IB Chemistry HL

Revision Notes

15.2.5 Free Energy & Equilibrium

Free Energy & Equilibrium

  • When ΔG < 0 for a reaction at constant temperature and pressure, the reaction is spontaneous
  • When a reversible reaction reaches equilibrium, the Gibbs free energy is changing as the ratio of reactants to products changes
  • For non-reversible reactions:
    • As the amount of products increases, the reaction moves towards completion
    • This leads to a decrease in Gibbs free energy
  • For reversible reactions:
    • As the amount of products increases, the reaction moves towards equilibrium
    • This causes a decrease in Gibbs free energy
  • At the point of equilibrium, Gibbs free energy is at its lowest as shown on the graph:

 

 

Gibbs free energy changes as the reaction proceeds

  • In section 1 of the graph, the forward reaction is favoured and the reaction proceeds towards a minimum value
  • Having reached a point of equilibrium, the Gibbs free energy increases
    • This is when the reaction becomes non-spontaneous (section 2)
  • The reverse reaction now becomes spontaneous and the Gibbs free energy again reaches the minimum value, so heads back towards equilibrium
  • The reaction will be spontaneous in the direction that results in a decrease in free energy (becomes more negative)
  • When the equilibrium constant, K, is determined for a given reaction, its value indicates whether the products or reactants are favoured at equilibrium
  • ΔG is an indication of whether the forward or backward reaction is favoured

 

 

 

Graph for a spontaneous reaction

 

 

 

Graph for a non-spontaneous reaction 

  • The quantitative relationship between standard Gibbs free energy change, temperature and the equilibrium constant is represented by:

ΔG = –RT In K 

 

 

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