i) and adenosine disphosphate (ADP) catalyzed by ATP synthatase: ADP + P_i → ATP + H₂O

The change in free energy for this reaction is ∆G = +7.3 kcal/mol under standard conditions.

The degradation/hydrolysis of ATP is a reverse process which is an exergonic process with ∆G = -7.3 kcal/mol: ATP + H₂O → ADP + P_i

But in reality, the reverse reaction does not always occur. For example, burning of a wood, rusting of an iron are irreversible reactions.

Common Mistakes

Do not confuse exergonic reactions with exothermic reactions. The former releases energy in various forms while the latter releases energy only in the form of heat. Exothermic is one type of exergonic reaction.

Practice Problems

1) Spontaneous reaction is/are: 

a) Endergonic 

b) Exergonic 

c) Both a & b. 

Answer: b) Exergonic 

2) Which of the following values must be negative in order for a reaction to be spontaneous? 

a) ∆G 

b) ∆S 

c) T∆S 

Answer: a) ∆G 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for  

• Bachelors. in Chemistry and Chemical Engineering 
• Masters. in Chemistry