Gibbs Free Energy and Its Role in Chemical Thermodynamics

Specific reactions are known to be spontaneous as they give off energy in the form of heat (H < 0). Interestingly, a few other spontaneous reactions result in an increase in the disorder of a system (S > 0). Therefore, calculating H and S can identify the actual forces behind such reactions.

Furthermore, when one such force behind a reaction is favoured, and others are not, Gibbs free energy (G) is used to identify those results. Moreover, it also reflects the balance between these reactions. 

What is Gibbs Free Energy?

In thermodynamics, Gibbs free energy is known as a thermodynamic potential. Moreover, this potential is used to calculate the optimum of reversible work that one thermodynamic system can perform at constant pressure and temperature. Additionally, the measuring unit of Gibbs free energy is Joules in SI. 

Furthermore, when a thermodynamic system transforms reversibly from its initial state towards its final state; the decrease in Gibbs energy is similar to the work done by this system and its surroundings. However, the work of pressure forces is not considered here.

Besides, this thermodynamic potential is minimised when a system reaches its chemical equilibrium at a constant temperature and pressure. Furthermore, the derivative of this system with respect to its reaction coordinate vanishes at this equilibrium point. Hence, a reduction in Gibbs free energy is needed to make such reactions spontaneous.

History of Gibbs Free Energy

The quantity termed as “free energy” is an advanced and accurate replacement for the archaic term “affinity”. This term was used by chemists in the initial years of physical chemistry to portray the forces behind chemical reactions.

Furthermore, in 1873, Josiah Willard Gibbs published his paper, “A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces”. In this journal, he mentioned the principles behind his new equation, which can predict or estimate the propensities of a different natural process which follows when these systems or bodies come in contact with each other. 

Moreover, in this paper, Gibbs identified three states of the above mentioned equilibrium. They are “necessarily stable”, “unstable”, and “neutral”. Additionally, he also mentioned whether any changes would follow or not. He arrived at this conclusion by understanding the interactions between homogeneous substances in contact. Furthermore, he used a three-dimensional volume-entropy-internal graph to study substances, which are part-solid, part-liquid and part-vapour.

Additionally, in another paper “Graphical Methods in the Thermodynamics of Fluids,” Gibbs outlined how his equation has the capability to assume the behaviour of systems when they are mixed. Moreover, this quantity here is associated with chemical reactions that can do the work. It also represents the sum of its enthalpy and product of its temperature and entropy. 

The Gibbs energy formula that defines this quantity is G=H -TS, or more completely as G=U+PV-TS. In this equation –

• U is internal energy with SI unit joule.

• P is pressure where the SI unit is pascal.

• V is a volume with SI unit m3T is temperature, and the SI unit is Kelvin.

• S is entropy with SI unit kelvin/joule.

• H is enthalpy where SI unit is a joule.

Gibbs Energy Reactions

Spontaneous Process

In chemistry, Gibbs energy change spontaneity of a process is the one that does not require any external energy. Moreover, it is considered natural as it occurs itself, without any outside influence. The spontaneous process can be quick or slow because it is not associated with kinetics rate. A prominent example of spontaneous reaction is diamonds turning into graphite. 

Additionally, over a long period, the carbon in the diamond slowly becomes more stable and less shiny, graphite. However, this process takes a long time, and it is hard for any human being to survive and witness this phenomenon. 

Furthermore, another point to remember here is that this process can be endothermic, as well as exothermic. 

How to Determine a Spontaneous Reaction?

The easiest way to understand this situation while solving an equation is if G is negative, then it is spontaneous. Otherwise, it is non-spontaneous, as it requires a continuous supply of external energy. Therefore, the Gibbs free energy symbol, i.e. G, can be ideally considered as “standard free energy charge”. 

Furthermore, as per the second law of thermodynamics, every spontaneous process raises the entropy of the universe. However, using this law to calculate a spontaneous reaction can be a little difficult. Chemists are usually interested in changes happening around them. Typically, it is a reaction in a beaker. Therefore, there is no need to investigate the entire universe, t understand a small change.

Thus, chemists use Gibbs free energy change to study such reactions. This new thermodynamic quantity helps researchers to determine entropy changes in the universe. Moreover, chemical reactions involving such thermodynamics quantities, variations of the following equations are often witnessed –

• ΔG (change in free energy) =ΔH (change in enthalpy) –TΔS (temperature change in entropy)

Moreover, this reaction, which does not have any subscript that specifies the thermodynamics values are for the system. Nevertheless, it is still considered that the values of H and S here are of the system of interest. 

Additionally, this equation is vital and exciting, as it permits to calculate the alterations using enthalpy and entropy changes. Furthermore, the G here can be used to figure out whether a reaction is spontaneous in forward or backward direction, or at equilibrium.

• Moreover, when G<0, this process is exergonic. It will move forward spontaneously and produce other products.

• However, if G>0, it is an endergonic process. Thus, it is not spontaneous in the forward direction. Instead, it will move freely in the reverse direction and produce other starting materials.

• On the other hand, when G=0, it reaches an equilibrium. Hence, the mixture of the products and reactants remains constant.

Spontaneity and Gibbs Free Energy

Furthermore, when any reaction occurs at a constant pressure P and constant temperature T, the second law of thermodynamics can be arranged for Gibbs energy define. Moreover, while using Gibbs free energy to determine the spontaneity of a process, the focus is on G. Thus, the absolute value is not considered here. Hence, the value of G in this process is the difference between its initial value and its final value.

Remarks on Gibbs Free Energy

The title “free energy” used to determine G has led to a lot of confusion. Thus, researchers these days primarily refer to it as Gibbs energy.

• Notably, this term “free” is a part of the older portrayal related to the steam engine origin of thermodynamics. It has only interest in converting heat into work. Here G stands for the optimum amount of energy, which can be extracted from this system to execute useful work. Furthermore, here ‘useful’ means any work which is not associated with the system expansion.

• Another serious difficulty of Gibbs energy change is in the framework of Chemistry. Even though G is calculated in the units of energy, it does not have a vital feature of energy, i.e. conservation. Even though energy levels fall due to any spontaneous chemical reactions, there is no increase of energy anywhere else. Hence, referring to G as energy is somewhat a misleading notion. 

• G has no thermodynamics quantities like H and S, as it has no physical reality like property of matter. Moreover, H and S stand for the quantity and distribution of energy in molecules, respectively. Hence, this free energy is just a useful construct, which serves as a term for a change to make the calculations easier. 

Furthermore, the Gibbs free energy is vital in researches as it enables one to predict the direction of a reaction. Moreover, this ability to calculate G plays a significant role in designing lab experiments. Apart from that, it is also an essential chapter of Chemistry, and students should prepare it well for their exam.

Additionally, students can seek assistance from Vedantu to prepare various chapters of Chemistry. They can download our official Vedantu app, and join the live classes by subject experts. Moreover, they can also access study materials, mocks tests, etc. to improve their preparations.