Concept of free energy

  • Free energy is a thermodynamic function like enthalpy and entropy.
  • It is also known as free enthalpy.
  • Free energy is used to determine the changes made by a system and how much work it produced.
  • It is expressed in two forms i.e. (1) Helmholtz free energy
                                                             (2) Gibb’s free energy

Gibbs free energy:

  • It is named in honor of the great American physicist J.W. Gibb’s(1839-1903).
  • Gibb’s free energy(G) is used to calculate the maximum reversible work done of a thermodynamic system at constant temperature and pressure.
  • Or this is the maximum amount of non expansion work, that can be extracted from a  completely reversible closed system .
  • Gibb’s free energy also tells weather the process is spontaneous or not, i.e.if the change in free energy is negative the process is spontaneous, and if positive the process is non spontaneous.
  • Mathematically
             G = H – TS
         => ∆G = ∆H – T∆S
            Where ∆G = change in Gibb’s free energy
                          ∆H = change in enthalpy
                          ∆S = change in entropy

Helmholtz free energy:

  • It is named in the honor of German physicist Hermann von Helmholtz(1821-1894).
  • It is also termed as Work function or Helmholtz function ( A) 
  • It is a type of thermodynamic potential that measures the useful work obtained from a closed system at constant temperature and volume.
  • If the change in Helmholtz free energy is negative, that means maximum amount of work is performed by a system at constant volume.
  • And if the volume is not constant the part of this work is performed as boundary work.
  • Mathematically, A = U – TS

                        =>∆A = ∆U – T∆S

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