AP Chemistry Unit 9.3 Gibbs Free Energy and Thermodynamic Favorability

Gibbs Free Energy and Thermodynamic Favorability

  • Gibbs Free Energy: available energy of substance that can be used for work
    • Used to predict if a chemical reaction will occur spontaneously
  • -ΔG° = the forward reaction is thermodynamically favored
    • The reaction favors the products
  • +ΔG° = not thermodynamically favored → not going to occur on its own
    • The reaction favors the reactants
    • Note: when see f in a question, use this equation
    •  of an element in its standard state is zero
      • Units: kJ
    • Method #1 for calculating Free Energy Change (for reactions at constant temp) 


      • ΔG° = 0 → are at equilibrium
        • Often when substance is at phase change → if at MP/BP or phase change are at equilibrium
        • Find MP/BP = set ΔG° equal to 0 and solve for T
        • Units must be the same!
    • Method #2: use Hess law

Effect of ΔH and ΔS on Spontaneity

Direction of Heat Flow

  1. Exothermic reactions usually increase the entropy of surroundings → are favored (bcuz heat is not required for the process)
  2. Endothermic reactions usually decrease the entropy of surroundings → are not favored (bcuz heat is required)



Favorability and Value of ΔG


Favored at all temp (ΔG always negative)



Favored at high temp (ΔG negative)

ΔG positive at low temp

Favored at low temp (ΔG negative)


Not favored at any temp (ΔG always positive) → never spontaneous

  • +ΔH = not favorable; -ΔH = favorable
  • Reactions can be ‘driven by enthalpy’ (where a very exothermic reaction (- ΔH)) overcomes a decrease in entropy) or ‘driven by entropy’ where an endothermic reaction occurs because of a highly positive ΔS
  • Note: Hess’s law works the same for ΔG° & ΔS° as it does for ΔH°
Scroll to Top