# Gibbs Free Energy

> Gibbs free energy is also referred to as the Gibbs function. Gibbs energy, also known as free enthalpy, is the measurement used to determine the highest...

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Last updated: July 30, 2026

![Gibbs Free Energy](https://biologynotesonline.com/wp-content/uploads/2024/03/Gibbs-Free-Energy.jpg)

Gibbs free energy is also referred to as the Gibbs function. Gibbs energy, also known as free enthalpy, is the measurement used to determine the highest amount of work that can be done in a thermodynamic process when temperatures and pressures remain constant. Gibbs free energy can be identified by the symbol "G". Its value is typically measured by Joules and Kilojoules. Gibbs free energy could be defined as the largest amount of work that can be taken from the closed system.

This property was discovered by an American researcher Josiah Willard Gibbs in the year 1876, when he was conducting experiments to understand the behavior of different systems when they were combined, or if a particular process could be performed simultaneously and in a spontaneous manner. Gibbs ' free energy was before known in the past as "available energy." It could be described as the amount of useful energy in a thermodynamic system which can be used to accomplish the work.

## What is gibbs free energy?

Gibbs Free Energy is the thermodynamic value of a system, which provides the energy needed for work. It's used to determine if the reaction is spontaneous. Simply put, spontaneous reactions are those that happen naturally in a natural way, while nonspontaneous reactions are those that don't. What I am referring to by "naturally" is that a reaction can occur in a given system without the net flow of energy that is free from its surroundings. For instance, ice at 10oC or 1atm can melt quickly, while ice at -10oC and 1atm won't.

We can see that in a process that is spontaneous, an organism will use up' a portion of its free energy, and changes in the Gibbs' free energy can be negative ( ΔG0) in a non-spontaneous process that will require the input of free energy from the environment. In addition, the increase in Gibbs' free energy can be counted as negative ( ΔG=0) for the reaction that has reached equilibrium. The results are listed in the table below.

![Gibbs Free Energy](https://biologynotesonline.com/wp-content/uploads/2024/04/image-326.png)Gibbs Free Energy

## ΔG = ΔH - TΔS

The variation in Gibbs free energy ( ΔG) for any system is dependent on the variation in the enthalpy ( ΔH) along with the changes in the entropy ( ΔS) in accordance with the equation below:

ΔG = ΔH - TΔS

ΔGo = ΔHo - TΔSo

The relation is valid under standard conditions , as well as in non-standard conditions. It is possible to draw few generalizations about the conditions under which a reaction can be spontaneous  (i.e. when  ΔG 0

But there are two parts to the universe, the system and the surroundings, and we could express the 2nd Law one final time as follows:

For a spontaneous process, ΔSsystem + ΔSsurroudings > 0

This is where Gibbs started.  But measuring quantities for the surroundings is problematic as it includes all the rest of the universe outside of the system being investigated.  So Gibbs set out to devise a way to determine the spontaneity of a process based only upon thermodynamic properties of the system alone.  For this he needed to define ΔSsurroundings in terms of the system and substitute it back into the 2nd Law.  The change in entropy is defined as ΔS = qrev/T. 

From this we can derive an expression for ΔSsurroundings:

ΔSsurroundings = ΔHsurroundings / T

However, the increase or decrease in the surrounding is caused by the flow of enthalpy into or from the system which is why ΔHsurroundings as well as ΔHsystem are the same in size however they are opposite in terms of sign:  ΔHsurroundings = -ΔHsystem.  We can substitute this into our definition of ΔSsurroundings.

ΔSsurroundings = ΔHsurroundings / T = -ΔHsystem / T

This can now be substituted back into the 2nd Law of Thermodynamics.

ΔSsystem + ΔSsurroudings > 0

ΔSsystem - ΔHsystem / T > 0

Finally multiplying all terms by -T yields Gibbs Free Energy equation (remember that multiplying or dividing an inequality by a negative number changes the sign).

-TΔSsystem + ΔHsystem  < 0     rearranged     ΔHsystem - TΔSsystem < 0

Gibbs now had a condition for spontaneity that relied only on thermodynamic properties of the system and then coined it 'Gibbs Free Energy.'

ΔGsystem= ΔHsystem - TΔSsystem

And therefore we have derived from the 2nd Law of Thermodynamics:

For a spontaneous process, ΔHsystem - TΔSsystem < 0

For a spontaneous process, ΔGsystem < 0

And there you have it; Gibbs had devised a method of predicting if/when a process is spontaneous based upon thermodynamic properties of the system alone.

## References

- [https://www.chadsprep.com/chads-general-chemistry-videos/gibbs-free-energy/](https://www.chadsprep.com/chads-general-chemistry-videos/gibbs-free-energy/)

- https://opentextbc.ca/introductorychemistry/chapter/gibbs-free-energy/

- https://chemistrytalk.org/what-is-gibbs-free-energy/

- https://www.chem.fsu.edu/chemlab/chm1046course/gibbs.html

- https://www.toppr.com/guides/chemistry/thermodynamics/gibbs-free-energy/

- https://www.thoughtco.com/definition-of-gibbs-free-energy-605869

- https://www.cliffsnotes.com/study-guides/chemistry/chemistry/thermodynamics/gibbs-free-energy

- https://www.chemguide.co.uk/physical/entropy/deltag.html

- https://www.corrosionpedia.com/definition/591/gibbs-free-energy-gfe

- https://en.wikipedia.org/wiki/Gibbs_free_energy

- https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/gibbs-free-energy/a/gibbs-free-energy-and-spontaneity

- https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Energies_and_Potentials/Free_Energy/Gibbs_(Free)_Energy

- https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/gibbs.php