Concept of Free Energy, Endergonic and Exergonic Reaction : Pharmaguideline

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Concept of Free Energy, Endergonic and Exergonic Reaction

The work that can be done in a thermodynamic system at constant temperature and pressure is defined by its free enthalpy.

Concept of free energy

The work that can be done in a thermodynamic system at constant temperature and pressure is defined by its free enthalpy. Gibb’s free energy, Gibb's function, and Gibb's energy can also be used to describe this. The symbol G is used to denote Gibb’s free energy. In general, people express it’s value in Joules or Kilojoules. A closed system has a Gibb’s free energy that is the maximum amount of energy it can produce. During experiments to predict how systems would behave when combined or whether a process could take place simultaneously and spontaneously, American scientist Josiah Willard Gibbs discovered this property in 1876. A thermodynamic system can use free energy to perform work when it is available. It was previously known as "available energy."

Gibbs free energy equals the product of the enthalpy of the system minus the temperature minus the entropy. The expression is as follows:

G = H – TS


Free Gibbs energy = G

Enthalpy is H

The temperature is T

Entropy is defined as S

G = U + PV – TS

Here, the internal energy is indicated as U (SI unit is Joule)

Pressure is p with SI unit as pascal

V is a volume with SI unit – a cube of m

The temperature has the SI unit as kelvin and here it is denoted as T

S is entropy with an SI unit of kelvin/joule

Endergonic and exergonic reactions

Chemical reactions, or processes, are classified as either endergonic or exergonic in thermochemistry, or physical chemistry. Both terms describe how energy is transformed during the reaction. Exotherms and endotherms are related to one another, however endergonic and exergonic pertain to reactions that utilize any kind of energy, while exothermic and endothermic refer to reactions that rely on heat sources.
  • A chemical reaction that releases energy will result in a negative number from the equation.
  • ∆ G < 0 indicates that reactions that release energy.
  • When the free energy of the products of a reaction is lower than that of the reactants, this is partly due to the energy released during the reaction.
  • A reaction that has a negative ∆ G and, as a result, produces free energy is called an exergonic reaction. These reactions release energy from the system.
  • In addition to being called spontaneous reactions, these reactions can also occur without the addition of external energy.
  • It is vital for biologists to study chemical reactions that release free energy so they can harness it inside cells to perform work.
  • We must distinguish between spontaneous reactions and reactions that occur immediately.
  • In our everyday lives, spontaneous reactions are not ones that occur suddenly or rapidly.
  • During the rusting process, copper and iron react in a slow, gradual manner.
  • Chemical reactions requiring energy input will have a positive ∆ G as opposed to reactions which release energy.
  • There is more free energy in the products in this case than in the reactants.
  • Thus, these molecules are capable of storing energy.
  • The reactions described here are nonspontaneous; they are called endergonic reactions.
  • endergonic reactions cannot occur on their own without the presence of free energy.
An exergonic reaction will decrease the free energy of a system while an endergonic reaction will increase it. The exergonic reaction releases energy, whereas the endergonic reaction consumes energy.

Endergonic reactions

  • Alternatively, endergonic reactions may also be referred to as unfavorable reactions or involuntary reactions. You must expend more energy than you receive.
  • Energy for this reaction is absorbed by the environment.
  • A reaction forms weaker chemical bonds than a reaction that breaks chemical bonds.
  • This leads to the system's free energy increase. An endergonic reaction has a positive (greater than zero) change in Gibbs free energy (G).
  • This decreases entropy (S).
  • Endergonic reactions do not occur spontaneously.
  • A photosynthesis reaction is an endothermic reaction as is the melting of ice into liquid water.
  • Those reactions cause endothermy to decrease the temperature of their surroundings.

Exergonic reactions

  • Exergonic reactions can also be called spontaneous reactions or favorable reactions.
  • Energy is released into the surrounding space during energetic reactions.
  • As a result of the reaction, chemical bonds are formed that are stronger than those that were formed by the reactants.
  • The amount of free energy decreases. Gibbs Free Energy (G) is decreased during an exergonic reaction.
  • S is the entropy of the reaction. It could also be said that the system becomes more disordered or random.
  • Unlike enzymatic reactions, exergonic reactions begin spontaneously.
  • As an example of an exothermic reaction, mixing sodium with chlorine to form table salt, combustion, and chemiluminescence (light is produced) all fall under the category of exergonic reactions.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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