Free Energy & Equilibrium (DP IB Chemistry: HL)

• 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^ꝋ = -RT In K