# 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)
 ΔS ΔH 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°
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