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Gibbs Energy Change and Equilibrium

Gibbs free energy is denoted as G.
It is used to predict the spontaneity of the chemical reaction.
It is used to predict the useful work that could be extracted from it.

  • Reversible system is a system at which all times it is in perfect equilibrium with its surroundings.
  • When applied to a chemical reaction, the term 'reversible' indicates that a given reaction can proceed in either direction simultaneously, so that a dynamic equilibrium is set up.
  • This means that the reaction in both the directions should proceed with a decrease in free energy which seems impossible.
  • It is possible only if at equilibrium the free energy of the system is minimum.
  • if it is not, the system would spontaneously change to configuration of lower free energy. So, the criterion for equilibrium,

rG = 0

Gibbs energy for a reaction in which all reactants and products are in standard state, ∆rG- is related to the equilibrium constant of the reaction as follows :-

0 = ∆rG- + RTlnK

or ∆rG- = -RTlnK

or ∆rG- = -2.303 RT log K

we also know that


rG- = ∆rH- - T∆rS - = -RTlnK


  • It is possible to obtain an estimate of ∆G- from the measurement of ∆H- and ∆H-
    ∆S- and then calculate K at any temperature for economic yields of the products
  • f K is measured directly in the laboratory, value of ∆G- at any other temperature can be calculated.

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