# 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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