Free Energy & Equilibrium (College Board AP Chemistry)

• 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