Free Energy & Equilibrium (College Board AP® Chemistry)

Free Energy & Equilibrium

• When ΔG° < 0 for a reaction at constant temperature and pressure, the reaction is thermodynamically favored

  • This means that the products are favored when the reaction reaches equilibrium 

  • We can say that the equilibrium constant K, is above 1

• When ΔG° is above 0, the reaction is not thermodynamically favored

  • The reaction will proceed in the reverse direction 

  • The equilibrium constant, K, is below 1

• When ΔG° is equal to zero the reaction is at equilibrium and no further reaction occurs 

  • The equilibrium constant will be close to 1 

• The qualitative relationship between ΔG°, the equilibrium constant, and temperature can be represented as:

  ΔG° = -RT In K 

  • R is the ideal gas constant

  • T represents the absolute temperature in Kelvin.

  • ln is the natural logarithm.

• To find the equilibrium constant we use:

K = e^-ΔG°/RT

Free energy diagrams

• Remember: ΔG is an indication of whether the forward or backward reaction is favoured

• For a thermodynamically favored reaction:

  • Most of the reactants are converted into products as the minimum is closer to the product side 

  • The slope of the curved line is to the left-hand side of the equilibrium point so ΔG° is negative and the reaction moves towards the products (and the minimum

Free energy diagram for a thermodynamically favored reaction

The reaction favors the products 

• For a not thermodynamically favored reaction:

  • A small amount of reactant is converted into products at equilibrium (where the minimum is)

  • The slope and ΔG° are positive so the reaction proceeds towards the reactants (and towards the minimum)

Free energy diagram for a not thermodynamically favored reaction

The reaction favors the reactants