Published by: Nuru

Published date: 25 Jun 2021

Gibbs free energy which is also known as Gibbs function or Gibbs energy or free enthalpy is a quantity used to measure the maximum amount of work done in a thermodynamic system when the temperature and pressure are constant. Gibbs free energy is denoted as G.

This property was determined by Josiah Willard Gibbs in 1876 when he was conducting experiments to predict the behaviour of systems when combined together or whether a process could occur simultaneously and spontaneously. Gibbs free energy was also previously known as “available energy.”

Gibbs free energy is equal to the enthalpy of the system minus the product of the temperature and entropy. The equation is given as;

G = H – TS

Where,

G = Gibbs free energy

H = enthalpy

T = temperature

S = entropy

Gibbs free energy is a state function hence it doesn’t depend on the path. So change in Gibbs free energy is equal to the change in enthalpy minus the product of temperature and entropy change of the system.

ΔG=ΔH-Δ(TS)

If the reaction is carried out under constant temperature{ΔT=O}

ΔG=ΔH-TΔS

This equation is called the Gibbs Helmholtz equation

ΔG>0; the reaction is non-spontaneous and endergonic

ΔG<0; the reaction is spontaneous and exergonic

ΔG=0; reaction is at equilibrium

Note:

- According to the second law of thermodynamics entropy of the universe always increases for a spontaneous process.
- ΔG determines the direction and extent of chemical change.
- ∆G is meaningful only for reactions in which the temperature and pressure remain constant. The system is usually open to the atmosphere (constant pressure) and we begin and end the process at room temperature (after any heat we have added or which is liberated by the reaction has dissipated).
- ∆G serves as the single master variable that determines whether a given chemical change is thermodynamically possible. Thus if the free energy of the reactants is greater than that of the products, the entropy of the world will increase when the reaction takes place as written, and so the reaction will tend to take place spontaneously. ΔS universe = ΔS system + ΔS surroundings
- If ΔG is negative, the process will occur spontaneously and is referred to as exergonic.
- Therefore spontaneity is dependent on the temperature of the system.