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Relationship between Free Energy, Enthalpy and Entropy; Redox potential

An enthalpy change is equal to the heat that is absorbed or released when a process takes place under constant pressure.

Relationship between free energy, enthalpy, and entropy


An enthalpy change is equal to the heat that is absorbed or released when a process takes place under constant pressure. In some cases, enthalpy is referred to as "heat content"; however, "enthalpy" is an intriguing and unusual term, so we usually use it instead. Historically, the words "entropy" and "enthalpy" come from the Greek, meaning "turning" and "warming", respectively. Entropy is usually pronounced with the first syllable stressed, whereas enthalpy is usually pronounced with the second syllable stressed.

H = U + PV

Enthalpy of the entire system is denoted as H

Volume = V

Pressure = P

When temperature and pressure are held constant, Gibbs free energy, also called Gibb's function or Gibb's energy, is used to measuring the amount of work that can be done in a thermodynamic system. 'G' is the symbol associated with Gibb's free energy. Joules or Kilojoules are usually used as units of measurement. A closed system can be thought of as having Gibbs free energy as the maximum amount of work that it can release. To predict the behavior of the systems when they are combined as well as to determine whether the process can occur spontaneously or simultaneously, Scientist Josiah Willard Gibbs conducted various experiments and determined this property in 1876. A thermodynamic system has Gibbs free energy which can be defined as the amount of workable energy present that can be utilized. Previously this term was referred to as "available energy."

Gibb's free energy comes from subtracting enthalpy from temperature and entropy. In other words;

G = H – TS


Entropy is S

Temperature is denoted as T

G is Gibb’s energy

H is enthalpy
  • For a spontaneous process, the entropy of the universe always increases according to the second law of thermodynamics.
  • Changes in entropy are determined by the G coefficient.
  • During constant temperatures and pressure reactions, ΔG is meaningful only. The reaction in this system starts as well as ends at room temperature whereas it is also open into the atmosphere at constant pressure.
  • An individual type of chemical change can be thermodynamically determined by ΔG as a single master variable. If the free energy of the reactants is greater than the products then the entropy of the system/world automatically increases and hence, the reaction occurs spontaneously when the free energy of the products is less than the reactants. A ΔS universe is the sum of a ΔS system and a ΔS environment
  • The process occurs spontaneously if ΔG is negative and is called exergonic.
  • To determine the spontaneity of the system, the temperature of the system is highly dependable.
For predicting spontaneity, the free energy change method is better than the entropy change method as the former only requires free energy changes of the system; the latter also requires entropy changes to the surroundings.

Redox potential

Redox potential is a specific indication of the degree to which both oxidizing and reducing properties of a substance have reached equilibrium, such as a mixture of reducing and oxidizing components. The redox potential can be expressed as follows:
  • The chemical reactivity of substances when exposed to environmental conditions
  • Various substances and systems can be predicted to exhibit corrosion protection
Oxidation-reduction is referred to as redox.

Redox potential is a measurement of the oxidizing-reducing ability of a system, according to the electrochemical balance. A substance's oxidation potential determines its ability to absorb oxygen, remove hydrogen, and lose electrons. An electron is attracted to hydrogen when it has a reduction potential.

By increasing the redox potential and turning positive, it becomes more capable of oxidizing. Quantitatively, it increases its reducing power when its value declines. A liquid's pH value can be compared to it. In redox reactions, material constituents are oxidized and process components are reduced in equal measures in one-half of the reaction. Reduction, for example, has a negative redox potential as a driving force. Positive redox potential is the driving force for oxidation. In addition to oxidation and reduction, corrosion involves an electrochemical activity which has the potential to lead to corrosion or resistance to corrosion.
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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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